Aircraft hardware:

The importance of aircraft hardware is often overlooked because of the small size of most items. However, the safe and efficient operation of any aircraft depends upon the correct selection and use of aircraft hardware. This chapter discusses the various types of threaded fasteners, quick-release fasteners, rivets, electrical hardware, and other miscellaneous hardware.
You must make sure that items of aircraft hardware remain tightly secured in the aircraft. Therefore, we will discuss proper safetying methods in this chapter. Aircraft hardware is identified for use by its specification number or trade name. Threaded fasteners and rivets are identified by Air Force-Navy (AN), National Aircraft Standard (NAS), and Military Standard (MS) numbers. Quick-release fasteners are identified by factory trade names and size designations.
When aircraft hardware is ordered from supply, the specification numbers and the factory part numbers are changed into stock numbers (SN). This change is identified by using a part-number cross-reference index.


In modern aircraft construction, thousands of rivets are used, but many parts require frequent dismantling or replacement. It is more practical for you to use some form of threaded fastener. Some joints require greater strength and rigidity than can be provided by riveting.
We use various types of bolts, screws, and nuts to solve this problem. Bolts and screws are similar in that both have a head at one end and a screw thread at the other.

However, there are several differences between them. The threaded end of a bolt is always relatively blunt. A screw may be either blunt or pointed. The threaded end
of a bolt must be screwed into a nut. The threaded end of the screw may fit into a nut or directly into the material being secured. A bolt has a fairly short threaded section and a comparatively long grip length (the unthreaded part). A screw may have a longer
threaded section and no clearly defined grip length. A bolt assembly is generally tightened by turning a nut.
The bolt head may or may not be designed to be turned. A screw is always designed to be turned by its head. Another minor difference between a screw and a bolt is that a screw is usually made of lower strength materials.
Threads on aircraft bolts and screws are of the American National Aircraft Standard type. This standard contains two series of threads—national coarse (NC) and national fine (NF). Most aircraft threads are of the NF series. Bolts and screws may have right- or left-hand
threads. A right-hand thread advances into engagement when turned clockwise. A left hand thread advances into engagement when turned counter clockwise.

However, there are several differences between them. The threaded end of a bolt is always relatively blunt. A screw may be either blunt or pointed. The threaded end
of a bolt must be screwed into a nut. The threaded end of the screw may fit into a nut or directly into the material being secured. A bolt has a fairly short threaded section and a comparatively long grip length (the unthreaded part). A screw may have a longer threaded section and no clearly defined grip length. A bolt assembly is generally tightened by turning a nut.
The bolt head may or may not be designed to be turned. A screw is always designed to be turned by its head. Another minor difference between a screw and a bolt is that a screw is usually made of lower strength materials. Threads on aircraft bolts and screws are of the
American National Aircraft Standard type. This standard contains two series of threads national coarse (NC) and national fine (NF). Most aircraft threads are of the NF series.
Bolts and screws may have right- or left-hand threads. A right-hand thread advances into engagement when turned clockwise. A left-hand thread advances into engagement when turned counter clockwise.


To choose the correct replacement for an unserviceable bolt, you must consider the length of the bolt. As shown in figure 5-1, the bolt length is the distance from the tip of the threaded end to the head of the bolt. Correct length selection is indicated when the bolt extends through the nut at least two full threads. See figure 5-2. If the bolt is too short, it will not extend out of the bolt hole far enough for the nut to be securely fastened. If it is too long, it may extend so far that it interferes with the movement of nearby parts.
In addition, if a bolt is too long or too short, its grip will usually be the wrong length. As shown in figure 5-2, the grip length should be approximately the same as the thickness of the material to be fastened. If the grip is too short, the threads of the bolt will extend into
the bolt hole. The bolt may act like a reamer when the material is vibrating. To prevent this, make certain that no more than two threads extend into the bolt hole.
Also, make certain that any threads that enter the bolt hole extend only into the thicker member that is being fastened. If the grip is too long, the nut will run out of threads before it can be tightened. In this event, a bolt with a shorter grip should be used. If the bolt grip
extends only a short distance through the hole, a washer may be used.
A second bolt dimension that must be considered is diameter. As shown in figure 5-1, the diameter of the bolt is the thickness of its shaft. The results of using a wrong diameter bolt should be obvious. If the bolt is too big, it cannot enter the bolt hole. If the diameter is too small, the bolt has too much play in the bolt hole.
The third and fourth bolt dimensions that should be considered when you choose a bolt replacement are head thickness and width. If the head is too thin or too narrow, it might not be strong enough to bear the load imposed on it. If the head is too thick or too wide, it
might extend so far that it interferes with the movement of adjacent parts.
AN bolts come in three head styles—hex head, clevis, and eyebolt. NAS bolts are available inimageimage

countersunk, internal wrenching, and hex head styles. MS bolts come in internal wrenching and hex head styles. Head markings indicate the material of which standard bolts are made. Head markings may indicate if the bolt is classified as a close-tolerance bolt. See figure 5-3. Additional information, such as bolt diameter, bolt length, and grip length, may be obtained from the bolt part number.

The most common threaded fastener used in aircraft construction is the screw. The three most used types are the machine screw, structural screw, and the self-tapping screw, as shown in figure 5-4. Figure 5-4 also shows the three head slots—straight, Phillips, and Reed and Prince.

Structural Screws
Structural screws are used for assembly of structural parts, as are structural bolts. They are made of alloy steel and are properly heat-treated. Structural screws have a definite grip length and the same shear and tensile strengths as the equivalent size bolt. They differ from structural bolts only in the type of head.

These screws are available in countersunk head, round head, and brazier head types. See figure 5-5.

Machine Screws
The commonly used machine screws are the round head, flat head, fillister head, pan head, truss head, and socket head types.

Self-Tapping Screws
A self-tapping screw is one that cuts its own internal threads as it is turned into the hole.
Self-tapping screws may be used only in comparatively soft metals and materials. Self-tapping screws may be further divided into two classes or groups—machine self-tapping screws and sheet metal self-tapping screws.
Machine self-tapping screws are usually used for attaching removable parts, such as nameplates, to castings. The threads of the screw cut mating threads in imageimage

the casting after a hole has been predrilled undersize. Sheet metal self-tapping screws are used for such purposes as temporarily attaching sheet metal in place for riveting. Sheet metal self-tapping screws may be used to permanently assemble non-structural units where it is necessary to insert screws in difficult to get to areas.
Self-tapping screws should never be used to replace standard screws, nuts, or rivets originally used in the structure.

Setscrews are used to position and hold components in place, such as gears on a shaft. Setscrews are available with many different point styles. They are classified as hexagon-socket and fluted-socket headless setscrews.

Aircraft nuts may be divided into two general groups—nonself-locking and self-locking nuts. Nonself-locking nuts are those that must be safetied by external locking devices, such as cotter pins, safety wire, or locknuts. The locking feature is an integral part
of self-locking nuts.

Nonself-locking Nuts
The most common of the nonself-locking nuts are the castle nut, the plain hex nut, the castellated shear nut, and the wing nut. Figure 5-6 shows these nonself-locking nuts.image

Castle nuts are used with drilled-shank AN hex-head bolts, clevis bolts, or studs. They are designed to accept a cotter pin or lockwire for safetying.
Castellated shear nuts are used on such parts as drilled clevis bolts and threaded taper pins. They are normally subjected to shearing stress only. They must not be used in installations where tension stresses are encountered. Plain hex nuts have limited use on aircraft structures. They require an auxiliary locking device such as a check nut or a lock washer.
Wing nuts are used where the desired tightness can be obtained by the fingers and where the assembly is frequently removed. Wing nuts are commonly used on battery connections.

Self-Locking Nuts
Self-locking nuts provide tight connections that will not loosen under vibrations. Self-locking nuts approved for use on aircraft meet critical specifications as to strength, corrosion resistance, and heat-resistant temperatures. New self-locking nuts must be used each time components are installed in critical areas throughout the entire aircraft. Self-locking nuts are found on all flight, engine, and fuel control linkage and attachments. There are two general types of self-locking nuts. They are the all-metal nuts and the
metal nuts with a non-metallic insert to provide the locking action. The Boots aircraft nut and the Flexloc nut are examples of the all-metal type. See figure 5-7.
The elastic stop and the non-metallic insert lock nut are examples of the non-metallic insert type. All-metal
self-locking nuts are constructed either of two ways. The threads in the load-carrying portion of the nut that is out of phase with the threads in the locking portion is one way. The second way is with a saw-cut top portion with a pinched-in thread. The locking action of these types depends upon the resiliency of the metal.imageimage

The elastic stop nut is constructed with a non-metallic (nylon) insert, which is designed to lock the nut in place. The insert is unthreaded and has a smaller diameter than the inside diameter of the nut.
Self-locking nuts are generally suitable for reuse in noncritical applications provided the threads have not been damaged. If the locking material has not been damaged or permanently distorted, it can be reused.


Turnlock fasteners are used to secure plates, doors, and panels that require frequent removal for inspection and servicing. Turnlock fasteners are also referred to as quick action panel fasteners. These fasteners are available in several different styles and are usually referred to by the manufacturer’s trade name. Some of the most common are the Camloc, Airloc, and Dzus.


The Camloc 4002 series fastener consists of four principal parts—receptacle, grommet, retaining ring, and stud assembly. See figure 5-9. The receptacle consists of an aluminum alloy forging mounted in a stamped sheet metal base. The receptacle assembly is riveted to the access door frame, which is attached to the structure of the aircraft. The grommet is a sheet metal ring held in the access panel by the retaining ring.
Grommets are available in two types—the flush type and the protruding type. In addition to serving as the grommet for the hole in the access panel, it also holds the stud assembly. The stud assembly consists of a stud, cross pin, spring, and spring cup. The assembly is
designed so that it can be quickly inserted into the grommet by compression of the spring. Once installed in the grommet, the stud assembly cannot be removed unless the spring is again compressed.image

The Camloc high-stress panel fastener, shown in figure 5-10, is a high-strength, quick-release, rotary-type fastener. It may be used on flat or curved, inside or outside panels. The fastener may have either a flush or protruding stud. The studs are held in the panel
with flat or cone-shaped washers. The latter being used with flush fasteners in dimpled holes. This fastener may be distinguished from screws by the deep No. 2 Phillips recess in the stud head and by the bushing in which the stud is installed.

Figure 5-11 shows the parts that make up an Airloc fastener. Similar to the Camloc fastener, the Airloc fastener consists of a receptacle, stud, and cross pin.
The stud is attached to the access panel and is held in place by the cross pin. The receptacle is riveted to the access panel frame.
Two types of Airloc receptacles are available—the fixed type (view A) and the floating type (view B). The floating type makes for easier alignment of the stud in the receptacle. Several types of studs are also available.
In each instance the stud and cross pin come as separate units so that the stud may be easily installed in the access panel.

Dzus fasteners are available in two types. One is the light-duty type, used on box covers, access hole covers, and lightweight fairing. The second is the heavy-duty type, which is used on cowling and heavy fairing. The main difference between the two types of Dzus
fasteners is a grommet, used with the heavy-duty fasteners. Otherwise their construction features are about the same.
Figure 5-12 shows the parts making up a light-duty Dzus fastener. Notice that they include a spring and a stud. The spring is made of cadmium-plated steel music wire and is usually riveted to an aircraft structural member. The stud comes in a number of designs (as shown in views A, B, and C) and mounts in a dimpled hole in the cover assembly.


Position the panel or plate on the aircraft before securing it in place. The spring riveted to the structural member enters the hollow center of the stud, which is retained in the plate or panel. Then, when the stud is turned about one-fourth turn, the curved jaws of the stud slip over the spring and compress it. The resulting tension locks the stud in place, thereby securing the panel or plate.


There are hundreds of thousands of rivets in the airframe of a modern aircraft. This is an indication of how important rivets are in the construction of aircraft.
A glance at any aircraft will disclose the thousands of rivets in the outer skin alone. In addition to being used in the skin, rivets are used in joining spar and rib sections. They are also used for securing fittings to various parts of the aircraft, and for fastening bracing
members and other parts together. Rivets that are satisfactory for one part of the aircraft are often unsatisfactory for another part.
Two of the major types of rivets used in aircraft construction are the solid rivet and the blind rivet. The solid rivet must be driven with a bucking bar. The blind rivet is installed when a bucking bar cannot be used.

Solid rivets are classified by their head shape, size, and the material from which they are manufactured.
Rivet head shapes and their identifying code numbers are shown in figure 5-13. The prefix MS identifies hardware under the control of the Department of Defense and that the item conforms to military standards. The prefix AN identifies specifications thatimage

are developed and issued under joint authority of the Air Force and the Navy. Solid rivets have five different head shapes. They are the round head, flat head, countersunk head, brazier head, and universal head rivets.

Round Head Rivets
Round head rivets are used on internal structures where strength is the major factor and streamlining is not important.

Flat Head Rivets:
Flat head rivets, like round head rivets, are used in the assembly of internal structures where maximum strength is required. They are used where interference of nearby members does not permit the use of round head rivets.

Countersunk Head Rivets
Countersunk head rivets, often referred to as flush rivets, are used where streamlining is important. On combat aircraft practically all external surfaces are flush riveted. Countersunk head rivets are obtainable with heads having an inclined angle of 78 and 100
degrees. The 100-degree angle rivet is the most commonly used type.

Brazier Head Rivets
Brazier head rivets offer only slight resistance to the airflow and are used frequently on external surfaces, especially on noncombat-type aircraft.

Universal Head Rivets
Universal head rivets are similar to brazier head rivets. They should be used in place of all other protruding-head rivets when existing stocks are depleted.

There are many places on an aircraft where access to both sides of a riveted structural part is impossible.
When attaching many non-structural parts, the full strength of solid-shank rivets is not necessary and their use adds extra weight. For use in such places, rivets have been designed that can be formed from the outside. They are lighter than solid-shank rivets but are amply strong. Such rivets are referred to as blind rivets.image

The rivnut is a hollow aluminum rivet that is counterbored and threaded on the inside. The rivet is installed with the aid of a special tool. Rivnuts are used primarily as a nut plate. They may be used as rivets in secondary structures such as instruments, brackets, and soundproofing materials. After rivnuts are installed, accessories can be fastened in place with screws. Rivnuts are manufactured in two head styles, countersunk and flat, and in two shank designs, open and closed ends. See figure 5-15.
Open-end rivnuts are the most widely used. They are preferred in place of the closed-end type. However, in sealed flotation or pressurized compartments, the closed-end rivnut must be used. Further information concerning rivets may be found in the Structural Hardware Manual, NAVAIR 01-1A-8.image

A turnbuckle is a mechanical screw device consisting of two threaded terminals and a threaded barrel. Figure 5-16 shows a typical turnbuckle assembly.image

Turnbuckles are fitted in the cable assembly for the purpose of making minor adjustments in cable length and for adjusting cable tension. One of the terminals has right-hand threads and the other has left-hand threads. The barrel has matching right- and left-hand
internal threads. The end of the barrel with the left-hand threads can usually be identified by a groove or knurl around that end.
When installing a turnbuckle in a control system, it is necessary to screw both of the terminals an equal number of turns into the barrel. It is also essential that you screw both turnbuckle terminals into the barrel until not more than three threads are exposed.
After you adjust a turnbuckle properly, it must be safetied. We will discuss the methods of safetying turnbuckles later in this chapter.

Taper pins are used in joints that carry shear loads and where the absence of clearance is essential. See figure 5-17. The threaded taper pin is used with a taper pin washer and a shear nut if the taper pin is drilled. Use a self-locking nut if the taper pin is undrilled. When a shear nut is used with the threaded taper pin and washer, the nut is secured with a cotter pin.

The flat head pin is used with tie-rod terminals or secondary controls, which do not operate continuously. The flat head pin should be secured with a cotter pin. The pin is normally installed with the head up. See figure 5-17, view C. This precaution is taken to
maintain the flat head pin in the installed position in case of cotter pin failure.



Push-pull tubes are used as linkage in various types of mechanically operated systems. This type linkage eliminates the problem of varying tension and permits the transfer of either compression or tension stress through a single tube.

A push-pull tube assembly consists of a hollow aluminum alloy or steel tube with an adjustable end fitting and a checknut at either end. (See figure 6-24.) The checknuts secure the end fittings after the tube assembly has been adjusted to its correct length. Push-pull tubes are generally made in short lengths to prevent vibration and bending under compression loads.


The three main types of pins used in aircraft structures are the taper pin, flathead pin, and cotter pin. Pins are used in shear applications and for safetying. Roll pins are finding increasing uses in aircraft construction.

Taper Pins

Plain and threaded taper pins (AN385 and AN386) are used in joints which carry shear loads and where absence of play is essential. The plain taper pin is drilled and usually safetied with wire. The threaded taper pin is used with a taper pin washer (AN975) and shear nut (safetied with cotter pin) or selflocking nut.

