At atmospheric condition we have sufficient pressure (3 p.s.i.) to breath freely. As the altitude increases pressure drops. Till 8,000 ft altitude the pressure variation won’t affect us but beyond that we will get affect (hypoxia), so in order to have same pressure to breath freely in the cabin of aircraft, we need to go for maintaining the pressure artificially.
It may be done by
- I. Cabin Superchargers
- II. Engine –Driven Compressors
- III. Compressed air from engine
As like pressure temperature also drops as the altitude increases, but up to certain layer of atmosphere which is stratosphere. Most of the aircraft are operating within Stratosphere. So we need to have a constant temperature in our cabin, so that passengers feel the comfort of their travel.
It can be done by
- i. Air cycle cooling system
- ii. Vapor cycle cooling system
Most of the aircraft have mingled the pressurization and air conditioning system in order to simplify the system.
AIR CYCLE SYSTEM IN AIRCRAFT
1. Left engine
2. Right engine
3. Flow limiter
4. Primary heat exchanger
5. Primary heat exchanger bypass valve
6. Shut off valve
8. Secondary heat exchanger
9. Water separator
10. Secondary heat exchanger bypass valve
11. Refrigeration unit
1. Air from the compressor sections of the two engines is taken for air conditioning and pressurization. We are making a cross connection so that it can supply uniform flow of air.
2. Air is going to the flow limiter. Flow limiter limits the amount of compressed air to entry into the system. Suppose if there is any pipeline ruptured in air cycle system, then flow limiter won’t allow the compressed air to enter into the air cycle system.
3. Partial amount of air from the flow limiter goes to the primary heat exchanger. Primary heat exchanger utilizes ram air for cooling purpose. The compressed air taken from the engine compressor may be at temp range of 200 deg to 400 deg Celsius. We are cooling the air without reducing much pressure by using heat exchanger, and heat exchanger works on Convective type.
4. Another amount of compressed air from flow limiter goes to mingle with the heat exchanger outlet air to make a constant temperature air of 300 °F. This constant temperature can be attained only by proper operation of Primary heat exchanger (PHE) bypass valve. For example if heat exchanger outlet air is at 200°F, but I need an output of 300°F at outlet portion of Primary heat exchanger bypass valve, so we are opening the bypass valve for some designated time and mixing the hot(directed from flow limiter) and cold (PHE outlet ) flows.
5 .This 300°F temperature air is going to split for three Purpose
a) For Anti-icing
b) To supply hot air to cabin(if required)
c) To refrigeration unit
6. This 300°F air can be directly used for anti-icing and de-icing purpose. This air will be taken by the tubes and will be sprayed on the leading edge through suitable arrangements. And one part of the 300°F air flow is directed to the cabin for hot air supply.
7. Remaining part of the air is directed towards refrigeration unit for further cooling. On the way there is main control that is “Main shut off Valve”. This can be directly controlled by the pilot.
8. After main shut off valve there is Refrigeration unit, this contains
c) Water Separator
Both turbine and compressor are connected by the same shaft.
9. The air flows to the compressor region; there it strikes the compressor blade and makes rotating at initial. After compressor starts it compresses the air and Pressurizes the air so some amount of heat may be added to the air. Then Air is going to the Secondary heat exchanger (SHE), where the heat from the air is taken by the ram air by convection. The air has been cooled now.
10. The output of SHE goes to the Turbine. Since because of the turbine, here the cold gas is allowed to expand, so that pressure drops, temperature again drops, it may be on some minus °F sometimes.
11. After expansion in the turbine, air which is in a circular motion is allowed to go for the water separator region for separation of water particles in the cold air. Since water particles are denser than air they get attached to the walls of the water separator due to centrifugal force. In some specified place there are some holes made on the water separator to drain the water particles attached.
12. In the cabin we need only a temperature range of 60°F-125°F (15°C-51°C) and a pressure of 3p.s.i. which is suitable for human.
13. By opening and closing the refrigerant bypass valve, we can mingle the pure cold air and 300°F air, to make a possible living temperature for human beings.
14. After maintaining to proper temperature and pressure the air is allowed to go to the cabin by suitable pipelines.
15. If there is any problem on the total system means, we can directly mix the hot air supply with ram air (which is taken near SHE) and maintain the proper temperature by proper mixing. But this method is only for emergency purpose. Please note when using this ram air method of cooling pressurization should be done separately by cabin superchargers or whatever the device builds up pressure.
