The A/C Pack exhaust air is ducted into the pressurized fuselage, where it is mixed with filtered air from the recirculation fans, and fed into the “mix manifold”. On nearly all modern jetliners, the airflow is approximately 50% “outside air” and 50% “filtered air.”
Air from the “mix manifold” is directed to overhead distribution nozzles in the various “zones” of the aircraft. Temperature in each zone may be adjusted by adding small amounts of “Trim Air”, which is low-pressure, high temperature air tapped off the A/C Pack upstream of the TCV. Air is also supplied to individual gaspers: small, circular vents above each passenger seat that can be adjusted by passengers for their personal comfort. A revolving control on the vent can be turned to adjust ventilation between no air output at all and a fairly substantial breeze.
Gaspers usually receive their air from the air conditioning packs aboard the aircraft, which in turn receive compressed, clean air from the compressor stages of the aircraft’s jet engines or when on the ground from the APU or a ground source. A master control for gaspers is located in the cockpit, and gaspers may be temporarily turned off during certain phases of flight, when the load on the engines from bleed-air demands must be minimized (e.g. take-off and climb).
Airflow into the fuselage is approximately constant, and pressure is maintained by varying the opening of the “Out Flow Valve” (OFV). Most modern jetliners have a single OFV located near the bottom aft end of the fuselage, although some larger aircraft like the 747 and 777 have two.
In the event the OFV should fail closed, at least two Positive Pressure Relief Valves (PPRV) and at least one Negative Pressure Relief Valve (NPRV) are provided to protect the fuselage from over- and under- pressurization.
Aircraft cabin pressure is commonly pressurized to a “cabin altitude” of 8000 feet or less. That means that the pressure is 10.9 psi (75 kPa), which is the ambient pressure at 8000 feet (2,400 m). Note that a lower cabin altitude is a higher pressure. The cabin pressure is controlled by a “Cabin Pressure Schedule,” which associates each aircraft altitude with a cabin altitude. The new airliners such as the Airbus A380 and Boeing 787 will have lower maximum cabin altitudes which help in passenger fatigue reduction during flights.
The atmosphere at typical jetliner cruising altitudes is generally very dry and cold; the outside air pumped into the cabin on a long flight has the potential to cause condensation which might in turn cause corrosion or electrical faults, and is thus is eliminated. Consequently when humid air at lower altitudes is encountered and drawn in, the ECS dries it through the warming and cooling cycle and the water separator mentioned above, so that even with high external relative humidity, inside the cabin it will usually be not much higher than 10% relative humidity.
Although low cabin humidity has health benefits of preventing the growth of fungus and bacteria, the low humidity causes drying of the skin, eyes and mucosal membranes and contributes to dehydration, leading to fatigue, discomfort and health issues. In one study the majority of flight attendants reported discomfort and health issues from low humidity.In a statement to Congress in 2003 a member of the Committee on Air Quality in Passenger Cabins of Commercial Aircraft said “low relative humidity might cause some temporary discomfort (e.g., drying eyes, nasal passages, and skin), but other possible short- or long-term effects have not been established”.
A cabin humidity control system may be added to the ECS of some aircraft to keep relative humidity from extremely low levels, consistent with the need to prevent condensation.Furthermore the Boeing 787 and Airbus 350, by using more corrosion-resistant composites in their construction, can operate with a cabin relative humidity of 16% on long flights.
Discharge Oxygen System:
1. Push oxygen control knob to ON position.
2. Insert fitting and hose in outlet in cabin and route hose outside cabin.
Purge Oxygen System:
1. Charge the oxygen system.
2. Move aircraft outdoors if possible. If unable to move aircraft outdoors, make sure area is roped off, no smoking or open flame permitted in the area, no grease or lubricant near area, cabin door and foul weather windows open.
3. Plug all masks into outlets and purge system by allowing the oxygen to flow for at least 10 minutes. Smell the oxygen flowing from the outlets and continue to purge until the oxygen is odourless. Refill cylinder as required during and after purging.
2. General Maintenance Precautions.
A. Before any maintenance is performed on the oxygen system personnel should read and thoroughly understand the following. Careful adherence to these instructions will aid in maintaining a trouble-free system.
B. If maintenance is performed on the aircraft oxygen system or on any other system in the aircraft requiring removal of any oxygen system component, strict adherence to the following procedures and precautions is required.