Flathead Pin

Commonly called a clevis pin, the flathead pin (MS20392) is used with tierod terminals and in secondary controls which are not subject to continuous operation. The pin is customarily installed with the head up so that if the cotter pin fails or works out, the pin will remain in place.

Cotter Pins

The AN380 cadmium plated, low carbon steel cotter pin is used for safetying bolts, screws, nuts, other pins, and in various applications where such safetying is necessary. The AN381 corrosion resistant steel cotter pin is used in locations where nonmagnetic material is required, or in locations where resistance to corrosion is desired.


The rollpin is a pressed fit pin with chamfered ends. It is tubular in shape and is slotted the full length of the tube. The pin is inserted with hand tools and is compressed as it is driven into place. Pressure exerted by the roll pin against the hole walls keeps it in place, until deliberately removed with a drift punch or pin punch.


Safetying is the process of securing all aircraft, bolts, nuts, screws, pins, and other fasteners so that they do not work loose due to vibration. A familiarity with the various methods and means of safetying equipment on an aircraft is necessary in order to perform maintenance and inspection.

There are various methods of safetying aircraft parts. The most widely used methods are safety wire, cotter pins, lockwashers, snaprings, and special nuts, such as selflocking nuts, pal nuts, and jamnuts. Some of these nuts and washers have been previously described in this chapter.

Safety Wiring

Safety wiring is the most positive and satisfactory method of safetying capscrews, studs, nuts, bolt heads, and turnbuckle barrels which cannot be safetied by any other practical means. It is a method of wiring together two or more units in such a manner that any tendency of one to loosen is counteracted by the tightening of the wire.

Nuts, Bolts, and Screws

Nuts, bolts, and screws are safety wired by the single wire or double twist method. The double twist method is the most common method of safety wiring. The single wire method may be used on small screws in a closely spaced closed geometrical pattern, on parts in electrical systems, and in places that are extremely difficult to reach.

Figure 6-25 is an illustration of various methods which are commonly used in safety wiring nuts, bolts, and screws. Careful study of figure 6-25 shows that:

a. Examples 1, 2, and 5 illustrate the proper method of safety wiring bolts, screws, squarehead plugs, and similar parts when wired in pairs.

b. Example 3 illustrates several components wired in series.

c. Example 4 illustrates the proper method of wiring castellated nuts and studs. (Note that there is no loop around the nut.)

d. Examples 6 and 7 illustrate a single threaded component wired to a housing or lug.

e. Example 8 illustrates several components in a closely spaced closed geometrical pattern, using a single wire method.

When drilled head bolts, screws, or other parts are grouped together, they are more conveniently safety wired to each other in a series rather than individually. The number of nuts, bolts, or screws that may be safety wired together is dependent on the application. For instance, when safety wiring widely spaced bolts by the double twist method, a group of three should be the maximum number in a series.

When safety wiring closely spaced bolts, the number that can be safety wired by a 24 inch length of wire is the maximum in a series. The wire is arranged so that if the bolt or screw begins to loosen, the force applied to the wire is in the tightening direction.

Parts being safety wired should be torqued to recommend values and the holes aligned before attempting the safetying operation. Never over torque or loosen a torqued nut to align safety wire holes.

Oil Caps, Drain Cocks, and Valves

These units are safety wired as shown in figure 6-26. In the case of the oil cap, the wire is anchored to an adjacent fillister head screw.

This system applies to any other unit which must be safety wired individually. Ordinarily, anchorage lips are conveniently located near these individual parts. When such provision is not made, the safety wire is fastened to some adjacent part of the assembly.

Electrical Connectors

Under conditions of severe vibration, the coupling nut of a connector may vibrate loose, and with sufficient vibration the connector may come apart. When this occurs, the circuit carried by the cable opens. The proper protective measure to prevent this occurrence is by safety wiring as shown in figure 6-27. The safety wire should be as short as practicable and must be installed in such a manner that the pull on the wire is in the direction which tightens the nut on the plug.


After a turnbuckle has been properly adjusted, it must be safetied. There are several methods of safetying turnbuckles; however, only two methods will be discussed in this section. These methods are illustrated in figure 6-28(A) and figure 6-28(B). The clip locking method is used only on the most modern aircraft. The older type aircraft still use the type turnbuckles that require the wire wrapping method.

Double Wrap Method

Of the methods using safety wire for safetying turnbuckles, the double wrap method is preferred, although the single wrap, methods described are satisfactory. The method of double wrap safetying is shown in figure 6-28(B). Use two separate lengths of the proper wire as shown in figure 6-29.

Run one end of the wire through the hole in the barrel of the turnbuckle and bend the ends of the wire towards opposite ends of the turnbuckle. Then pass the second length of the wire into the hole in the barrel and bend the ends along the barrel on the side opposite the first. Then pass the wires at the end of the turnbuckle in opposite directions through the holes in the turnbuckle eyes or between the jaws of the turnbuckle fork, as applicable. Bend the laid wires in place before cutting off the wrapped wire. Wrap the remaining length of safety wire at least four turns around the shank and cut it off. Repeat the procedure at the opposite end of the turnbuckle.

When a swaged terminal is being safetied, pass the ends of both wires, if possible, through the hole provided in the terminal for this purpose and wrap both ends around the shank as described above.

If the hole is not large enough to allow passage of both wires, pass the wire through the hole and loop it over the free end of the other wire, and then wrap both ends around the shank as described.

Single Wrap Method

The single wrap safetying methods described in the following paragraphs are acceptable but are not the equal of the double wrap methods.

Pass a single length of wire through the cable eye or fork, or through the hole in the swaged terminal at either end of the turnbuckle assembly. Spiral each of the wire ends in opposite directions around the first half of the turnbuckle barrel so that the wires cross each other twice. Thread both wire ends through the hole in the middle of the barrel so that the third crossing of the wire ends is in the hole. Again, spiral the two wire ends in opposite directions around the remaining half of the turnbuckle, crossing them twice. Then, pass one wire end through the cable eye or fork, or through the hole in the swaged terminal. In the manner described above, wrap both wire ends around the shank for at least four turns each, cutting off the excess wire.

An alternate to the above method is to pass one length of wire through the center hole of the turnbuckle and bend the wire ends toward opposite ends of the turnbuckle. Then pass each wire end through the cable eye or fork, or through the hole in the swaged terminal and wrap each wire end around the shank for at least four turns, cutting off the excess wire. After safetying, no more than three threads of the turnbuckle threaded terminal should be exposed.

General Safety Wiring Rules

When using the safety wire method of safetying, the following general rules should be followed:

1. A pigtail of 1/4 to 1/2 inch (three to six twists) should be made at the end of the wiring. This pigtail must be bent back or under to prevent it from becoming a snag.

2. The safety wire must be new upon each application.

3. When castellated nuts are to be secured with safety wire, tighten the nut to the low side of the selected torque range, unless otherwise specified, and if necessary, continue tightening until a slot aligns with the hole.

4. All safety wires must be tight after installation, but not under such tension that normal handling or vibration will break the wire.

5. The wire must be applied so that all pull exerted by the wire tends to tighten the nut.

6. Twists should be tight and even, and the wire between the nuts as taut as possible without overtwisting.

7. The safety wire should always be installed and twisted so that the loop around the head stays down and does not tend to come up over the bolt head, causing a slack loop.

Cotter Pin Safetying

Cotter pin installation is shown in figure 6-30. Castellated nuts are used with bolts that have been drilled for cotter pins. The cotter pin should fit neatly into the hole, with very little sideplay. The following general rules apply to cotter pin safetying:

1. The prong bent over the bolt end should not extend beyond the bolt diameter. (Cut it off if necessary.)

2. The prong bent down should not rest against the surface of the washer. (Again, cut it off if necessary.)

3. If the optional wraparound method is used, the prongs should not extend outward from the sides of the nut.

4. All prongs should be bent over a reasonable radius. Sharp angled bends invite breakage. Tapping lightly with a mallet is the best method of bending the prongs.


A snapring is a ring of metal, either round or flat in cross section, which is tempered to have springlike action. This springlike action will hold the snapring firmly seated in a groove. The external types are designed to fit in a groove around the outside of a shaft or cylinder. The internal types fit in a groove inside a cylinder. A special type of pliers is designed to install each type of snapring.

Snaprings can be reused as long as they retain their shape and springlike action.

External-type snaprings may be safety wired, but internal types are never safetied. Safety wiring of an external type snapring is shown in figure 6-31.

Identification of all types of fluid line fittings:

Aircraft fluid lines are usually made of metal tubing or flexible hose. Metal tubing (also called rigid fluid lines) is used in stationary applications and where long, relatively straight runs are possible. They are widely used in aircraft for fuel, oil, coolant, oxygen, instrument, and hydraulic lines. Flexible hose is generally used with moving parts or where the hose is subject to considerable vibration.

Occasionally, it may be necessary to repair or replace damaged aircraft fluid lines. Very often the repair can be made simply by replacing the tubing. However, if replacements are not available, the needed parts may have to be fabricated. Replacement tubing should be of the same size and material as the original tubing. All tubing is pressure tested prior to initial installation, and is designed to withstand several times the normal operating pressure to which it will be subjected. If a tube bursts or cracks, it is generally the result of excessive vibration, improper installation, or damage caused by collision with an object. All tubing failures should be carefully studied and the cause of the failure determined.

Rigid Fluid Lines

Tubing Materials


In the early days of aviation, copper tubing was used extensively in aviation fluid applications. In modern aircraft, aluminum alloy, corrosion resistant steel or titanium tubing have generally replaced copper tubing.

Aluminum Alloy Tubing

Tubing made from 1100 H14 (1/2-hard) or 3003 H14 (1/2-hard) is used for general purpose lines of low or negligible fluid pressures, such as instrument lines and ventilating conduits. Tubing made from 2024-T3, 5052-O, and 6061-T6 aluminum alloy materials is used in general purpose systems of low and medium pressures, such as hydraulic and pneumatic 1,000 to 1,500 psi systems, and fuel and oil lines.


Corrosion resistant steel tubing, either annealed CRES 304, CRES 321 or CRES 304-1/8-hard, is used extensively in high pressure hydraulic systems (3,000 psi or more) for the operation of landing gear, flaps, brakes, and in fire zones. Its higher tensile strength permits the use of tubing with thinner walls; consequently, the final installation weight is not much greater than that of the thicker wall aluminum alloy tubing. Steel lines are used where there is a risk of foreign object damage (FOD); i.e., the landing gear and wheel well areas. Although identification markings for steel tubing differ, each usually includes the manufacturer’s name or trademark, the Society of Automotive Engineers (SAE) number, and the physical condition of the metal.

Titanium 3AL-2.5V

This type of tubing and fitting is used extensively in transport category and high performance aircraft hydraulic systems for pressures above 1,500 psi. Titanium is 30 percent stronger than steel and 50 percent lighter than steel. Cryofit fittings or swaged fittings are used with titanium tubing. Do not use titanium tubing and fittings in any oxygen system assembly. Titanium and titanium alloys are oxygen reactive. If a freshly formed titanium surface is exposed in gaseous oxygen, spontaneous combustion could occur at low pressures.

Material Identification

Before making repairs to any aircraft tubing, it is important to make accurate identification of tubing materials. Aluminum alloy, steel, or titanium tubing can be identified readily by sight where it is used as the basic tubing material. However, it is difficult to determine whether a material is carbon steel or stainless steel, or whether it is 1100, 3003, 5052-O, 6061-T6 or 2024-T3 aluminum alloy. To positively identify the material used in the original installation, compare code markings of the replacement tubing with the original markings on the tubing being replaced.

On large aluminum alloy tubing, the alloy designation is stamped on the surface. On small aluminum tubing, the designation may be stamped on the surface; but more often it is shown by a color code, not more than 4″ in width, painted at the two ends and approximately midway between the ends of some tubing. When the band consists of two colors, one-half the width is used for each color. [Figure 7-1]


Metal tubing is sized by outside diameter (o.d.), which is measured fractionally in sixteenths of an inch. Thus, number 6 tubing is 6/16″ (or 3/8″) and number 8 tubing is 8/16″ (or 1/2″), and so forth. The tube diameter is typically printed on all rigid tubing. In addition to other classifications or means of identification, tubing is manufactured in various wall thicknesses. Thus, it is important when installing tubing to know not only the material and outside diameter, but also the thickness of the wall. The wall thickness is typically printed on the tubing in thousands of an inch. To determine the inside diameter (i.d.) of the tube, subtract twice the wall thickness from the outside diameter.

For example, a number 10 piece of tubing with a wall thickness of 0.063″ has an inside diameter of 0.625″ – 2(0.063″) = 0.499″.

Fabrication of Metal Tube Lines

Damaged tubing and fluid lines should be repaired with new parts whenever possible. Unfortunately, sometimes replacement is impractical and repair is necessary. Scratches, abrasions, or minor corrosion on

the outside of fluid lines may be considered negligible and can be smoothed out with a burnishing tool or aluminum wool. Limitations on the amount of damage that can be repaired in this manner are discussed in this chapter under “Rigid Tubing Inspection and Repair.” If a fluid line assembly is to be replaced, the fittings can often be salvaged; then the repair will involve only tube forming and replacement.

Tube forming consists of four processes: Cutting, bending, flaring, and beading. If the tubing is small and made of soft material, the assembly can be formed by hand bending during installation. If the tube is 1/4″ diameter or larger, hand bending without the aid of tools is impractical.

Tube Cutting

When cutting tubing, it is important to produce a square end, free of burrs. Tubing may be cut with a tube cutter or a hacksaw. The cutter can be used with any soft metal tubing, such as copper, aluminum, or aluminum alloy. Correct use of the tube cutter is shown in Figure 7-2. Special chipless cutters are available for cutting aluminum 6061-T6, corrosion resistant steel and titanium tubing.

A new piece of tubing should be cut approximately 10 percent longer than the tube to be replaced to provide for minor variations in bending. Place the tubing in the cutting tool, with the cutting wheel at the point where the cut is to be made. Rotate the cutter around the tubing, applying a light pressure to the cutting wheel by intermittently twisting the thumbscrew. Too much pressure on the cutting wheel at one time could deform the tubing or cause excessive burring. After cutting the tubing, carefully remove any burrs from inside and outside the tube. Use a knife or the burring edge attached to the tube cutter. The deburring operation can be accomplished by the use of a deburring tool as shown in Figure 7-3. This tool is capable of removing both the inside and outside burrs by just turning the tool end for end.

When performing the deburring operation, use extreme care that the wall thickness of the end of the tubing is not reduced or fractured. Very slight damage of this type can lead to fractured flares or defective flares which will not seal properly. Use a fine-tooth file to file the end square and smooth.

If a tube cutter is not available, or if tubing of hard material is to be cut, use a fine-tooth hacksaw, preferably one having 32 teeth per inch. The use of a saw will decrease the amount of work hardening of the tubing during the cutting operation. After sawing, file the end of the tube square and smooth, removing all burrs.

An easy way to hold small diameter tubing, when cutting it, is to place the tube in a combination flaring tool and clamp the tool in a vise. Make the cut about onehalf inch from the flaring tool. This procedure keeps sawing vibrations to a minimum and prevents damage to the tubing if it is accidentally hit with the hacksaw frame or file handle while cutting. Be sure all filings and cuttings are removed from the tube.

Tube Bending

The objective in tube bending is to obtain a smooth bend without flattening the tube. Tubing under 1/4″ in diameter usually can be bent without the use of a bending tool. For larger sizes, either portable hand benders or production benders are usually used. Table 7-1 shows preferred methods and standard bend radii for bending tubing by tube size.

Using a hand bender, insert the tubing into the groove of the bender, so that the measured end is left of the form block. Align the two zeros and align the mark on the tubing with the L on the form handle. If the measured end is on the right side, then align the mark on the tubing with the R on the form handle. With a steady motion, pull the form handle till the zero mark on the form handle lines up with the desired angle of bend, as indicated on the radius block. [Figure 7-4]

Bend the tubing carefully to avoid excessive flattening, kinking, or wrinkling. A small amount of flattening in bends is acceptable, but the small diameter of the flattened portion must not be less than 75 percent of the original outside diameter. Tubing with flattened, wrinkled, or irregular bends should not be installed. Wrinkled bends usually result from trying to bend thin wall tubing without using a tube bender. Excessive flattening will cause fatigue failure of the tube. Examples of correct and incorrect tubing bends are shown in Figure 7-5.

Tube bending machines for all types of tubing are generally used in repair stations and large maintenance shops. With such equipment, proper bends can be made on large diameter tubing and on tubing made from hard material. The production CNC™ tube bender is an example of this type of machine. [Figure 7-6]

The ordinary production tube bender will accommodate tubing ranging from 1/4″ to 11/2″ outside diameter. Benders for larger sizes are available, and the principle of their operation is similar to that of the hand tube bender. The radius blocks are so constructed that the radius of bend will vary with the tube diameter. The radius of bend is usually stamped on the block.