Vapor cycle cooling system in Aircraft
Liquids can be vaporized at any temperature by changing the pressure acting on it. To clearly understand this concept, we will take an example of water contained in a vessel. When the vessel is at atmospheric pressure, the water will be boiling at 100°C when heating. If I am pressurizing the vessel to more than the atmospheric pressure, then water will not boil at 100°C. If I am creating a drop in pressure in the vessel by a vacuum pump, then water will boil at a temperature less than 100°C.
Basic law of thermodynamics states that heat will always flow from hot body to cold body. If I need a reverse of this, I have to add some work.
Typical System operation:
1. We are having the FREON as refrigerant in the vapor cycle cooling system. It has a boiling point of 4°C.
2. At the receiver I am having high pressure, so that FREON will have high boiling point.
3. When the vapor cycle system is switched on, the compressor starts delivering the pressure and thus making flow.
4. The highly pressurized FREON at the receiver is in liquid phase. When the Freon flows through the circuit, first it expands at the Expansion valve. So pressure has been dropped (i.e. Boiling point decreased).
5. The less pressure Freon then goes to the evaporator stage. Evaporator will be exposed to Cabin. We blow the warm air of cabin over the evaporator coils by fan, and thus doing a forced convection.
6. The heat transferred to the Freon makes it to change the phase which is from liquid to vapor.
7. The less pressure Freon vapor is then compressed by the Compressor and thus it delivers high temperature high pressure Freon vapor.
8. Now this high pressure and high temperature Freon vapor enters the Condenser coils where the cool air from atmosphere will be blown over the coils (here too making a forced convection). Condenser will be exposed to the Atmosphere. Because of heat transfer the Freon losses heat and returns to liquid phase.
9. Then it goes to the receiver (high pressure low temperature Freon liquid)
10. The cycle continues as stated. Before doing any type of maintenance activities to the vapor cycle system, we have to purge the system with inert gas in a open atmosphere.
11. Freon is colorless, odorless, and non toxic; however, being heavier than air, it will displace oxygen and cause suffocation. When heated over an open flame, it converts to phosgene which is deadly!
12. To know the Freon level in the circuit a sight glass arrangement will be employed between Receiver to Expansion valve. If the unit requires additional refrigerant, bubbles will be present in the sight glass otherwise steady.
When using this method air conditioning, pressurization have to be done separately, either by using Cabin superchargers or Engine driven compressor or by using your future design that delivers pressure with least energy taken as input.
FIRE PROTECTION SYSTEM
Because fire is one of the most dangerous threats to an aircraft, the potential fire zones of modern multi-engine aircraft are protected by a fixed fire protection system. A ‘fire zone’ is an area of region of an aircraft designed by the manufacturer to require fire detection and or fire extinguishing equipment and a high degree of inherent fire resistance. The term ‘fixed’ describes a permanently installed system in contrast to any type of portable fire extinguishing equipment, such as hand-held Co2 fire extinguisher.
Fire suppression system includes
I. Fire detection system
II. Smoke detection system
III. Fire extinguishing system
Fire detection system should signal the presence of fire. Units of the system are installed in locations where there are greatest possibilities of a fire. The common fire detection systems are
v Thermal Switch
v Continuous loop Systems
Thermal switches are heat –sensitive units that complete electrical circuits at a certain temperature. They are connected in parallel with each other. If the temperature rises above a set value in any one section of the circuit, the thermal switch will close the electrical circuit and gives indication to the pilot through indication lights.
A thermocouple gives the rate of temperature rise.
A Thermocouple is the junction of two dissimilar metals which generates a small electric current that varies according to the temperature of the junction. For this reason it does not require an external power source. The dissimilar metals can be constantan and iron, Alumel and Chromel, or some other combination of metals or alloys which will produce the required results. The complete thermocouple circuit consists of the ‘cold’ junction, the ‘hot’ junction, electric leads (made from the same material as the thermocouple), and a galvanometer-type indicating instrument as illustrated below.
Constantan is chosen as one of the metals because its resistance is affected very little by changes in temperature. When the hot junction is at higher or lower temperature than the cold junction, a current will flow through the circuit and instrument. The value of current is depending on the difference in temperature between the two junctions.
Continuous loop systems:
A continuous loop detector or sensing system permits more complete coverage of a fire hazard area than any type of detectors. There is no rate of heat rise sensitivity in a continuous loop system.
In the kidde continuous loop system, two wires are imbedded in a special ceramic core within a inconel tube. One of the two wires in the kidde sensing system is welded to the case at each end and acts as an internal ground. The second wire is a hot lead (above ground potential) that provides a current signal when ceramic core material changes its resistance with change in temperature. If any fire is near this instrument causes the ceramic core resistance to drop and thereby wire will conduct more current than usual, from that we can find the fire.
