Air conditioning, pressurisation and oxygen systems
Scope
This domain covers bleed-air air conditioning, cabin pressurisation and crew/passenger oxygen. Questions target pack operation, outflow-valve control, differential pressure limits and oxygen delivery modes.
Air conditioning
- Bleed air from the engine compressor, APU or ground cart passes a precooler and pack. The air-cycle machine turbine expands and cools it; a water separator removes moisture. A temperature control valve and trim air set zone temperature.
- On the ground, packs may use APU bleed or a ground cart; engine bleed may be limited to avoid compressor surge.
Pressurisation
- Cabin altitude is controlled by the outflow valve; the controller schedules cabin altitude and rate. Maximum differential pressure is typically about 8-9 psi, protected by positive-pressure relief valves; negative-pressure relief prevents inward damage.
- Cabin altitude warning is at 10,000 ft; passenger masks deploy automatically at about 14,000 ft.
- The cabin climbs at a controlled rate; descent rate is also limited to protect the structure and ears.
Oxygen
- Crew use 100% pressure-demand masks; passengers use diluter-demand masks that mix cabin air with oxygen.
- Chemical generators (sodium chlorate) supply about 15-20 min; gaseous cylinders are typically charged to 1,800-2,000 psi; LOX converters are used on some types.
- Preflight checks: mask, supply pressure, emergency selector and regulator function.
Worked example
At FL350 ambient pressure is about 3.5 psi. With the cabin at 8,000 ft (10.9 psi), differential is 7.4 psi, within an 8.6 psi limit. If the outflow valve jammed closed in a climb, differential would rise until the positive-pressure relief valve opened.
Traps
Cabin altitude is not aircraft altitude. The outflow valve is not the safety valve. Diluter-demand masks do not deliver 100% at all times. Chemical generators cannot be switched off once started.