Alternative Bending Methods

When hand or production tube benders are not available or are not suitable for a particular bending operation, a filler of metallic composition or of dry sand may be used to facilitate bending. When using this method, cut the tube slightly longer than is required. The extra length is for inserting a plug (which may be wooden) in each end. The tube can also be closed by flattening the ends or by soldering metal disks in them.

After plugging one end, fill and pack the tube with fine, dry sand and plug tightly. Both plugs must be tight so they will not be forced out when the bend is made. After the ends are closed, bend the tubing over a forming block shaped to the specified radius. In a modified version of the filler method, a fusible alloy is used instead of sand. In this method, the tube is filled under hot water with a fusible alloy that melts at 160 °F. The alloy-filled tubing is then removed from the water, allowed to cool, and bent slowly by hand around a forming block or with a tube bender. After the bend is made, the alloy is again melted under hot water and removed from the tubing. When using either filler methods, make certain that all particles of the filler are removed. Visually inspect with a borescope to make certain that no particles will be carried into the system in which the tubing is installed. Store the fusible alloy filler where it will be free from dust or dirt. It can be remelted and reused as often as desired. Never heat this filler in any other way than the prescribed method, as the alloy will stick to the inside of the tubing, making them both unusable.


Rigid tubing may be joined to either an end item (such as a brake cylinder), another section of either rigid tubing, or to a flexible hose (such as a drain line). In the case of connection to an end item or another tube, fittings are required, which may or may not necessitate flaring of the tube. In the case of attachment to a hose, it may be necessary to bead the rigid tube so that a clamp can be used to hold the hose onto the tube.

Flareless Fittings

Although the use of flareless tube fittings eliminates all tube flaring, another operation, referred to as presetting, is necessary prior to installation of a new flareless tube assembly. Flareless tube assemblies should be preset with the proper size presetting tool or operation. Figure 7-11 (steps 1, 2, and 3) illustrates the presetting operation, which is performed as follows:

Cut the tube to the correct length, with the ends perfectly square. Deburr the inside and outside of the tube. Slip the nut, then the sleeve, over the tube (step 1), lubricate the threads of the fitting and nut with hydraulic fluid. Place the fitting in a vise (step 2), and hold the tubing firmly and squarely on the seat in the fitting. (The tube must bottom firmly in the fitting.) Tighten the nut until the cutting edge of the sleeve grips the tube. To determine this point, slowly turn the tube back and forth while tightening the nut. When the tube no longer turns, the nut is ready for tightening. Final tightening depends upon the tubing (step 3). For aluminum alloy tubing up to and including 1/2″ outside diameter, tighten the nut from 1 to 11/6 turns. For steel tubing and aluminum alloy tubing over 1/2″ outside diameter, tighten from 11/6 to 11/2 turns.

After presetting the sleeve, disconnect the tubing from the fitting and check the following points: The tube should extend 3/32″ to 1/8″ beyond the sleeve pilot; otherwise, blowoff may occur. The sleeve pilot should contact the tube or have a maximum clearance of 0.005″ for aluminum alloy tubing or 0.015″ for steel tubing. A slight collapse of the tube at the sleeve cut is permissible. No movement of the sleeve pilot, except rotation, is permissible.


Tubing may be beaded with a hand beading tool, with machine beading rolls, or with grip dies. The method to be used depends on the diameter and wall thickness of the tube and the material from which it was made.

The hand beading tool is used with tubing having 1/4″ to 1″ outside diameter. The bead is formed by using the beader frame with the proper rollers attached. The inside and outside of the tube is lubricated with light oil to reduce the friction between the rollers during beading. The sizes, marked in sixteenths of an inch on the rollers, are for the outside diameter of the tubing that can be beaded with the rollers. [Figure 7-12]

Separate rollers are required for the inside of each tubing size, and care must be taken to use the correct parts when beading. The hand beading tool works somewhat like the tube cutter in that the roller is screwed down intermittently while rotating the beading tool around the tubing. In addition, a small vise (tube holder) is furnished with the kit.

Other methods and types of beading tools and machines are available, but the hand beading tool is used most often. As a rule, beading machines are limited to use with large diameter tubing, over 115/16″, unless special rollers are supplied. The grip-die method of beading is confined to small tubing.

Fluid Line Identification

Fluid lines in aircraft are often identified by markers made up of color codes, words, and geometric symbols. These markers identify each line’s function, content, and primary hazard. Figure 7-13 illustrates the various color codes and symbols used to designate the type of system and its contents.

Fluid lines are marked, in most instances with 1″ tape or decals, as shown in Figure 7-14(A). On lines 4″ in diameter (or larger), lines in oily environment, hot lines, and on some cold lines, steel tags may be used in place of tape or decals, as shown in Figure 7-14(B). Paint is used on lines in engine compartments, where there is the possibility of tapes, decals, or tags being drawn into the engine induction system.

In addition to the above-mentioned markings, certain lines may be further identified regarding specific function within a system; for example, drain, vent, pressure, or return. Lines conveying fuel may be marked FLAM; lines containing toxic materials are marked TOXIC in place of FLAM. Lines containing physically dangerous

materials, such as oxygen, nitrogen, or Freon™, may be marked PHDAN. [Figure 7-14]

Aircraft and engine manufacturers are responsible for the original installation of identification markers, but the aviation mechanic is responsible for their replacement when it becomes necessary. Tapes and decals are generally placed on both ends of a line and at least once in each compartment through which the line runs. In addition, identification markers are placed immediately adjacent to each valve, regulator, filter, or other accessory within a line. Where paint or tags are used, location requirements are the same as for tapes and decals.

Fluid Line End Fittings

Depending on the type and use, fittings will have either pipe threads or machine threads. Pipe threads are similar to those used in ordinary plumbing and are tapered, both internal and external. External threads are referred to as male threads and internal threads are female threads.

When two fittings are joined, a male into a female, the thread taper forms a seal. Some form of pipe thread lubricant approved for the particular fluid application should be used when joining pipe threads to prevent seizing and high-pressure leakage. Use care when applying thread lubricant so that the lubricant will not enter and contaminate the system. Do not use lubricants on oxygen lines. Oxygen will react with petroleum products and can ignite (special lubricants are available for oxygen systems).

Machine threads have no sealing capability and are similar to those used on common nuts and bolts. This type of fitting is used only to draw connections together or for attachment through bulkheads. A flared tube connection, a crush washer, or a synthetic seal is used to make the connection fluid tight. Machine threads have no taper and will not form a fluid-tight seal. The size of these fittings is given in dash numbers, which equal the nominal o.d. in sixteenths of an inch.

Universal Bulkhead Fittings

When a fluid line passes through a bulkhead, and it is desired to secure the line to the bulkhead, a bulkhead fitting should be used. The end of the fitting that passes through the bulkhead is longer than the other end(s), which allows a locknut to be installed, securing the fitting to the bulkhead.

Fittings attach one piece of tubing to another, or to system units. There are four types: (1) bead and clamp, (2) flared fittings, (3) flareless fittings, and (4) permanent fittings (Permaswage™, Permalite™, and Cyrofit™). The amount of pressure that the system carries and the material used are usually the deciding factors in selecting a connector.

The beaded type of fitting, which requires a bead and a section of hose and hose clamps, is used only in low- or medium-pressure systems, such as vacuum and coolant systems. The flared, flareless, or permanenttype fittings may be used as connectors in all systems, regardless of the pressure.

AN Flared Fittings

A flared tube fitting consists of a sleeve and a nut, as shown in Figure 7-15. The nut fits over the sleeve and, when tightened, draws the sleeve and tubing flare tightly against a male fitting to form a seal. Tubing used with this type of fitting must be flared before installation. The male fitting has a cone-shaped surface with the same angle as the inside of the flare. The sleeve supports the tube so that vibration does not concentrate

at the edge of the flare, and distributes the shearing action over a wider area for added strength.

Fitting combinations composed of different alloys should be avoided to prevent dissimilar metal corrosion. As with all fitting combinations, ease of assembly, alignment, and proper lubrication should be assured when tightening fittings during installation.

Standard AN fittings are identified by their black or blue color. All AN steel fittings are colored black, all AN aluminum fittings are colored blue, and aluminum bronze fittings are cadmium plated and natural in appearance. A sampling of AN fittings is shown in Figure 7-16. Table 7-2 contains additional information on sizes, torques, and bend radii.

MS Flareless Fittings

MS flareless fittings are designed primarily for highpressure (3,000 psi) hydraulic systems that may be subjected to severe vibration or fluctuating pressure. Using this type of fitting eliminates all tube flaring, yet provides a safe and strong, dependable tube connection. [Figure 7-17] The fitting consists of three parts: a body, a sleeve, and a nut.

[Figure 7-18] The internal design of the body causes the sleeve to cut into the outside of the tube when the body and nut are joined.

The counterbore shoulder within the body is designed with a reverse angle of 15° for steel connectors and 45° for aluminum fittings. This reverse angle prevents inward collapse of the tubing when tightened and provides a partial sealing force to be exerted against the periphery of the body counterbore.

Swaged Fittings

A popular repair system for connecting and repairing hydraulic lines on transport category aircraft is the use of Permaswage™ fittings. Swaged fittings create a permanent connection that is virtually maintenance free. Swaged fittings are used to join hydraulic lines in areas where routine disconnections are not required and are often used with titanium and corrosion resistant steel tubing. The fittings are installed with portable hydraulically powered tooling, which is compact enough to be used in tight spaces. [Figure 7-19] If the fittings need to be disconnected, cut the tubing with a tube cutter.

Special installation tooling is available in portable kits. Always use the manufacturer’s instructions to install swaged fittings. One of the latest developments is the Permalite™ fitting. Permalite™ is a tube fitting that is mechanically attached to the tube by axial swaging. The movement of the ring along the fitting body results in deformation of the tube with a leak-tight joint. [Figure 7-20]

Cryofit Fittings

Many transport category aircraft use Cryofit fittings to join hydraulic lines in areas where routine disconnections are not required. Cryofit fittings are standard fittings with a cryogenic sleeve. The sleeve is made of a shape memory alloy, Tinel™. The sleeve is manufactured 3 percent smaller, frozen in liquid nitrogen, and expanded to 5 percent larger than the line. During installation, the fitting is removed from the liquid nitrogen and inserted onto the tube. During a 10 to 15 second warming up period, the fitting contracts to its original size (3 percent smaller), biting down on the tube, forming a permanent seal. Cryofit fittings can only be removed by cutting the tube at the sleeve, though this leaves enough room to replace it with a swaged fitting without replacing the hydraulic line. It is frequently used with titanium tubing. The shape memory technology is also used for end fittings, flared fittings, and flareless fittings. [Figure 7-21]

Rigid Tubing Installation and Inspection

Before installing a line assembly in an aircraft, inspect the line carefully. Remove dents and scratches, and be sure all nuts and sleeves are snugly mated and securely fitted by proper flaring of the tubing. The line assembly should be clean and free of all foreign matter.

Connection and Torque

Never apply compound to the faces of the fitting or the flare, for it will destroy the metal-to-metal contact between the fitting and flare, a contact which is necessary to produce the seal. Be sure that the line assembly is properly aligned before tightening the fittings. Do not pull the installation into place with torque on the nut. Correct and incorrect methods of installing flared tube assemblies are illustrated in Figure 7-22.

Proper torque values are given in Table 7-2. Remember that these torque values are for flared-type fittings only. Always tighten fittings to the correct torque value when installing a tube assembly. Overtightening a fitting may badly damage or completely cut off the tube flare, or it may ruin the sleeve or fitting nut. Failure to tighten sufficiently also may be serious, as this condition may allow the line to blow out of the assembly or to leak under system pressure. The use of torque wrenches and the prescribed torque values prevents overtightening or undertightening. If a tube fitting assembly is tightened properly, it may be removed and retightened many times before reflaring is necessary.

Flareless Tube Installation

Tighten the nut by hand until an increase in resistance to turning is encountered. Should it be impossible to run the nut down with the fingers, use a wrench, but be alert for the first signs of bottoming. It is important that the final tightening commence at the point where the nut just begins to bottom. Use a wrench and turn the nut one-sixth turn (one flat on a hex nut). Use a wrench on the connector to prevent it from turning while tightening the nut. After the tube assembly is installed, the system should be pressure tested. It is permissible to tighten the nut an additional one-sixth turn (making a total of one-third turn), should a connection leak. If leakage still occurs after tightening the nut a total of one-third turn, remove the assembly and inspect the components for scores, cracks, presence of foreign material, or damage from overtightening. Several aircraft manufacturers include torque values in their maintenance manuals to tighten the flareless fittings.

The following notes, cautions, and faults apply to the installation of rigid tubing.

Note: Overtightening a flareless tube nut drives the cutting edge of the sleeve deeply into the tube, causing the tube to be weakened to the point where normal in-flight vibration could cause the tube to shear. After inspection (if no discrepancies are found), reassemble the connections and repeat the pressure test procedures.

Caution: Never tighten the nut beyond one-third turn (two flats on the hex nut); this is the maximum the fitting may be tightened without the possibility of permanently damaging the sleeve and nut.

Common faults: Flare distorted into nut threads; sleeve cracked; flare cracked or split; flare out of round; inside of flare rough or scratched; and threads of nut or union dirty, damaged, or broken.

Rigid Tubing Inspection and Repair

Minor dents and scratches in tubing may be repaired. Scratches or nicks not deeper than 10 percent of the wall thickness in aluminum alloy tubing, which are not in the heel of a bend, may be repaired by burnishing with hand tools. The damage limits for hard, thinwalled corrosion-resistant steel and titanium tubing are considerably less than for aluminum tubing, and might depend on the aircraft manufacturer. Consult the aircraft maintenance manual for damage limits. Replace lines with severe die marks, seams, or splits in the tube. Any crack or deformity in a flare is unacceptable and is cause for rejection. A dent of less than 20 percent of the tube diameter is not objectionable, unless it is in the heel of a bend. To remove dents, draw a bullet of proper size through the tube by means of a length of cable, or push the bullet through a short straight tube by means of a dowel rod. In this case, a bullet is a ball bearing or slug normally made of steel or some other hard metal. In the case of soft aluminum tubing, a hard wood slug or dowel may even be used as a bullet. [Figure 7-23] A severely damaged line should be replaced.

However, the line may be repaired by cutting out the damaged section and inserting a tube section of the same size and material. Flare both ends of the undamaged and replacement tube sections and make the connection by using standard unions, sleeves, and tube nuts. Aluminum 6061-T6, corrosion resistant steel 304-1/8h and Titanium 3AL-2.5V tubing can be repaired by swaged fittings. If the damaged portion is short enough, omit the insert tube and repair by using one repair union. [Figure 7-24] When repairing a damaged line, be very careful to remove all chips and burrs.

Any open line that is to be left unattended for some time should be sealed, using metal, wood, rubber, or plastic plugs or caps.

When repairing a low-pressure line using a flexible fluid connection assembly, position the hose clamps carefully to prevent overhang of the clamp bands or chafing of the tightening screws on adjacent parts. If chafing can occur, the hose clamps should be repositioned on the hose. Figure 7-25 illustrates the design of a flexible fluid connection assembly and gives the maximum allowable angular and dimensional offset.

When replacing rigid tubing, ensure that the layout of the new line is the same as that of the line being replaced. Remove the damaged or worn assembly, taking care not to further damage or distort it, and use it as a forming template for the new part. If the old length of tubing cannot be used as a pattern, make a wire template, bending the pattern by hand as required for the new assembly. Then bend the tubing to match the wire pattern. Never select a path that does not require bends in the tubing. A tube cannot be cut or flared accurately enough so that it can be installed without bending and still be free from mechanical strain. Bends are also necessary to permit the tubing to expand or contract under temperature changes and to absorb vibration. If the tube is small (under 1/4″) and can be hand formed, casual bends may be made to allow for this. It the tube must be machine formed, definite bends must be made to avoid a straight assembly. Start all bends a reasonable distance from the fittings because the sleeves and nuts must be slipped back during the fabrication of flares and during inspections. In all cases, the new tube assembly should be so formed prior to installation that it will not be necessary to pull or deflect the assembly into alignment by means of the coupling nuts.

Flexible Hose Fluid Lines

Flexible hose is used in aircraft fluid systems to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed. It can also serve as a connector in metal tubing systems.

Hose Materials and Construction

Pure rubber is never used in the construction of flexible fluid lines. To meet the requirements of strength, durability, and workability, among other factors, synthetics are used in place of pure rubber. Synthetic materials most commonly used in the manufacture of flexible hose are Buna-N, neoprene, butyl, ethylene propylene diene rubber (EPDM) and Teflon™. While Teflon™ is in a category of its own, the others are synthetic rubber.

Buna-N is a synthetic rubber compound which has excellent resistance to petroleum products. Do not confuse with Buna-S. Do not use for phosphate ester base hydraulic fluid (Skydrol).

Neoprene is a synthetic rubber compound which has an acetylene base. Its resistance to petroleum products is not as good as Buna-N, but it has better abrasive resistance. Do not use for phosphate ester base hydraulic fluid (Skydrol).