It uses a single wire surrounded by a continuous string of ceramic beads in an inconel tube. The beads in the Fenwal detector are wetted with a eutectic salt which posses the characteristics of suddenly lowering its electrical resistance as the sensing element reaches its alarm temperature.
If the volume of a gas is held constant, its pressure increases as temperature increases; thus the helium between the two tubing walls exerts a pressure proportional to the average absolute temperature along the entire length of the tube. One end of the tube is connected to a small chamber containing a metal diaphragm switch. One side of the diaphragm is therefore exposed to the sensor tube pressure and one to the ambient pressure. When system pressure exceeds some predetermined value, diaphragm will activate the electrical contacts which in-turn activates the alarm. When the temperature is reduced to the normal levels after action has been taken to suppress the fire, the gas absorption material reabsorbs the gas and pressure drops, thus opening the switch and cutting off the alarm.
Smoke detection System
Pure air just contains 78% of nitrogen, 21% of oxygen and 1% rest of other gases. When there is fire prevailing in the aircraft the air will get dusted because of reaction between Carbon and oxygen thus forms CO, CO2 and other gases. In order to eliminate the fire, smoke has to be identified first. It can be done by
a) Light detection
b) Light Refraction
d) Solid state semiconductor
Light detection type:
We all know photo voltaic cell converts light energy into electrical energy. The pure light rays will be fall on photo voltaic panel and produces some electrical energy at normal. When the area is surrounded by smoke the intensity of light rays falling on the panel gets differ. From that we gets alarm.
Light Refraction type smoke detector:
Here a controlled mass of air is taken for smoke detection by use of fan or whatever blowing device. Source of light is the Electric bulb. It is made to pass it rays from a single point in order to satisfy our requirement. The light which fall on the wall of the detection chamber gets reflected and the reflected rays which falls on photo voltaic panel induces some current based on the intensity of rays fallen. When smoke particles comes into picture, they itself reflect the rays before colliding by the wall and makes different electrical energy by making different intensity, thus alarm gets activated.
Ionization type smoke detector:
The Radioactive material (Americium) emits Alpha rays continuously. The Alpha rays makes reaction with the clean cabin air (under normal conditions) and ionize the incoming air ingredients to Positive and Negative charges. Those Positive charged ions are attracted by negative charged pole, and Negative charged ions are attracted by positive charged poles. Thus it gives some reading. When smoked air comes into picture the amount of attraction on poles varies and thus we are getting smoke detection alarm activated.
Solid State Smoke Detector:
A solid filament coated with semi-conductor material is along the length of the fuselage to detect the fire. An electrical current is given over the filament by the wrap around coils. At normal conditions the filament will have some resistance and it produces some output value. When smoke comes into picture, the semi conductor has ability to absorb carbon particles, so that resistance of the filament varies, thus getting different output from that of we got on normal.
If suppose the atmospheric air itself getting too much carbon particles, then we may commit a false smoke warning, so that in order to activate alarm we are just comparing the filament exposed to atmospheric air and cabin.
FIRE EXTINGUISHING SYSTEM
After finding out the fired place or region we have to use suitable extinguishing agent in order to maintain safety. Types of fire and the suitable extinguishing agent filled extinguisher are figure below.
For Aircraft fire extinguishing operation, we are having extinguishing agent stored at an arrangement called Fire Bottle. It will be routed to the region in need of extinguishing by suitable hoses and Sprayed through Nozzles. The Common shapes of fire bottle are Cylindrical and Spherical.
If the aircraft gets fire in the ground, we may use ordinary Fire extinguishers. But make sure that the fire extinguisher type which you use should be specified as per the figure shown at first. For example if you use water filled extinguisher for a fire caused by electrical sources, then it is useless. After selection of specified extinguisher spray the extinguishing agent in the base of the fire by opening the Safety pin and by pressing the trigger.
Anti-icing equipment is designed to prevent the formation of ice, while deicing equipment is designed to remove ice once it has formed. These systems protect the leading edge of wing and tail surfaces, pitot and static port openings, fuel tank vents, stall warning devices, windshields, and propeller blades. Ice detection lighting may also be installed on some aircraft to determine the extent of structural icing during night flights.
Most light aircraft have only a heated pitot tube and are not certified for flight in icing. These light aircraft have limited cross-country capability in the cooler climates during late fall, winter, and early spring. Noncertificated aircraft must exit icing conditions immediately. Refer to the AFM/POH for details.