Butyl is a synthetic rubber compound made from petroleum raw materials. It is an excellent material to use with phosphate ester base hydraulic fluid (Skydrol). Do not use with petroleum products.

rubber inner tube covered with layers of cotton braid and wire braid and an outer layer of rubber-impregnated cotton braid. This type of hose is suitable for use in fuel, oil, coolant, and hydraulic systems. The types of hose are normally classified by the amount of pressure they are designed to withstand under normal operating conditions.

Low, Medium, and High Pressure Hoses

  • Low pressure — below 250 psi. Fabric braid reinforcement.
  • Medium pressure — up to 3,000 psi. One wire braid reinforcement. Smaller sizes carry up to 3,000 psi. Larger sizes carry pressure up to 1,500 psi.
  • High pressure — all sizes up to 3,000 psi operating pressures.

Hose Identification

Lay lines and identification markings consisting of lines, letters, and numbers are printed on the hose. [Figure 7-26] Most hydraulic hose is marked to identify its type, the quarter and year of manufacture, and a 5-digit code identifying the manufacturer. These

markings are in contrasting colored letters and numerals which indicate the natural lay (no twist) of the hose and are repeated at intervals of not more than 9 inches along the length of the hose. Code markings assist in replacing a hose with one of the same specifications or a recommended substitute. Hose suitable for use with phosphate ester base hydraulic fluid will be marked Skydrol use. In some instances, several types of hose may be suitable for the same use. Therefore, to make the correct hose selection, always refer to the applicable aircraft maintenance or parts manual.

Teflon™ is the DuPont trade name for tetrafluoroethylene resin. It has a broad operating temperature range (-65 °F to +450 °F). It is compatible with nearly every substance or agent used. It offers little resistance to flow; sticky, viscous materials will not adhere to it. It has less volumetric expansion than rubber, and the shelf and service life is practically limitless. Teflon™ hose is flexible and designed to meet the requirements of higher operating temperatures and pressures in present aircraft systems. Generally, it may be used in the same manner as rubber hose. Teflon™ hose is processed and extruded into tube shape to a desired size. It is covered with stainless steel wire, which is braided over the tube for strength and protection. Teflon™ hose is unaffected by any known fuel, petroleum, or synthetic base oils, alcohol, coolants, or solvents commonly used in aircraft. Teflon™ hose has the distinct advantages of a practically unlimited storage time, greater operating temperature range, and broad usage (hydraulic, fuel, oil, coolant, water, alcohol, and pneumatic systems). Medium-pressure Teflon™ hose assemblies are sometimes preformed to clear obstructions and to make connections using the shortest possible hose length. Since preforming permits tighter bends that eliminate the need for special elbows, preformed hose assemblies save space and weight. Never straighten a preformed hose assembly. Use a support wire if the hose is to be removed for maintenance. [Figure 7-27]

Flexible Hose Inspection

Check the hose and hose assemblies for deterioration at each inspection period. Leakage, separation of the cover or braid from the inner tube, cracks, hardening, lack of flexibility, or excessive “cold flow” are apparent signs of deterioration and reason for replacement. The term “cold flow” describes the deep, permanent impressions in the hose produced by the pressure of hose clamps or supports.

When failure occurs in a flexible hose equipped with swaged end fittings, the entire assembly must be replaced. Obtain a new hose assembly of the correct size and length, complete with factory installed end fittings. When failure occurs in hose equipped with reusable end fittings, a replacement line can be fabricated with the use of such tooling as may be necessary to comply with the assembly instructions of the manufacturer.

Fabrication and Replacement of Flexible Hose

To make a hose assembly, select the proper size hose and end fitting. [Figure 7-28] MS-type end fittings for flexible hose are detachable and may be reused if determined to be serviceable. The inside diameter of the fitting is the same as the inside diameter of the hose to which it is attached.

Flexible Hose Testing

All flexible hose must be proof-tested after assembly and applying pressure to the inside of the hose assembly. The proof-test medium may be a liquid or gas. For example, hydraulic, fuel, and oil lines are generally tested using hydraulic oil or water, whereas air or instrument lines are tested with dry, oil-free air or nitrogen. When testing with a liquid, all trapped air is bled from the assembly prior to tightening the cap or plug. Hose tests, using a gas, are conducted underwater. In all cases, follow the hose manufacturer’s instructions for proof-test pressure and fluid to be used when testing a specific hose assembly. [Table 7-3]

When a flexible hose has been repaired or overhauled using existing hardware and new hose material, and before the hose is installed on the aircraft, it is recommended that the hose be tested to at least 1.5 system pressure. A hydraulic hose burst test stand is used for testing flexible hose. [Figure 7-29] A new hose can be operationally checked after it is installed in the aircraft using system pressure.

Size Designations

Hose is also designated by a dash number, according to its size. The dash number is stenciled on the side of the hose and indicates the size tubing with which the hose is compatible. It does not denote inside or outside diameter. When the dash number of the hose

corresponds with the dash number of the tubing, the proper size hose is being used. Dash numbers are shown in Figure 7-26.

Hose Fittings

Flexible hose may be equipped with either swaged fittings or detachable fittings, or they may be used with beads and hose clamps. Hoses equipped with swaged fittings are ordered by correct length from the manufacturer and ordinarily cannot be assembled by the mechanic. They are swaged and tested at the factory and are equipped with standard fittings. The detachable fittings used on flexible hoses may be detached and reused if they are not damaged; otherwise, new fittings must be used. [Figure 7-30]

Installation of Flexible Hose Assemblies

Slack—Hose assemblies must not be installed in a manner that will cause a mechanical load on the hose. When installing flexible hose, provide slack or bend in the hose line from 5 to 8 percent of its total length to provide for changes in length that will occur when pressure is applied. Flexible hose contracts in length and expands in diameter when pressurized. Protect all flexible hoses from excessive temperatures, either by locating the lines so they will not be affected or by installing shrouds around them.

Flex—When hose assemblies are subject to considerable vibration or flexing, sufficient slack must be left between rigid fittings. Install the hose so that flexure does not occur at the end fittings. The hose must remain straight for at least two hose diameters from the end fittings. Avoid clamp locations that will restrict or prevent hose flexure.

Twisting—Hoses must be installed without twisting to avoid possible rupture of the hose or loosening of the attaching nuts. Use of swivel connections at one or both ends will relieve twist stresses. Twisting of the hose can be determined from the identification stripe running along its length. This stripe should not spiral around the hose.

Bending—To avoid sharp bends in the hose assembly, use elbow fittings, hose with elbow-type end fittings, or the appropriate bend radii. Bends that are too sharp will reduce the bursting pressure of flexible hose considerably below its rated value. [Figure 7-31]

Clearance—The hose assembly must clear all other lines, equipment, and adjacent structure under every operating condition.

Flexible hose should be installed so that it will be subject to a minimum of flexing during operation. Although hose must be supported at least every 24 inches, closer supports are desirable. Flexible hose must never be stretched tightly between two fittings. If clamps do not seal at specified tightening, examine hose connections and replace parts as necessary. The above is for initial installation and should not be used for loose clamps.

For retightening loose hose clamps in service, proceed as follows: Non-self-sealing hose—if the clamp screw cannot be tightened with the fingers, do not disturb unless leakage is evident. If leakage is present, tighten one-fourth turn. Self-sealing hose—if looser than finger-tight, tighten to finger-tight and add one-fourth turn. [Table 7-4]

Hose Clamps

To ensure proper sealing of hose connections and to prevent breaking hose clamps or damaging the hose, follow the hose clamp tightening instructions carefully. When available, use the hose clamp torque-limiting wrench. These wrenches are available in calibrations of 15 and 25 in-lb limits. In the absence of torquelimiting wrenches, follow the finger-tight-plus-turns method. Because of the variations in hose clamp design and hose structure, the values given in Table 7-4 are approximate. Therefore, use good judgment when tightening hose clamps by this method. Since hose connections are subject to “cold flow” or a setting process, a follow-up tightening check should be made for several days after installation.

Support clamps are used to secure the various lines to the airframe or powerplant assemblies. Several types of support clamps are used for this purpose. The most commonly used clamps are the rubber-cushioned and plain. The rubber-cushioned clamp is used to secure lines subject to vibration; the cushioning prevents chafing of the tubing. [Figure 7-32] The plain clamp is used to secure lines in areas not subject to vibration.

A Teflon™-cushioned clamp is used in areas where the deteriorating effect of Skydrol, hydraulic fluid, or fuel is expected. However, because it is less resilient, it does not provide as good a vibration-damping effect as other cushion materials.

or oil lines in place. Unbonded clamps should be used only for securing wiring. Remove any paint or anodizing from the portion of the tube at the bonding clamp location. Make certain that clamps are of the correct size. Clamps or supporting clips smaller than the outside diameter of the hose may restrict the flow of fluid through the hose. All fluid lines must be secured at specified intervals. The maximum distance between supports for rigid tubing is shown in Table 7-5.


Flexible hose is used in aircraft plumbing to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed. It can also serve as a connector in metal tubing systems.


Synthetic materials most commonly used in the manufacture of flexible hose are: Buna-N, Neoprene, Butyl and Teflon (trademark of DuPont Corp.). Buna-N is a synthetic rubber compound which has excellent resistance to petroleum products. Do not confuse with Buna-S. Do not use for phosphate ester base hydraulic fluid (Skydrol). Neoprene is a synthetic rubber compound which has an acetylene base. Its resistance to petroleum products is not as good as Buna-N but has better abrasive resistance. Do not use for phosphate ester base hydraulic fluid (Skydrol). Butyl is a synthetic rubber compound made from petroleum raw materials. It is an excellent material to use with phosphate ester based hydraulic fluid (Skydrol). Do not use with petroleum products. Teflon is the DuPont trade name for tetrafluoroethylene resin. It has a broad operating temperature range (-65° F to +450° F). It is compatible with nearly every substance or agent used. It offers little resistance to flow; sticky viscous materials will not adhere to it. It has less volumetric expansion than rubber and the shelf and service life is practically limitless.

Rubber Hose

Flexible rubber hose consists of a seamless synthetic rubber inner tube covered with layers of cotton braid and wire braid, and an outer layer of rubber impregnated cotton braid. This type of hose is suitable for use in fuel, oil, coolant, and hydraulic systems. The types of hose are normally classified by the amount of pressure they are designed to withstand under normal operating conditions.

1. Low pressure, any pressure below 250 psi.
Fabric braid reinforcement.

2. Medium pressure, pressures up to 3,000 psi.
One wire braid reinforcement.
Smaller sizes carry pressure up to 3,000 psi.
Larger sizes carry pressure up to 1,500 psi.

3. High pressure (all sizes up to 3,000 psi operating pressures).

Identification markings consisting of lines, letters, and numbers are printed on the hose. (See figure 5-2.) These code markings show such information as hose size, manufacturer, date of manufacture, and pressure and temperature limits. Code markings assist in replacing a hose with one of the same specification or a recommended substitute. Hose suitable for use with phosphate ester base, hydraulic fluid will be marked “Skydrol ^R use”. In some instances several types of hose may be suitable for the same use. Therefore, in order to make the correct hose selection, always refer to the maintenance or parts manual for the particular airplane.

Teflon Hose

Teflon hose is a flexible hose designed to meet the requirements of higher operating temperatures and pressures in present aircraft systems. It can generally be used in the same manner as rubber hose. Teflon hose is processed and extruded into tube shape to a desired size. It is covered with stainless steel wire, which is braided over the tube for strength and protection.

Teflon hose is unaffected by any known fuel, petroleum, or synthetic base oils, alcohol, coolants, or solvents commonly used in aircraft. Although it is highly resistant to vibration and fatigue, the principle advantage of this hose is its operating strength.

Size Designation

The size of flexible hose is determined by its inside diameter. Sizes are in one-sixteenth inch increments and are identical to corresponding sizes of rigid tubing, with which it can be used.

Identification of Fluid Lines

Fluid lines in aircraft are often identified by markers made up of color codes, words, and geometric symbols. These markers identify each line’s function, content, and primary hazard, as well as the direction of fluid flow.Figure 5-3 illustrates the various

color codes and symbols used to designate the type of system and its contents. In most instances, fluid lines are marked with 1 inch tape or decals, as shown in figure 5-4 (A).

On lines 4 inches in diameter (or larger), lines in oily environment, hot lines, and on some cold lines, steel tags may be used in place of tape or decals, as shown in figure 5-4(B). Paint is used on lines in engine compartments, where there is the possibility of tapes, decals, or tags being drawn into the engine induction system.

In addition to the above mentioned markings, certain lines may be further identified as to specific function within a system; for example, DRAIN, VENT, PRESSURE, or RETURN.

Lines conveying fuel may be marked FLAM; lines containing toxic materials are marked TOXIC in place of FLAM. Lines containing physically dangerous materials, such as oxygen, nitrogen, or freon, are marked PHDAN.

The aircraft and engine manufacturers are responsible for the original installation of identification markers, but the aviation mechanic is responsible for their replacement when it becomes necessary.

Generally, tapes and decals are placed on both ends of a line and at least once in each compartment through which the line runs. In addition, identification markers are placed immediately adjacent to each valve, regulator, filter, or other accessory within a line. Where paint or tags are used, location requirements are the same as for tapes and decals.


Plumbing connectors, or fittings, attach one piece of tubing to another or to system units. There are four types: (1) Flared fitting, (2) flareless fitting, (3) bead and clamp, and (4) swaged. The amount of pressure that the system carries is usually the deciding factor in selecting a connector. The beaded type of joint, which requires a bead and a section of hose and hose clamps, is used only in low or medium pressure systems, such as vacuum and coolant systems. The flared, flareless, and swaged types may be used as connectors in all systems, regardless of the pressure.

Flared Tube Fittings

A flared tube fitting consists of a sleeve and a nut, as shown in figure 5-5. The nut fits over the sleeve and, when tightened, draws the sleeve and tubing flare tightly against a male fitting to form a seal. Tubing used with this type of fitting must be flared before installation.

The male fitting has a cone shaped surface with the same angle as the inside of the flare. The sleeve supports the tube so that vibration does not concentrate at the edge of the flare, and distributes the shearing action over a wider area for added strength. Tube flaring and the installation of flared tube fittings are discussed in detail later in this chapter.

The AC (Air Corps) flared tube fittings have been replaced by the AN (Army/Navy) Standard and MS (Military Standard) fittings. However, since AC fittings are still in use in some of the older aircraft, it is important to be able to identify them. The AN fitting has a shoulder between the end of the threads and the flare cone. (See figure 5-6.) The AC fitting does not have this shoulder.

Other differences between the AC and AN fittings include the sleeve design, the AC sleeve being noticeably longer than the AN sleeve of the same size. Although certain flared tube fittings are interchangeable, the pitch of the threads is different in most cases. Figures 5-7a and b show the AN and AC811 fittings that can be safely interchanged. Combinations of end connections, nuts,

sleeves, and tube flares are allowed to make up a complete fitting assembly. The use of dissimilar metals should be avoided since their contact will cause corrosion. When combining AC and AN end connections, nuts, sleeves, or tube flares, if the nut will not move more than two threads by hand, stop and investigate for possible trouble.

The AN standard fitting is the most commonly used flared tubing assembly for attaching the tubing to the various fittings required in aircraft plumbing systems. The AN standard fittings include the AN818 nut and AN819 sleeve. (See figure 5-8.) The AN819 sleeve is used with the AN818 coupling nut. All these fittings have straight threads, but they have different pitch for the various types.

Flared tube fittings are made of aluminum alloy, steel, or copper base alloys. For identification purposes, all AN steel fittings are colored black, and all AN aluminum alloy fittings are colored blue. The AN 819 aluminum bronze sleeves are cadmium plated and are not colored. The size of these fittings is given in dash numbers, which equal the nominal tube outside diameter (O.D.) in sixteenths of an inch.

Threaded flared tube fittings have two types of ends, referred to as male and female. The male end of a fitting is externally threaded, whereas the female end of a fitting is internally threaded.

Flareless Tube Fittings

The MS (Military Standard) flareless tube fittings are finding wide application in aircraft plumbing systems. Using this type fitting eliminates all tube flaring, yet provides a safe, strong, dependable tube connection. The fitting consists of three parts: a body, a sleeve, and a nut. The body has a counterbored shoulder, against which the end of the tube rests. (See figure 5-9.) The angle of the counterbore causes the cutting edge of the sleeve to cut into the outside of the tube when the two are joined. Installation of flareless tube fittings is discussed later in this chapter.

Quick Disconnect Couplings

Quick disconnect couplings of the self-sealing type are used at various points in many fluid systems. The couplings are installed at locations where frequent uncoupling of the lines is required for inspection and maintenance.

Quick disconnect couplings provide a means of quickly disconnecting a line without loss of fluid or entrance of air into the system. Each coupling assembly consists of two halves, held together by a union nut. Each half contains a valve that is held open when the coupling is connected, allowing fluid to flow through the coupling in either direction. When the coupling is disconnected, a spring in each half closes the valve, preventing the loss of fluid and entrance of air.