Airfoil Anti-Ice and Deice
Inflatable deicing boots consist of a rubber sheet bonded to the leading edge of the airfoil. When ice builds up on the leading edge, an engine-driven pneumatic pump inflates the rubber boots. Many turboprop aircraft divert engine bleed air to the wing to inflate the rubber boots. Upon inflation, the ice is cracked and should fall off the leading edge of the wing. Deicing boots are controlled from the flight deck by a switch and can be operated in a single cycle or allowed to cycle at automatic, timed intervals. [Figure 6-48]
Figure 6-48. Deicing boots on the leading edge of the wing.
In the past it was believed that if the boots were cycled too soon after encountering ice, the ice layer would expand instead of breaking off, resulting in a condition referred to as ice “bridging.” Consequently, subsequent deice boot cycles would be ineffective at removing the ice buildup. Although some residual ice may remain after a boot cycle, “bridging” does not occur with any modern boots. Pilots can cycle the boots as soon as an ice accumulation is observed. Consult the AFM/POH for information on the operation of deice boots on an aircraft.
Many deicing boot systems use the instrument system suction gauge and a pneumatic pressure gauge to indicate proper boot operation. These gauges have range markings that indicate the operating limits for boot operation. Some systems may also incorporate an annunciator light to indicate proper boot operation.
Proper maintenance and care of deicing boots are important for continued operation of this system. They need to be carefully inspected during preflight.
Another type of leading edge protection is the thermal anti-ice system. Heat provides one of the most effective methods for preventing ice accumulation on an airfoil. High performance turbine aircraft often direct hot air from the compressor section of the engine to the leading edge surfaces. The hot air heats the leading edge surfaces sufficiently to prevent the formation of ice. A newer type of thermal anti-ice system referred to as thermawing uses electrically heated graphite foil laminate applied to the leading edge of the wing and horizontal stabilizer. Thermawing systems typically have two zones of heat application. One zone on the leading edge receives continuous heat; the second zone further aft receives heat in cycles to dislodge the ice allowing aerodynamic forces to remove it. Thermal anti-ice systems should be activated prior to entering icing conditions.
An alternate type of leading edge protection that is not as common as thermal anti-ice and deicing boots is known as a weeping wing. The weeping-wing design uses small holes located in the leading edge of the wing to prevent the formation and build-up of ice. An antifreeze solution is pumped to the leading edge and weeps out through the holes. Additionally, the weeping wing is capable of deicing an aircraft. When ice has accumulated on the leading edges, application of the antifreeze solution chemically breaks down the bond between the ice and airframe, allowing aerodynamic forces to remove the ice. [Figure 6-48]
Figure 6-48. TKS weeping wing anti-ice/deicing system.
There are two main types of windscreen anti-ice systems. The first system directs a flow of alcohol to the windscreen. If used early enough, the alcohol will prevent ice from building up on the windscreen. The rate of alcohol flow can be controlled by a dial in the flight deck according to procedures recommended by the aircraft manufacturer.
Another effective method of anti-icing equipment is the electric heating method. Small wires or other conductive material is imbedded in the windscreen. The heater can be turned on by a switch in the flight deck, causing an electrical current to be passed across the shield through the wires to provide sufficient heat to prevent the formation of ice on the windscreen. The heated windscreen should only be used during flight. Do not leave it on during ground operations, as it can overheat and cause damage to the windscreen. Warning: the electrical current can cause compass deviation errors by as much as 40°.
Propellers are protected from icing by the use of alcohol or electrically heated elements. Some propellers are equipped with a discharge nozzle that is pointed toward the root of the blade. Alcohol is discharged from the nozzles, and centrifugal force drives the alcohol down the leading edge of the blade. The boots are also grooved to help direct the flow of alcohol. This prevents ice from forming on the leading edge of the propeller. Propellers can also be fitted with propeller anti-ice boots. The propeller boot is divided into two sections—the inboard and the outboard sections. The boots are imbedded with electrical wires that carry current for heating the propeller. The prop anti-ice system can be monitored for proper operation by monitoring the prop anti-ice ammeter. During the preflight inspection, check the propeller boots for proper operation. If a boot fails to heat one blade, an unequal blade loading can result, and may cause severe propeller vibration. [Figure 6-49]
Figure 6-49. Prop ammeter and anti-ice boots.
Other Anti-Ice and Deice Systems
Pitot and static ports, fuel vents, stall-warning sensors, and other optional equipment may be heated by electrical elements. Operational checks of the electrically heated systems are to be checked in accordance with the AFM /POH.
Operation of aircraft anti-icing and deicing systems should be checked prior to encountering icing conditions. Encounters with structural ice require immediate action. Anti-icing and deicing equipment are not intended to sustain long-term flight in icing conditions.