The union nut has a quick lead thread which permits connecting or disconnecting the coupling by turning the nut. The amount the nut must be turned varies with different style couplings. One style requires a quarter turn of the union nut to lock or unlock the coupling while another style requires a full turn.

Some couplings require wrench tightening; others are connected and disconnected by hand. The design of some couplings is such that they must be safetied with safety wire. Others do not require lock wiring, the positive locking being assured by the teeth on the locking spring, which engage ratchet teeth on the union nut when the coupling is fully engaged. The lock spring automatically disengages when the union nut is unscrewed. Because of individual differences, all quick disconnects should be installed according to instructions in the aircraft maintenance manual.

Flexible Connectors

Flexible connectors may be equipped with either swaged fittings or detachable fittings, or they may be used with beads and hose clamps. Those equipped with swaged fittings are ordered by correct length from the manufacturer and ordinarily cannot be assembled by the mechanic. They are swaged and tested at the factory and are equipped with standard fittings.

The fittings on detachable connectors can be detached and reused if they are not damaged; otherwise new fittings must be used.

The bead and hose clamp connector is often used for connecting oil, coolant, and low pressure fuel system tubing. The bead, a slightly raised ridge around the tubing or the fitting, gives a good gripping edge that aids in holding the clamp and hose in place. The bead may appear near the end of the metal tubing or on one end of a fitting.


Many factors affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors can be controlled and some cannot.


The environmental conditions under which an aircraft is maintained and operated greatly affect corrosion characteristics. In a predominately marine environment (with exposure to sea water and salt air), moisture laden air is considerably more detrimental to an aircraft than it would be if all operations were conducted in a dry climate. Temperature considerations are important because the speed of electrochemical attack is increased in a hot, moist climate.

Size and Type of Metal

It is a well known fact that some metals will corrode faster than others. It is a less known fact that variations in size and shape of a metal can indirectly affect its corrosion resistance.

Thick structural sections are more susceptible to corrosive attack than thin sections because variations in physical characteristics are greater. When large pieces are machined or chemically milled after heat treatment, the thinner areas will have different physical characteristics than the thicker areas. (See figure 6-59.)

From a corrosion control standpoint, the best approach is to recognize the critical nature of the integrity and strength of major structural parts and to maintain permanent protection over such areas at all times to prevent the onset of deterioration.

Foreign Material

Among the controllable factors which affect the onset and spread of corrosive attack is foreign material which adheres to the metal surfaces. Such foreign material includes:

(1) Soil and atmospheric dust.
(2) Oil, grease, and engine exhaust residues.
(3) Salt water and salt moisture condensation.
(4) Spilled battery acids and caustic cleaning solutions.
(5) Welding and brazing flux residues.

It is important that aircraft be kept clean. How often and to what extent an aircraft should be cleaned depends on several factors, such as location, model of aircraft, and type of operation.

The Federal Aviation Regulations (FAR) Part 43, Maintenance, Preventive Maintenance, Rebuilding, and Alteration, permits the holder of a pilot certificate issued under FAR Part 61 to perform specified preventive maintenance on any aircraft owned or operated by that pilot as long as the aircraft is not used under FAR Part 121, 127, 129, or 135. FAR Part 43, Appendix A, Subpart C, Preventive Maintenance, lists the authorized preventive maintenance work. One restriction on such work is that it cannot involve complex assembly operations.

Although the following examples of preventive maintenance authorized by FAR Part 43 can be done by a certificated pilot under the conditions listed in the FAR, each individual planning on doing such work should make a self-analysis as to whether or not he or she has the ability to perform the work satisfactorily and safely. If any of the preventive maintenance authorized by FAR Part 43 is done, the person doing the work must make an entry in the appropriate logbook or record system to document the work done. The entry shall contain:

1. A description of the work performed (or references to data that is acceptable to the Administrator).
2. Date of completion.
3. Signature, certificate number, and kind of certificate held by the person performing the work.

The signature constitutes approval for return to service ONLY for work performed.


The following is a partial list of what a certificated pilot who meets the conditions in FAR Part 43 can do.

1. Removal, installation, and repair of landing gear tires.

2. Servicing landing gear wheel bearings, such as cleaning and greasing.

3. Servicing landing gear shock struts by adding oil, air, or both.

4. Replacing defective safety wire or cotter keys.

5. Lubrication not requiring disassembly other than removal of nonstructural items such as cover plates, cowling, and fairings.

6. Replenishing hydraulic fluid in the hydraulic reservoir.

7. Applying preservative or protective material to components where no disassembly of any primary structure or operating system is involved and where such coating is not prohibited or is not contrary to good practices.

8. Replacing safety belts.

9. Replacing bulbs, reflectors, and lenses of position an landing lights.

10. Replacing or cleaning spark plugs and setting of spark plug gap clearance.

11. Replacing any hose connection, except hydraulic connections.

12. Replacing and servicing batteries.

13. Making simple fabric patches not requiring rib stitching or the removal of structural parts or control surfaces. In the case of balloons, the making of small fabric repairs to envelopes (as defined in, and in accordance with, the balloon manufacturers’ instructions) not requiring load tape repair or replacement.

14. Replacing any cowling not requiring removal of the propeller or disconnection of flight controls.


The following suggested checklist is one example of how to conduct a preventive maintenance check of a typical general aviation airplane. It is not the only way to check an aircraft. As in the case of any suggested checklist, you should always follow the manufacturer’s operating checklist for your specific aircraft. Persons performing preventive maintenance on any aircraft must follow good safety procedures when checking any type of aircraft. This is especially true when checking an installed propeller on an operational aircraft. There is always the remote chance that a defective magneto ground wire could allow a moving prop to start the engine.


1. Spinner and back plate for cracks or looseness.

2. Blades for nicks or cracks.

3. Hub for grease or oil leaks.

4. Bolts for security and safety wire.


1. Preflight engine.

2. Run-up engine to warm-up and check:

a. Magnetos for RPM drop and ground-out.

b. Mixture and throttle controls for operation and ease of movement.

C. Propeller control for operation and ease of movement.

d. Engine idle for proper RPM.

e. Carburetor heat or alternate air.

f. Alternator output under a load (landing light, etc., in the “on” position).

g. Vacuum system (if installed) for output.

h. Temperatures (CHT, oil, etc.) within proper operating range.

i. Engine and electric fuel pumps for fuel flow or fuel pressure.

j. Fuel selector, in all positions, for free and proper operation.

3. Remove engine cowling. Clean and check for cracks, loose fasteners, or damage.

4. Check engine oil for quantity and condition. Change oil and oil filter, check screens.

5. Check oil temperature “sensing” unit for leaks, security, and broken wires.

6. Check oil lines and fittings for condition, leaks and security, and evidence of chafing.

7. Check oil cooler for condition (damage, dirt and air blockage), security, leaks, and winterization plate (if applicable.

8. Clean engine.

9. Remove, clean, and check spark plugs for wear. Regap and reinstall plugs, moving “top to bottom,” and “bottom to top” of cylinders. Be sure to gap and torque plugs to manufacturer’s specifications.

10. Check magnetos for security, cracks, and broken wires or insulation.

11. Check ignition harness for chafing, cracked insulation and cleanliness.

12. Check cylinders for loose or missing nuts and screws, cracks around cylinder hold-down studs, and for broken cooling fins.

13. Check rocker box covers for evidence of oil leaks and loose nuts or screws.

14. Remove air filter and tap gently to remove dirt particles.

15. Replace air filter.

16. Check all air-inlet ducts for condition (no air leaks, holes, etc.).

17. Check intake seals for leaks (fuel stains) and clamps for security.

18. Check condition of priming lines and fittings for leaks (fuel stains) and clamps for security.

19. Check condition of exhaust stacks, connections, clamps, gaskets, muffler, and heat box for cracks, security, condition, and leaks.

20. Check condition of fuel lines for leaks (fuel stains) and security.

21. Drain at least one pint of fuel into a clean transparent container from each fuel filter, each fuel tank sump, and any other aircraft fuel drain to check for water, dirt, the wrong type of fuel, or other type of contamination.

22. Visually check vacuum pump and lines for missing nuts, cracked pump flanges, and security.

23. Check crankcase breather tubes and clamps for obstructions and security

24. Check crankcase for cracks, leaks, and missing nuts.

25. Check engine mounts for cracks or loose mountings.

26. Check engine baffles for cracks, security, and foreign objects.

27. Check wiring for security, looseness, broken wires, and condition of insulation.

28. Check firewall and firewall seals.

29. Check generator or alternator belt for proper tension and fraying.

30. Check generator (or alternator) and starter for security and safety of nuts and bolts.

31. Check brake fluid for level and proper type.

32. Lubricate engine controls: propeller, mixture, and throttle.

33. Check alternate air source “door” or carburetor heat to ensure when “door” is closed it has a good seal. Check “door” operation.

34. Reinstall engine cowling.


1. Cabin door, latch, and hinges for operation and worn door seals.

2. Upholstery for tears.

3. Seats, seat belts, and adjustment hardware.

4. Trim operation for function and ease of movement.

5. Rudder pedals and toe brakes for operation and security.

6. Parking brake.

7. Control wheels, column, pulleys, and cables for security, operation and ease of movement.

8. Lights for operation.

9. Heater and defroster controls for operation and ducts for condition and security.

10. Air vents for general condition and operation.

11. Windshield, doors, and side windows for cracks, leaks, and crazing.

12. Instruments and lines for proper operation and security.

Fuselage and Empennage-Check:

1. Baggage door, latch, and hinges for security and opera-tion; baggage door seal for wear.

2. Battery for water, corrosion, and security of cables.

3. Antenna mounts and electric wiring for security and corrosion.

4. Hydraulic system for leaks, security, and fluid level.

5. ELT for security, switch position, and battery condition and age.

6. Rotating beacon for security and operation.

7. Stabilizer and control surfaces, hinges, linkages, trim tabs, cables, and balance weights for condition, cracks, frayed cables, loose rivets, etc.

8. Control hinges for appropriate lubrication.

9. Static ports for obstructions.


1. Wing tips for cracks, loose rivets, and security.

2. Position lights for operation.

3. Aileron and flap hinges and actuators for cleanliness and lubrication.

4. Aileron balance weights for cracks and security.

5. Fuel tanks, caps and vents, and placards for quantity and type of fuel.

6. Pitot or Pitot-static port or ports for security and obstruction.

Landing Gear-Check:

1. Strut extension.

2. Scissors and nose gear shimmy damper for leaks and loose or missing bolts.

3. Wheels and tires for cracks, cuts, wear and pressure.

4. Hydraulic lines for leaks and security.

5. Gear structure for cracks, loose or missing bolts, and security.

6. Retracing mechanism and gear door for loose or miss-ing bolts and for abnormal wear.

7. Brakes for wear, security, and hydraulic leaks.

Functional Check Flight (FCF)-Check:

1. Brakes for proper operation during taxi.

2. Engine and propeller for power, smoothness, etc., during run-up.

3. Engine instruments for proper reading.

4. Power output (on takeoff run).

5. Flight instruments.

6. Gear retraction and extension for proper operation and warning system.

7. Electrical system (lights, alternator output).

8. Flap operation.

9. Trim functions.

10. Avionics equipment for proper operation (including a VOR or VOT check for all VOR receivers).

11. Operation of heater, defroster, ventilation, and air conditioner.


1. Ensure that the aircraft is in compliance with all applicable Airworthiness Directives (AD) and the compliance has been properly recorded in the aircraft records. If the AD involves recur-ring action, you should know when the next action is required.

2. Comply with recommended service bulletins and service letters. These are recommendations unless an AD requires compliance.

3. See that a current FAA approved Flight Manual or Pilot’s Operating Handbook with all required changes is aboard and that all required placards are properly installed.

4. Check that the Certificate of Airworthiness and Aircraft Registration are displayed. Check for a FCC radio station license is aboard if any type transmitter is installed.

5. Verify that all FAA required tests involving the transponder, the VOR, and static system have been made and entered in the appropriate aircraft records.


It pays to take good care of your engine. Good maintenance is not cheap, but poor performance can be disastrously expensive.

If you are unqualified or unable to do a particular authorized job, you must depend on competent and certificated aircraft maintenance technicians to perform the job. Always use FAA-approved parts.

You can save money and have a better understanding of your aircraft, if you participate in the maintenance yourself.

If you do some of your own preventive maintenance, do it properly. Make sure you complete the job you start and make all of the required record entries.

Money, time, and effort spent on maintenance pays off. It also ensures your aircraft will have a higher resale value, if you decide to sell.

Remember, a well cared for aircraft is a safe aircraft. A safe air-craft needs to be flown by a competent and proficient pilot. Maintain both your aircraft and yourself in top-notch condition.

Additional Reading: Advisory Circular (AC) AC 20-106, Aircraft Inspection for the General Aviation Aircraft Owner

Federal Aviation Regulations (FAR) Part 39, Airworthiness Directives

FAR Part 43, Maintenance, Preventive Maintenance, Rebuilding, and Alteration

Corrosion Removal Techniques



General safety precautions, outlined in the following paragraphs, contain guidelines for handling materials with hazardous physical properties and emergency procedures for immediate treatment of personnel who have inadvertently come into contact with one of the harmful materials. Materials having hazardous physical properties are referenced to the pertinent safety precautions and emergency safety procedures. All personnel responsible for using or handling hazardous materials should be thoroughly familiar with the information in the following paragraphs.

a. Safety precautions.

When required to use or handle any of the solvents, special cleaners, paint strippers (strong alkalies and acids), etchants (corrosion removers containing acids), or surface activation material (alodine 1200), observe the following safety precautions:

(1) Avoid prolonged breathing of solvents’ or acids’ vapors. Solvents and acids should not be used in confined spaces without adequate ventilation or approved respiratory protection;

(2) Never add water to acid. Always add acid to water;

(3) Mix all chemicals per the manufacturers’ instructions;

(4) Clean water for emergency use should be available in the immediate work area before starting work;

(5) Avoid prolonged or repeated contact of solvents, cleaners, etchants (acid), or conversion coating material (alodine solution) with skin. Rubber or plastic gloves should be worn when using solvents, cleaners, paint strippers, etchants, or conversion coating materials. Goggles or plastic face shields and suitable protective clothing should be worn when cleaning, stripping, etching, or conversion coating overhead surfaces;

(6) When mixing alkalies with water or other substance, use containers which are made to withstand heat generated by this process;

(7) Wash any paint stripper, etchant, or conversion coating material immediately from body, skin, or clothing;

(8) Materials splashed in the eyes should be promptly flushed out with water, and medical aid obtained for the injured person;

(9) Do not eat or keep food in areas where it may absorb poisons. Always wash hands before eating or smoking;

(10) Verify that the area within 50 ft. of any cleaning or treating operations where low flash point (140 °F or below) materials are being used is clear and remains clear of all potential ignition sources;

(11) Suitable fire extinguishing equipment should be available to the cleaning/treating area;

(12) Where any flammable materials are being used, equipment should be effectively grounded;

(13) If materials (acid, alkali, paint remover, or conversion coatings) are spilled on equipment and/or tools, treat immediately by rinsing with clean water, if possible, and/or neutralizing acids with baking soda and alkalies with a weak (5 percent) solution of acetic acid in water;

(14) Solvents with a low flash point (below 100 °F), such as methyl ethyl ketone (MEK) and acetone, should not be used in any confined locations;

(15) All equipment should be cleaned after work has been completed;

(16) Implement all company safety precautions;

(17) Check local environmental regulations for restrictions on the use of solvents, primers, and top coats; and

(18) Ensure disposal of waste material is per local environmental requirements.

b. Emergency safety procedures.


(1) If exposed to physical contact with any of the following materials:

Methyl alcohol Xylene
Methyl ethyl ketone Petroleum naphthas
Methyl isobutyl ketone Chromates
Toluene Dichromates
Trichloroethylene Acetates
Epoxy resin Cyclohexanone
Methylene chloride Cellosolve
Brush alodine Carbon tetrachloride

Treat as follows:

(i) If splashed into eyes, do not rub.


(ii) Flush eyes immediately with water for at least 15 minutes. Lift upper and lower eyelids frequently to ensure complete washing.

(iii) If splashed on clothing or large areas of body, immediately remove contaminated clothing and wash body with plenty of soap and water. Wash clothing before rewearing.

(iv) If splashed onto an easily accessible part of the body, immediately wash with soap and water.

(v) If suffering headache or other obvious symptoms resulting from overexposure, move to fresh air immediately.

(vi) If vapors are inhaled and breathing has slowed down or stopped, remove person from exposure and start artificial respiration at once. Call ambulance and continue this treatment until ambulance arrives.

(2) If exposed to physical contact with any of the following materials:

Hydrofluoric acid Phenol
Nitric acid Cresols
Phosphoric acid Tricresyl phosphate

Treat as follows:

(i) If splashed into eyes, quickly wipe eyelids with a soft cleaning tissue and immediately flush eye with gentle stream from a drinking fountain, cup, or other convenient water outlet while holding lids open. Call an ambulance and continue the flushing procedure until an ambulance arrives.

(ii) If splashed onto an easily accessible part of body, immediately drench affected area with water until ambulance arrives.

(iii) If splashed onto clothing or large area of body, immediately drench body and remove clothing while drenching until ambulance arrives.

(iv) If taken internally, begin following treatment immediately:

(A) If the person is conscious, cause vomiting by placing finger in back of the person’s throat. Encourage the person to drink large quantities of water and repeatedly wash out the mouth.

(B) If the person is unconscious, do not give any liquid. Start artificial respiration at once. Continue until ambulance arrives. If person regains consciousness before ambulance arrives, proceed as in subparagraph (A).


The effectiveness of corrosion control depends on how well basic work procedures are followed. The following common work practices are recommended:

a. If rework procedures or materials are unknown, contact the aircraft manufacturer or authorized representative before proceeding;

b. The work areas, equipment, and components should be clean and free of chips, grit, dirt, and foreign materials.

c. Do not mark on any metal surface with a graphite pencil or any type of sharp, pointed instrument. Temporary markings (defined as markings soluble in water or methyl chloroform) should be used for metal layout work or marking on the aircraft to indicate corroded areas.

d. Graphite should not be used as a lubricant for any component. Graphite is cathodic to all structural metals and will generate galvanic corrosion in the presence of moisture, especially if the graphite is applied in dry form.

e. Footwear and clothing should be inspected for metal chips, slivers, rivet cuttings, dirt, sand, etc., and all such material removed before walking or working on metal surfaces such as wings, stabilizers, fuel tanks, etc.

f. Do not abrade or scratch any surface unless it is an authorized procedure. If surfaces are accidentally scratched, the damage should be assessed and action taken to remove the scratch and treat the area.

g. Coated metal surfaces should not be polished for aesthetic purposes. Buffing would remove the protective coating and a brightly polished surface is normally not as corrosion resistant as a nonpolished surface unless it is protected by wax, paint, etc. A bare skin sheet polished to a mirror finish is more resistant than a bare mill finished sheet when both are given regular maintenance.

h. Protect surrounding areas when welding, grinding, drilling, etc., to prevent contaminating them with residue from these operations. In those areas where protective covering cannot be used, action should be taken to remove the residue by cleaning.

i. Severely corroded screws, bolts, and washers should be replaced. When a protective coating, such as a cadmium plating on bolts, screws, etc., is damaged, immediate action should be taken to apply an appropriate protective finish to prevent additional corrosion damage.

602 – 609 RESERVED.



When active corrosion is visually apparent, a positive inspection and rework program is necessary to prevent any further deterioration of the structure. The following methods of assessing corrosion damage and procedures for rework of corroded areas could be used during cleanup programs. In general, any rework would involve the cleaning and stripping of all finish from the corroded area, the removal of corrosion products, and restoration of surface protective film.

a. Repair of corrosion damage includes removal of all corrosion and corrosion products. When the corrosion damage exceeds the damage limits set by the aircraft manufacturer in the structural repair manual, the affected part must be replaced or an FAA approved engineering authorization for continued service for that part must be obtained.

b. For corrosion damage on large structural parts which is in excess of that allowed in the structural repair manual and where replacement is not practical, contact the aircraft manufacturer for rework limits and procedures.


Several standard methods are available for corrosion removal. The methods normally used to remove corrosion are mechanical and chemical. Mechanical methods include hand sanding using abrasive mat, abrasive paper, or metal wool, and powered mechanical sanding, grinding, and buffing, using abrasive mat, grinding wheels, sanding discs, and abrasive rubber mats. However, the method used depends upon the metal and the degree of corrosion. The removal method to use on each metal for each particular degree of corrosion is outlined in the following paragraphs.


All corrosion products should be removed completely when corroded structures are reworked as the corroding process will continue even though the affected surface is refinished. Before starting rework of corroded areas, carry out the following:

a. Position airplane in wash rack or provide washing apparatus for rapid rinsing of all surfaces.
b. Connect a static ground line to the airplane.
c. Prepare the aircraft for safe ground maintenance.

(1) Remove the aircraft battery(s), LOX container (if installed), and external hydraulic and electric power.
(2) Install all applicable safety pins, flags, and jury struts.

d. Protect the pitot static ports, louvers, airscoops, engine opening, wheels, tires, magnesium skin panels, and airplane interior from moisture and chemical brightening agents.

e. Protect the surfaces adjacent to rework areas from chemical paint strippers, corrosion removal agents, and surface treatment materials.


a. For small areas of metallic surfaces, paint may be removed by hand, using a medium grade abrasive mat. For larger areas, chemical paint removal is the preferred method.

b. Phenolic and nonphenolic chemical paint removers containing methylene chloride are recommended for paint striping on metallic surfaces only. Chemical paint strippers containing acid should not be used as hydrogen embrittlement on high strength steel and some stainless steels will occur. The following procedure is recommended when using chemical paint remover:

(1) Mask acrylic windows and canopies, other plastic parts, rubber hoses, exposed wiring, composite surfaces, wheels and tires, and any other areas where the paint remover residue cannot be removed and refinishing cannot be accomplished;

(2) Remove sealants when required, by cutting away excess sealant with a sharp plastic scraper;

(3) Apply a thick, continuous coating of paint remover to cover the surface to be stripped;

(4) Allow paint remover to remain on the surface for a sufficient length of time to wrinkle and completely lift the paint. Reapply paint remover as necessary in the areas where paint remains tight or where the material has dried. Micarta scrapers, abrasive pads, or fiber brushes may be used to assist in removing persistent paint;

(5) Remove loosened paint and residual paint remover by washing and scrubbing the surface with fresh water and a stiff nylon bristle brush or an abrasive pad; and

(6) After thorough rinsing, remove masking materials and thoroughly clean the area with a solution of aircraft cleaning compound to remove paint remover residues.

c. For composite surfaces, paint removal should be done by mechanical removal techniques (scuff sanding) only. Composite surfaces include fiberglass, kevlar, carbon, graphite, and others. Due to the irregularities in composite surfaces (fiber weave), complete removal of the paint system can not be accomplished without surface fiber damage.

d. Mechanical paint removal may be done by hand or with fine or very fine abrasive mats or flap brushes on power tools. If power tools are used, care must be used to prevent removal of the base material.


In special instances, a particular or specific method may be required to remove corrosion. Depending upon rework criteria, corrosion in a hole may be removed by enlarging the hole. Abrasive blasting may be required for removing corrosion from steel fasteners, side skins, or irregularly shaped parts or surfaces. Whenever such special cases occur, the specified method for corrosion removal should be observed.


All depressions resulting from corrosion rework should be faired or blended with the surrounding surface. Fairing can be accomplished as follows:

a. Remove rough edges and all corrosion from the damaged area. All dish outs should be elliptically shaped with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. (SELECT THE PROPER ABRASIVE FOR FAIRING OPERATIONS FROM TABLE 6-1.)



8 – X
9 – X
10 – X
11 – X
4 – 400
8 – X
9 – X
10 – X

8 – X
10 – X
11 – X
4 – 400
8 – X
10 – X

10 – X
11 – X
4 – 400
10 – X

8 – X
10 – X
11 – X
4 – 400
8 – X
10 – X

9 – X
10 – X
11 – X

b. Rework depressions by forming smoothly blended dish outs, using a ratio of 20:1, length to depth (see Figures 6-1 and 6-2). In areas having closely spaced multiple pits, intervening material should be removed to minimize surface irregularity or waviness (see Figures 6-3 and 6-4). Steel nutplates and steel fasteners should be removed before blending corrosion out of aluminum structure. Steel or copper particles embedded in aluminum can become a point of future corrosion (see Figure 6-5). All corrosion products must be removed during blending to prevent reoccurrence of corrosion (seeFigures 6-6 and 6-7).

c. In critical and highly stressed areas, all pits remaining after removal of corrosion products by any method should be blended out to prevent stress risers which may cause stress corrosion cracking. On noncritical structure it is not necessary to blend out pits remaining after removal of corrosion products by abrasive blasting, since this results in unnecessary metal removal.


a. A serious problem encountered in corrosion control is the identification of the metal on which corrosion occurs. The importance of this identification arises from the fact that all metals possess certain chemical characteristics that are common only to themselves and which vary greatly from metal to metal and from alloy to alloy of the same metal. Since these characteristics are common to all metals and their alloys, chemical cleaning solutions and chemical protective films will react differently with various metals. In some cases, this produces adverse reactions which can severely weaken or destroy the structural capabilities of the metal.

(1) The primary method of determining material identification is in the aircraft structural repair manual. When the structural repair manual is limited or when more information is required, i.e., heat treat and protective finishes, the best source of material identification is the aircraft manufacturer, where an aircraft drawing can be reviewed.

(2) Chemical testing can be used when all other methods have been exhausted and when the following precautions are followed:

(i) Personnel should become thoroughly familiar with the safety precautions and emergency safety procedures, prior to performing any chemical testing.

(ii) Chemical spot testing should be accomplished by qualified personnel only.

(iii) Fasteners should not be identified by chemical spot tests.

(iv) High strength steel should not be identified by chemical spot tests.

b. Chemical testing for each type of metal should be accomplished on test samples before any tests are accomplished on the actual part. The preliminary surface preparation and primary classification of the metal may be determined by the following procedure.

(1) On the surface to be tested, choose an area where there is no corrosion and remove paint (if present) from a 1 inch square. Paint may be removed using hand sanding or an approved paint remover.


(2) Clean area of surface to be tested.

(3) Tentatively identify the exposed metal surface by visually comparing it with samples of previously identified materials, if available.

(4) Identify the metal as ferrous or nonferrous by placing a magnet on the exposed surface.

(i) Magnetic attraction classifies the base metal as a ferrous magnetic material (iron or steel).

(ii) The absence of magnetic attraction classifies the base metal as either an austenitic stainless steel or a nonferrous metal (aluminum, magnesium, etc.)

(5) Hardness test magnetic metals by a qualified person prior to chemical spot testing. If the metal is nonmagnetic, proceed with paragraph 618.


The magnetic metals usually employed in aircraft construction are ferrous alloys (high strength steels and some stainless steels). These magnetic alloys, when plated, are generally plated with either chromium, nickel, zinc, cadmium, silver, or with a combination of these platings.

a. If a magnetic alloy has been plated with cadmium, zinc, or chromium, it will exert magnetic attraction. Nickel plating will show slight magnetic attraction even if the substrate or base metal is not magnetic.

b. If positive identification of the metal plating is necessary, the identification should be made after accomplishing a hardness test.


c. Place a drop of 10 percent hydrochloric acid (HCL) on the prepared metal surface. Ensure that the surface is dry before applying acid.

(1) A rapid reaction producing a dark deposit indicates that the metal is zinc.

(2) A slow or no reaction indicates that the metal may be cadmium, chromium, nickel, or steel.


d. After 1 minute, add a drop of Na2S to the drop of HCL.

(1) A white precipitate identifies the metal as zinc.
(2) A yellow ring formed around a white precipitate identifies the metal as cadmium.
(3) A black ring formed around a white precipitate identifies the metal as iron or steel.
(4) A black precipitate indicates that the metal is chromium or nickel.

e. Confirm the cadmium, zinc, iron, or steel test by placing a drop of 20 percent nitric acid (HNO3) on a fresh spot. After 1 minute, add a drop of Na2S to the drop of HNO3.

(1) A white precipitate identifies the metal as zinc.
(2) A yellow precipitate identifies the metal as cadmium.
(3) A black spot identifies the metal as iron or steel.

f. Confirm the chromium test by placing a drop of 10 percent HCL on a fresh spot. Add a drop of concentrated sulfuric acid (H2SO4) to the drop of HCL. A color change to green after 1 or 2 minutes identifies the metal as chromium.

g. Confirm the nickel test by placing a drop of dimethylglyoxime solution on a fresh spot. Add a drop of ammonium hydroxide (NH4OH) to the drop of dimethylglyoxime solution. A pink to red precipitate identifies the metal as nickel.

h. Clean and refinish as detailed in paragraph 620.


The most common nonmagnetic metals used in aircraft construction are aluminum, magnesium, and austenitic steels (generally used as 18-8 stainless steel). The positive identification of these nonmagnetic metals is accomplished by the following procedure:

a. Place a drop of 10 percent HCL on the prepared metal surface and allow to stand for 1 minute. Ensure that the surface is dry before applying acid. (Zinc deposits on nonmagnetic metals will react with 10 percent HCL but will not produce a black spot.)

(1) A rapid or violent reaction that produces a black spot indicates that the metal is magnesium.
(2) A slow reaction indicates that the metal is aluminum.
(3) No reaction indicates that the metal is an austenitic steel or a nonmagnetic plating material.

b. If a reaction did not produce a blackspot as noted in paragraph 618.a.(1), determine if zinc is present as detailed in paragraph 617.e.

c. If the results of paragraph 618.b. are negative (zinc not present), confirm the magnesium and aluminum tests by placing a drop of 10 percent sodium hydroxide (NaOH) on a fresh spot. Check for the following:

(1) No reaction which will identify the metal as magnesium.

(2) A reaction that produces a colorless spot will identify the metal as a bare aluminum alloy.

d. If an aluminum alloy is identified as outlined in paragraph 618.c., further test to distinguish the different alloys by placing a drop of 10 percent cadmium chloride on a fresh spot.

(1) A dark gray deposit forming within a few seconds will identify the metal as 7075 or 7178 bare aluminum alloy.

(2) A dark gray deposit forming within 2 minutes will identify the metal as 7075 or 7178 clad aluminum alloy.

(3) No deposit formation in the time specified for 7075 or 7178 clad will identify the metal as 2024 aluminum alloy (a faint deposit will form after 15 or 20 minutes).

e. Confirm the austenitic steel test by dissolving 10 grams of cupric chloride (CUCl2 2H2O) in 100 cubic centimeters of HCL and placing a drop of the solution on a fresh spot. After 2 minutes, add three or four drops of distilled water to the drop of HCL solution and dry the surface. The appearance of a brown spot identifies the metal as an austenitic steel.

f. If no reaction was noted as outlined in paragraph 618.a or e., test for a plating material as detailed in paragraph 617.

g. If step f. reveals the presence of plating on the nonmagnetic metal, the plating should be removed by mechanical abrasion and the base metal identified by the visual and/or chemical methods outlined in paragraph 617.

h. Clean and refinish as required in paragraph 620.


The most common types of surface treatment for metals used in aircraft construction are: chemical conversion coatings, phosphate treatments for steels, and chromate treatments for aluminum. Other surface treatments include lacquer and chromate films. The identification of these surface treatments may be accomplished by the following procedures:

a. Phosphate treatment.

The presence of a phosphate treatment on steel, zinc, cadmium, or aluminum can be confirmed by placing a drop of 20 percent nitric acid (HNO3) on the surface and following this with two drops of ammonium molybdate solution. If the metal surface has had a phosphate treatment, a yellow precipitate will form.

b. Chromate treatment.

Surface chromate treatments on zinc, cadmium, aluminum, or magnesium are highly colored and are indicative of the application of these treatments. However, a bleached chromate treatment may have been applied and then coated with lacquer to mask any residual iridescence for the sake of appearance. If so, visual detection of the chromate is impossible. To test for lacquer, proceed as directed in paragraph 619.c. It should be noted that the bleaching process used in a bleached chromate treatment lowers the corrosion resistance provided by the chromate film.

c. Lacquer finish.

To test for lacquer, place a drop of concentrated sulfuric acid on the surface. If lacquer is present, the spot will rapidly turn brown with no effervescence. If lacquer is not present, the spot will not turn brown. If the metal is zinc, there will be no rapid effervescence. If the metal is cadmium, there will be no reaction.

d. Chromate film.

To detect a chromate film on zinc and cadmium, place a drop of 5 percent aqueous solution of lead acetate on the surface. If the metal has been treated, the surface will show no discoloration for 10 seconds. If there is no surface treatment, an immediate dark spot will appear.



After identification of the metal is completed, clean the area as follows:

a. Blot any remaining chemicals with a dry cloth.

b. Swab the area several times with a water moistened cloth.

c. Test the surface by placing a piece of litmus paper on the moistened surface. If the litmus paper changes color, repeat steps a. and b. until no color change occurs.

d. Dry surface thoroughly.

e. Remove corrosion, if present, and refinish the surface, as applicable.

621 – 625 RESERVED.



Abrasive blasting is a process for cleaning or finishing metals, plastics, and other materials by directing a stream of abrasive particles against the surface of the parts. Abrasive blasting is used for the removal of rust and corrosion and for cleaning prior to further processing, such as painting or plating. Standard blast cleaning practices should be adopted with the following requirements being made:

a. Any form of blast cleaning equipment may be used, but in cabinet blasting is preferred.
b. External gun blasting may be used if adequate confinements and recovery are provided for the abrasives.


Operators should be adequately protected with complete face and head covering equipment, and provided with pure breathing air. Magnesium creates a fire hazard when abrasive blasted. Dry abrasive blasting of titanium alloys and high tensile strength steel creates sparking. Care should be taken to assure that no hazardous concentration of inflammable vapors exists. Static ground the dry abrasive blaster and the material to be blasted.

a. The part to be blast cleaned should be removed from the aircraft, if possible. Otherwise, areas adjacent to the part should be masked or protected from abrasive impingement and system (hydraulic, oil, fuel, etc.) contamination.

b. Parts should be clean of oil, grease, dirt, etc., and dry prior to blast cleaning.

c. Close tolerance surfaces, such as bushings, bearing shaft, etc., should be masked.

d. Blast clean only enough to remove corrosion coating. Proceed immediately with finishing requirement using surface treatments as required.

628 – 639 RESERVED.



Corrosion evaluation will be required after general inspection and cleaning to determine the nature and extent of repair or rework. Local blending of corroded areas may be required to determine the total extent of the corrosion problem (see Figures 6-1, 6-2, 6-3 and 6-4). Corrosion damage classifications are defined as follows:

a. Light corrosion.

Characterized by discoloration or pitting to a depth of approximately 0.001 inch maximum. This type of damage is normally removed by light, hand sanding or a minimum of chemical treatment.

b. Moderate corrosion.

Appears similar to light corrosion except there may be some blisters or evidence of scaling and flaking. Pitting depths may be as deep as 0.010 inch. This type of damage is normally removed by extensive mechanical sanding.

c. Severe corrosion.

General appearance may be similar to moderate corrosion with severe blistering exfoliation and scaling or flaking. Pitting depths will be deeper than 0.010 inch. This type of damage is normally removed by extensive mechanical sanding or grinding. Severe corrosion damage beyond the limits of the aircraft structural repair manual will require FAA approved engineering authorization and may include the following typical corrosion repairs: trimming out of cracked and corroded areas (see Figure 6-8) or spot facing of fastener locations (see Figure 6-9).


There are two basic methods of corrosion removal, mechanical and chemical. The method used depends upon the type of structure, its location, the type and severity of corrosion, and the availability of maintenance equipment. Mechanical methods of corrosion removal are the most commonly used including: sanding, buffing, grinding, and section removal. Avoid the use of soft metal wire brushes (i.e., copper alloys) as residual traces of copper on cleaned metal will contribute to future corrosion. If brushes are used they should be stainless steel or nonmetallic. Mechanical methods of corrosion removal are used on all three levels of corrosion damage. Chemical methods of corrosion removal are limited to light corrosion and only in areas where the chemicals cannot migrate to other areas.


Determine degree of corrosion damage (light, moderate, or severe) with a depth dial gauge, straight edge, or a molding compound. The depth of corrosion cannot be measured until all the corrosion is removed. Before measurements are made, visually determine if corrosion is in an area which has previously been reworked. If corrosion is in the recess of a faired or blended area, measure damage to include the material which has previously been removed. The following method outlines the process for taking measurements with the depth gauge.

a. Remove loose corrosion products, if present.

b. Position depth gauge as illustrated in Figure 6-10 and determine the measurement reading.

NOTE: The base of the depth gauge should be flat against the undamaged surface on each side of the corrosion. When taking measurements on concave or convex surfaces, place the base perpendicular to the radius of the surface as shown in Figure 6-10.

c. Take several additional depth readings.

d. Select deepest reading as being the depth of the corrosion damage.


The maximum allowable amount of material removed from any damaged surface may be determined from criteria contained in the allowable damage limit chart in the manufacturer’s repair manual. If no criteria is given, contact the aircraft manufacturer for cleanup limits.


The amount of material which may be removed from a part or panel during corrosion cleanup is usually available in the manufacturer’s allowable damage limit charts. To ensure that the allowable limits are not exceeded, an accurate measurement should be made of the material removed or material thickness remaining in the reworked area.

a. Measurement of panel thickness after rework can be made using an ultrasonic tester. This method requires a qualified NDI operator and suitable test standards for calibration.

b. Measurement of the depth of blended pits (material removed) can be made using a depth dial gauge (see Figure 6-10). If the depth dial gauge will not work, clay impressions, liquid rubber, or other similar means which will give accurate results may be used to determine material removed. In the event that material removal limits have been exceeded, the area or part should be repaired or replaced. If replacement or repair criteria is not contained in the repair manual, contact the manufacturer or the FAA.

645 – 649 RESERVED.



In general, corrosion of aluminum can be more effectively treated in place rather than removing structural parts from the aircraft for corrosion treatment. Treatment includes the mechanical removal of the corrosion products, the inhibition of residual materials by chemical means, and the restoration of permanent surface coating. Details of treatment vary depending on whether the aluminum surfaces are to be left bare in use or are to be protected by paint coatings.


a. Bare aluminum surfaces. While few unpainted aircraft are used under marine conditions, some general information is included on the nature of alclad surfaces and their treatment. Relatively speaking, pure aluminum has greater corrosion resistance than the stronger aluminum alloys. Advantage is taken of this by laminating a thin sheet of relatively pure aluminum, one to five mils thick, over the base higher strength aluminum alloy surface. The protection obtained is good, and the alclad surface can be maintained in a polished condition (see Figure 6-12). In cleaning such surfaces, however, care should be taken to prevent staining and marring of the exposed aluminum, and more important from a protection standpoint, to avoid unnecessary mechanical removal of the protective alclad layer and the exposure of more susceptible, but stronger, aluminum alloy base material.

b. Additional processing of aluminum surfaces prior to paint finishes. Aluminum surfaces that are to be subsequently painted can be exposed to more severe cleaning procedures and can also be given more thorough corrective treatment prior to painting. Application of a paint finish requires proper prepaint treatment for good paint adhesion.

c. Special treatment of anodized surfaces. Anodizing is the most common surface treatment of aluminum alloy surfaces. Tank processing is accomplished during manufacture or rework of a part or component and frequently prior to its fabrication from sheet stock. The aluminum sheet or casting is made as a positive pole in an electrolytic bath in which chromic acid or other oxidizing agents produce a supplemental protective oxide film on the aluminum surface. Aluminum oxide is naturally protective and anodizing merely increases the thickness and density of the natural oxide film. When this coating is damaged in service, it can only be partially restored by chemical surface treatment. Therefore, any processing of anodized surfaces should avoid unnecessary destruction of the oxide film.

(1) Steel wool, steel wire brushes, copper alloy brushes, or severe abrasive materials should not be used on any aluminum surface. Aluminum wool, fiber bristle brushes, and mild abrasives are acceptable tools for cleaning anodized surfaces, but care should be exercised in any cleaning process to avoid unnecessary breaking of the protective film, particularly at the edges of the aluminum sheet (see Figures 6-11 and 6-13).

(2) Tampico fiber brushes are preferred and are adequate to remove most corrosion. Producing a buffed or wire brush finish by any means should be prohibited. Take every precaution to maintain as much of the protective coating as practicable. Otherwise, treat anodized surfaces in the same manner as other aluminum finishes. Vacuum blasting is an acceptable corrosion removal method to remove surface corrosion. Vacuum blasting should not be used to remove intergranular corrosion.

(3) Chemical conversion coating (Specification MIL-C-81706) is a chemical surface treatment used on aluminum alloys to inhibit corrosion and to provide a proper surface for paint finishing.

d. Special processing of intergranular corrosion in heat treated aluminum alloy surfaces. Intergranular corrosion is usually more severe in heat treated aluminum alloys and is a corrosive attack along grain boundaries of the alloyed aluminum, where the grain boundaries differ from the metal within the grain. When in contact with an electrolyte, rapid corrosion occurs at the grain boundaries. In its most severe form, actual lifting of metal layers (exfoliation) occurs. The mechanical removal of all corrosion products and visible delaminated metal layers should be accomplished in order to determine the extent of the damage and to evaluate the remaining structural strength of the component.

(1) Use metal scrapers, rotary files, or abrasive wheels to assure that all corrosion products are removed and that only structurally sound aluminum remains.


(2) Rotary files should be sharp to insure that they cut the metal without excessive smearing. A dull cutting tool will smear the metal over corrosion cracks or fissure and give the appearance that corrosion has been removed, when, in fact, it may not have been.

(3) Carbide tip rotary files or metal scrapers should be utilized since they stay sharp longer. Abrasive blasting should not be used to remove intergranular corrosion.

(4) Inspection with a 5 to 10 power magnifying glass or the use of dye penetrant will assist in determining if all unsound metal and corrosion products have been removed.

(5) When complete removal of corrosion has been accomplished, blend or fair using a ratio of 20:1 (length to depth) in the area of corrosion removal. Blending, where required, can best be accomplished by using aluminum oxide impregnated, rubber base wheels.

(6) Chemical conversion coat the exposed surfaces completely and restore paint coatings in the same manner as on any other aluminum surface (see Figures 6-14, 6-15, and 6-16).

(7) Corrosion damage beyond the limits set in the structural repair manual should be repaired per a cognizant engineer or aircraft manufacturer’s instructions that are FAA approved.


After extensive corrosion removal, the following procedures should be followed:

a. If water can be trapped in blended areas, chemical conversion coat and fill the blended area with structural adhesive or sealant to the same level and contour as the original skin. When areas are small enough that structural strength has not been significantly decreased, no other work is required prior to applying the protective finish.

b. When corrosion removal exceeds the limits of the structural repair manual, contact a cognizant engineer or the aircraft manufacturer for repair instructions.

c. Where exterior doublers are allowed, it is necessary to seal and insulate them adequately to prevent further corrosion.

d. Doublers should be made from alclad, when available, and the sheet should be anodized (preferred) or chemical conversion coated after all cutting, drilling, and countersinking has been accomplished.

e. All rivet holes should be drilled, countersunk, surface treated, and primed prior to installation of the doubler.

f. Apply a suitable sealing compound, in the area to be covered by the doubler. Apply sufficient thickness of sealing compound to fill all voids in the area being repaired.

g. Install rivets wet with sealant. Sufficient sealant should be squeezed out into holes so that all fasteners, as well as all edges of the repair plate, will be sealed against entrance of moisture.

h. Remove all excess sealant after fasteners are installed. Apply a fillet sealant bead around the edge of the repair. After the sealant has cured apply the protective paint finish to the reworked area.


Intergranular corrosion in aluminum alloys often originates at countersunk areas where steel fasteners are used. Removal of corrosion in a countersink is impossible to accomplish with the fastener in place.

a. When corrosion is found around a fixed fastener head, the fastener must be removed to ensure corrosion removal. It is imperative that all corrosion be removed to prevent further corrosion and loss of structural strength. To reduce the reoccurrence of corrosion, the panel should receive a chemical conversion coating, be primed, and have the fasteners installed wet with sealant.

b. Each time removable steel fasteners are removed from access panels, they should be inspected for material condition including the condition of the plating. If mechanical or plating damage is evident, replace the fastener. Upon installation, one of the following fastener installation methods should be followed:

(1) Brush a corrosion preventive compound on the substructure around and in the fastener hole, start the fastener, apply a bead of sealant to the fastener countersink, and set and torque the fastener within the working time of the sealant (this is the preferred method);

(2) Apply the corrosion preventive compound to the substructure and fastener, set and torque the fastener; or

(3) Apply a coating of primer to the fastener, and while wet with primer set and torque the fastener.


a. Prepare the aircraft for corrosion rework as provided in paragraph 612 and remove corrosion products as follows. Observe the work procedures of paragraph 601.

b. Positively identify the metal as aluminum.

c. Clean the area to be reworked. Strip paint if required.

d. Determine extent of corrosion damage as covered in paragraph 642. To remove light corrosion, proceed with paragraph 654.e. To remove moderate or severe corrosion, proceed with paragraph 654.f.

e. Remove light corrosion by hand rubbing the corroded surface with tools, abrasives or by chemical means as follows. Do not use the chemical removal process at temperatures above 100 °F or below 40 °F.

(1) Protectively mask adjacent areas to prevent brighteners from contacting magnesium, anodized aluminum, glass, plexiglass, fabric surfaces, and all steel. Wear acid resistant gloves, protective mask, and protective clothing when working with corrosion removing compounds. If corrosion removing compounds accidentally contact the skin or eyes, flush off immediately with plenty of clear water. Refer to safety procedures in paragraph 600.

(2) Dilute corrosion removing compound (Specification MIL-C-38334, Type I) with an equal volume of water. Mix the compound only in wood, plastic, or plastic lined containers. The diluted solution of corrosion removing compound may be applied by flowing, mopping, sponging, brushing, or wiping.

(3) Apply diluted solution to large areas with a circular motion to disturb the surface film and ensure proper coverage. The diluted solution should be applied starting at the lowest area and working upwards. The solution will be more effective if applied warm (140 °F maximum) followed by vigorous agitation with a nonmetallic, acid resistant brush or aluminum oxide abrasive nylon mat.

(4) Leave the solution on surface for approximately 12 minutes. Do not allow solution to dry on surface, as streaking will result. (On large exterior surfaces, remove solution by high pressure water rinse.)

(5) Wipe off solution with a clean, moist cloth; frequently rinse the cloth in clear water. Wipe the area several additional times with a fresh cloth dampened and rinsed frequently in clear water.

(6) Dry the area with a clean, dry cloth and inspect for corrosion.

(7) Repeat the procedure outlined in paragraph 654.e. if any corrosion remains.


(8) After all corrosion has been removed, proceed with paragraph 654.f.(4).

f. Mechanically remove moderate or severe corrosion by the appropriate methods as follows.


(1) Remove loose corrosion products by hand rubbing the corroded surface with tools or abrasives. Dry abrasive blasting using glass beads (Specification MIL-G-9954) sizes 10, 11, 12, 13, or grain abrasive (Specification MIL-G-5634) types I and III, may be used as an alternate method of removing corrosion from clad and nonclad aluminum alloys. Abrasive blasting should not be used to remove heavy corrosion products. Direct pressure machines should have the nozzle pressure set at 30 to 40 psi for clad aluminum alloys and 40 to 45 psi for nonclad aluminum alloys. Engineering approval from the aircraft manufacturer should be obtained prior to abrasive blasting metal thinner than 0.0625 inch.

(2) Remove residual corrosion by hand sanding or with an approved hand-operated power tool. Corrosion removal using power tools is generally done with flapbrush, rotary file, sanding pad, or abrasive wheel attachments. Rotary files should not be used on skin thinner than 0.0625. Select an appropriate abrasive from Table 6-1.

(3) Using a blend ratio of 20:1 (length to depth) blend and finish the corrosion rework area with progressively finer abrasive paper until 400 grit abrasive paper is used.

(4) Clean reworked area using dry cleaning solvent; do not use kerosene.

(5) Determine depth of faired depressions to ensure that rework limits have not been exceeded.

(6) Apply a chemical conversion coating to the corrosion rework area.

(7) Apply paint finish to the corrosion rework area.

655 – 659 RESERVED.

{p170 through 180 blank}



Corrosive attack on magnesium skins will usually occur around edges of skin panels, underneath holddown washers, or in areas physically damaged by shearing, drilling, abrasion, or impact. Entrapment of moisture under and behind skin crevices is frequently a contributing factor. If the skin section can be easily removed, this should be accomplished in order to assure complete inhibition and treatment.

a. Complete mechanical removal of corrosion products should be practiced when practicable. Such mechanical cleaning shall normally be limited to the use of stiff bristle brushes and similar nonmetallic cleaning tools, particularly during treatment in place under field conditions.

b. Any entrapment of steel particles from steel wire brushes, steel tools, or contamination of treated surfaces by dirty abrasives, can cause more trouble than the initial corrosive attack.

c. When aluminum insulating washers are used and they no longer adhere to magnesium panels, corrosion is likely to occur under the washers if corrective measures are not taken.

(1) When machine screw fasteners are used, they should be removed from all loose insulating washer locations in order to surface treat the magnesium panel.

(2) Where permanent fasteners other than machine screws are used, the insulating washer and fastener should be removed to ensure complete corrosion removal.

(3) When located so that water can be trapped in the counterbored area where the washer was located, use sealants to fill the counterbore. If necessary to fill several areas adjacent to each other, it may be advantageous to cover with a strip of sealant.


The same general instructions apply when making repairs in magnesium as in aluminum alloy skin, except that two coats of epoxy primer may be required on both the doubler and skin being patched instead of only one coat. Where it is difficult to form magnesium alloys in the contour, aluminum alloy may be utilized. When this is done, it is necessary to insure effective dissimilar metal insulation. Vinyl tape will insure positive separation of dissimilar metals, but edges will still have to be sealed to prevent entrance of moisture between mating surfaces at all points where repairs are made. It is recommended that only noncorrosive type sealant be used, since it serves a dual purpose of material separation and sealing.


Magnesium castings, in general, are more porous and more prone to penetrating attack than wrought magnesium skin. However, treatment in the field is, for all practical purposes, the same for all magnesium areas. Engine cases are among the most common examples of cast magnesium encountered in modern aircraft. Bellcranks, fittings, and numerous covers, plates, and handles may also be magnesium castings. When attack occurs on a casting, the earliest practicable treatment is required if dangerous corrosive penetration is to be avoided. Engine cases in salt water can develop “moth holes” and complete penetration overnight.

a. If it is at all practicable, faying surfaces involved should be separated in order to effectively treat the existing attack and prevent its further progress. The same general treatment sequence detailed for magnesium skin should be followed. Where engine cases are concerned, baked enamel overcoats are usually involved rather than other topcoat finishes. A good air drying enamel can be used to restore protection.

b. If extensive removal of corrosion products from a structural casting is involved, a decision from the aircraft manufacturer may be necessary in order to evaluate the adequacy of structural strength remaining. Structural repair manuals usually include dimensional tolerance limits for critical structural members. The FAA should be consulted if any questions of safety are involved.


If possible, corroded magnesium parts should be removed from aircraft. When impossible to remove the part, make aircraft preparations detailed in paragraph 612. When using that procedure, observe the safety precautions and procedures of paragraph 600.

a. Positively identify metal as magnesium. (Refer to paragraph 618)

b. Clean area to be reworked.

c. Strip paint if required.

d. Determine extent of corrosion damage as detailed in paragraph 642. To remove light corrosion, proceed with paragraph 663.e. To remove moderate or severe corrosion, proceed with paragraph 663.f.

e. Remove light corrosion by light hand sanding or chemically, as follows. Do not use the following procedure for adhesive bonded parts or assemblies, areas where the brush on solution might become lodged, or local areas bared specifically for grounding or electrical bonding purpose.

(1) Remove loose corrosion with aluminum wool or abrasive mat, paper, or cloth.

(2) Mask off other materials and parts, especially rubber parts, bearings, and cast or pressed inserts to prevent contact with the treating solution or its fumes.

(3) Prepare corrosion treating solution in the following proportions: 1 1/2 pounds of sodium dichromate and 1 1/2 pints of concentrated nitric acid (HNO3) per gallon of water. Mix as follows, but prepare and store the solution in clean polyethylene or glass containers:

(i) Fill a suitable container with a volume of water equal to 1/4 the desired total quantity of solution.

(ii) Add full quantity of sodium dichromate in proportions indicated and agitate solution until the chemical is dissolved.

(iii) Add water until quantity of solution is equal to approximately 2/3 the desired total quantity.

(iv) Slowly add total volume of nitric acid (HNO3) to solution and mix thoroughly.

(v) Add remaining water until total desired quantity of solution is reached and stir until entire solution concentration is equal.

(4) Remove remaining corrosion by swabbing the corroded surface 1 to 2 minutes with the nitric acid (HNO3) solution, then wipe dry.

(5) Rinse thoroughly with clean water while scrubbing with a mop, brush, or abrasive mat and wipe dry.

(6) Repeat the preceding sequence, as necessary, until all corrosion has been removed.

(7) After all corrosion has been removed, proceed with paragraph 663.g.

f. Mechanically remove moderate or severe corrosion. Wear goggles or a face shield to preclude injury from corrosion particles breaking loose and flying off. Protect adjacent areas to prevent additional damage from corrosion products removed when using this procedure.


(1) Remove heavy corrosion products by hand brushing with a stainless steel or fiber brush followed by vacuum abrasive blasting with glass beads, (Specification MIL-G-9954) sizes 10, 11, 12, 13; or grain abrasive (Specification MIL-G-5634) Types I or III. An air pressure at the nozzle of 10 to 35 psi should be used for direct pressure machines. For suction type blast equipment, use 50 percent higher pressure.

(2) Remove residual corrosion by hand sanding or with approved hand operated power tool.

(3) After removing all corrosion visible through a magnifying glass, apply corrosion treating solution.

g. Fair depressions resulting from rework using a blend ratio of 20:1. Clean rework area using 240 grit abrasive paper. Smooth with 300 grit and final polish with 400 grit abrasive paper.

h. Determine depth of faired depressions to ensure that rework limits have not been exceeded.

i. Clean reworked area using a solvent to provide a water breakfree surface. Do not use kerosene.

j. Prepare and apply magnesium conversion coat conforming to MIL-M-3171, TYPE VI (DOW-19) as follows:

(1) Measure 1 gallon of distilled water into a clean polyethylene or glass container.

(2) Add 1.3 ounces (dry) of chromium trioxide or 1.3 ounces of technical grade chromic acid.

(3) Add 1 ounce of calcium sulfate dehydrate (CaSO4.2H2O)

(4) Vigorously stir for at least 15 minutes to ensure that the solution is saturated with calcium sulfate. (Let chromate solution stand for 15 minutes prior to decanting.)

(5) Prior to use, decant solution (avoid transfer of undissolved calcium sulfate) into suitable usage containers (polyethylene or glass).

(6) Apply solution by swabbing until the metal surface becomes a dull color (the color can vary from green-brown, brassy, yellow-brown to dark brown). Under optimum conditions of temperature at 70 °F or above and fresh materials, the time required to properly apply magnesium pretreatment is usually 1 to 5 minutes. Under these conditions, 1 to 2 minutes of treatment should produce a brassy film, and 3 to 5 minutes a dark brown coating. Under adverse conditions, and if the desired specified finish color is not produced in the specified time, the treatment may have to be prolonged up to 20 to 30 minutes in some instances until the proper finish is effected. For good paint adhesion, a dark brown color, free of powder, is considered best. The color may vary in using different vendors’ materials. Too long exposure to the brush on solution produces a coating which will powder and impair adhesion of applied paint finish/films. Use caution in swabbing on the solution. Severe rubbing of the wet surface will damage the coating.


(7) Rinse with clean water, then allow to dry at ambient temperature for a minimum of 1 hour (more in high humidity areas).

k. Apply primer and topcoat finish.

l. Remove masking and protective covering.

664 – 669 RESERVED.

{p186 through p190 blank}



One of the most familiar kinds of corrosion is red iron rust, generally resulting from atmospheric oxidation of steel surfaces. Some metal oxides protect the underlying base metal, but red rust is not a protective coating. Its presence actually promotes additional attack by attracting moisture from the air and acting as a catalyst in causing additional corrosion to take place.

a. Red rust first shows on bolt heads, holddown nuts, and other unprotected aircraft hardware. Red rust will often occur under nameplates which are secured to steel parts. Its presence in these areas is generally not dangerous and has no immediate effect on the structural strength of any major components. However, it is indicative of a general lack of maintenance and possible attack in more critical areas.

b. When paint failures occur or mechanical damage exposes highly stressed steel surfaces to the atmosphere, even the smallest amount of rusting is potentially dangerous in these areas and should be removed and controlled.


The most practicable means of controlling the corrosion of steel is the complete removal of corrosion products by mechanical means (see Figures 6-17 and 6-18). On high strength steel, corrosion removal by hand sanding is recommended. The use of powered tools is not recommended on high strength steel because of the danger of local overheating and the formation of notches which could lead to failure. However, it should be recognized that in any such use of abrasives, residual iron rust usually remains in the bottom of small pits and other crevices.

a. The best method to use on exterior surfaces is abrasive blasting which has the ability of removing nearly all rust.

b. Paint the cleaned metal surface as soon as possible after corrosion removal, and in any event do not allow the surface to become wet before painting.



a. There are acceptable methods for converting iron rust to phosphates and other protective coatings. Parco Lubrizing and use of phosphoric zinc preparations are examples of such treatment. However, these processes require shop installed hot tanks and are impracticable for use in the field.

b. Other preparations (i.e. phosphoric acid, naval jelly, etc.) are effective rust converters where tolerances are not critical and where thorough rinsing and neutralizing of residual acid is also possible.

c. These situations are generally not applicable to assembled aircraft and use of chemical inhibitors on installed steel parts is not only undesirable, but very dangerous. The possibility of entrapping of corrosive solutions, resulting in uncontrolled attack which could occur when such materials are used under field conditions, outweighs any advantage to be gained from their use.


Do not use wire brushes on high stressed steel parts. Any corrosion on the surface of a highly stressed steel part is potentially dangerous, and the careful removal of corrosion products is mandatory. Surface scratches or change in surface structure from overheating can cause sudden failure of these parts. Removal of corrosion products may be accomplished by careful use of mild abrasive papers, such as fine grit aluminum oxide, or fine buffing compounds, such as rouge, on cloth buffing wheels.

a. It is essential that steel surfaces not be overheated.

b. Abrasive blasting is also a satisfactory corrosion removal method for high strength steel located on aircraft exteriors.

c. After careful removal of surface corrosion, protective paint finishes should be applied immediately.


Do not use chemical cleaners on stainless steels. Stainless steels are of two general types: magnetic and nonmagnetic. Magnetic steels are of the ferritic or martensitic types and are identified by numbers in the 400 series. Corrosion often occurs on 400 series stainless steels and treatment is the same as specified in paragraph 673. Nonmagnetic steels are of the austenitic type and are identified by numbers in the 300 series. They are much more corrosion resistant than the 400 series steels, particularly in a marine environment.

a. Austenitic steels develop corrosion resistance by an oxide film which should not be removed even though the surface is discolored. The original oxide film is normally formed at time of fabrication by passivation. If this film is broken accidentally or by abrasion, it may not restore itself without repassivation.

b. If any deterioration or corrosion does occur on austenitic steels, and the structural integrity or serviceability of the part is affected, it will be necessary to remove the part.


If possible, corroded steel parts should be removed from the aircraft. When impossible to remove the part, observe the aircraft preparations and safety precautions in paragraph 601, 602, and 603. Chemical removal or chemical conversion coatings are not allowed on steel parts.

a. Positively identify the metal as steel as detailed in paragraph 617 and establish its heat treated value.

b. Clean area to be reworked.


c. Strip paint if required.

d. Remove all degrees of corrosion damage from steels heat treated below 220,000 psi as required by paragraph 655.e. Corrosion removal on steels treated to 220,000 psi and above should be accomplished only by hand sanding or dry abrasive blasting. (Refer to section 6 of this chapter.)

e. Mechanically remove all degrees of corrosion from steel parts heat treated below 220,000 psi as follows.


(1) Remove heavy deposits of corrosion products using a stainless steel hand brush. Abrasive blasting may be used as an alternate method of corrosion removal. Abrasive blasting should be accomplished using aluminum oxide (Specification MIL-G-21380) Type I, Grades A or B, grit sizes 25, 50, or 120; or No. 13 glass beads (Specification MIL-G-9954). An air pressure at the nozzle of 40 to 50 psi should be used for direct pressure machines when removing corrosion by abrasive blasting. Engineering approval from the aircraft manufacturer should be obtained prior to abrasive blasting metal thinner than 0.0625 inch.

(2) Remove residual corrosion by hand sanding or with approved hand operated power tool.

(3) The surface is highly reactive immediately following corrosion removal; consequently, primer coats should be applied within 1 hour after sanding. After removing all corrosion visible through a magnifying glass, continue with paragraph 675.f.

f. Fair depressions resulting from rework using a blend ratio of 20:1. Clean rework area using 240 grit abrasive paper. Smooth with 300 grit and final polish with 400 grit abrasive paper.

g. Determine depth of faired depressions as required to ensure that rework limits have not been exceeded.

h. Clean reworked area with dry, cleaning solvent. Do not use kerosene.

i. Apply protective finish or specific organic finish as required.

j. Remove masking and protective covering.

676 – 679 RESERVED.

{p196 through p200 blank}



Nickel and chromium platings are used extensively as protective and wear resistant coatings over high strength steel parts (landing gear journals, shock strut pistons, etc.). Chromium and nickel plate provide protection by forming a somewhat impervious physical coat over the underlying base metal. When breaks occur in the surface, the protection is destroyed.

a. The amount of reworking that can be performed on chromium and nickel plated components is limited. This is due to the critical requirements to which such components are subjected.

b. The rework should consist of light buffing to remove corrosion products and produce the required smoothness. This is permissible, provided the buffing does not take the plating below the minimum allowable thickness.

c. Whenever a chromium or nickel plated component requires buffing, coat the area with a corrosion preventive compound, if possible.

d. When buffing exceeds the minimum thickness, or the base metal has sustained corrosive attack, the component should be removed and replaced.

e. The removed component can be restored to serviceable condition by having the old plating completely stripped and replated in accordance with acceptable methods and specifications.


Cadmium plating is used extensively in aircraft construction as a protective finish over both steel and copper alloys. Protection is provided on a sacrificial basis in which the cadmium is attacked rather than the underlying base material. Properly functioning cadmium surface coatings may well show mottling, ranging from white to brown to black spots on their surfaces. These are indicative of the sacrificial protection being offered by the cadmium coat, and under no condition should such spotting be removed merely for appearance’s sake. In fact, cadmium will continue to protect even when actual breaks in the coating develop and bare steel or exposed copper surfaces appear.

a. Where actual failures of the cadmium plate occur and the initial appearance of corrosion products of the base metal develops, some mechanical cleaning of the area may be necessary but should be limited to removal of the corrosion products from the underlying base material.

b. Under no condition should such a coating be cleaned with a wire brush. If protection is needed, a touchup with primer or a temporary preservative coating should be applied. Restoration of the plate coating is impracticable in the field.

c. Zinc coatings offer protection in an identical manner to cadmium, and the corrective treatment for failure is generally the same as for cadmium plated parts. However, the amount of zinc on aircraft structures is very limited and usually does not present a maintenance problem.

682 – 689 RESERVED.

{p203 through p210 blank}



Silver, platinum, and gold finishes are generally used in aircraft assemblies because of their resistance to ordinary surface attack and their improved electrical or heat conductivity. Silver plated electrodes can be cleaned of brown or black sulfide tarnish, as necessary, by placing them in contact with a piece of magnesium sheet stock while immersed in a warm water solution of common table salt mixed with baking soda or by using a fine grade abrasive mat or pencil eraser followed by solvent cleaning. If assemblies are involved, careful drying and complete displacement of water is necessary. In general, cleaning of gold or platinum coatings is not recommended in the field.


Copper and copper alloys are relatively corrosion resistant, and attacks on such components will usually be limited to staining and tarnish. Generally, such change in surface condition is not dangerous and should ordinarily have no effect on the function of the part. However, if it is necessary to remove such staining, a chromic acid solution of 8 to 24 ounces per gallon of water containing a small amount of battery electrolyte (not to exceed 50 drops per gallon) is an effective brightening bath. Staining may also be removed using a fine grade abrasive mat or pencil eraser followed by solvent cleaning.

a. The stained part should be immersed in the cold solution. However, surfaces can also be treated in place by applying the solution to the stained surface with a small brush.

b. Care should be exercised to avoid any entrapment of the solution after treatment. The part should be cleaned thoroughly following treatment with all residual solution removed.

c. Serious copper corrosion is evident by the accumulation of green to blue copper salts on the corroded part. These products should be removed mechanically using a stiff bristle brush, brass wire brush, 400 grit abrasive paper or bead blast with glass beads, (Specification MIL-G-9954, size 13). When bead blasting, air pressure at the nozzle should be 20 to 30 psi for direct pressure machines. Do not bead blast braided copper flexible lines. A surface coating should be reapplied over the reworked area. Again, chromic acid treatment will tend to remove the residual corrosion products.

d. Most brass and bronze structural parts will be protected by cadmium surface plate. The mottling of the protective cadmium coat should not be removed, and mechanical surface cleaning should not be attempted unless actual copper corrosion products are beginning to appear. Under these conditions, any mechanical removal of the protective cadmium should be held to a minimum and limited to the immediate area of the copper attack.


Titanium and its alloys are highly corrosion resistant because a protective oxide film forms on their surfaces upon contact with air. When titanium is heated, different oxides having different colors form on the surface. A blue oxide coating will form at 700 to 800 °F; a purple oxide at 800 to 950 °F; and a gray or black oxide at 1,000 °F or higher. Corrosive attack on titanium surfaces is difficult to detect. It may show deterioration from the presence of salt deposits and metal impurities at elevated temperatures so periodic removal of surface deposits is required.

a. Clean titanium surfaces until all traces of corrosion or surface deposits are removed using one of the following:

(1) Stainless steel wire brush;

(2) Hand sand with aluminum oxide abrasive paper or abrasive mat;

(3) Dry blast with glass beads, (Specification MIL-G-9954) sizes 10, 11, 12, or 13, or aluminum oxide (Specification MIL-G-21380 Type I) grades A or B, using 40 to 50 psi air pressure at the nozzle for direct pressure machines; or

(4) Hand polish with aluminum polish and soft cloth.

b. Chlorinated Hydrocarbon cleaners should not be used on titanium alloys which are subject to elevated temperatures in service. Such solvents can cause stress corrosion in titanium.



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