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Air Conditioning & Cabin Pressurisation
Air supply, the air-cycle machine, pressurisation control by the outflow valve, and safety/warning devices.
Air conditioning & pressurisation — summary notes
FreeMain ideas
- Conditioned air normally comes from engine BLEED AIR (hot, high-pressure), supplemented by the APU or a ground cart; it is cooled and its temperature controlled before being distributed to the cabin and flight deck.
- Most jets cool with an AIR-CYCLE MACHINE (the 'pack'): bleed air is pre-cooled, compressed, cooled again in heat exchangers, then EXPANDED through a turbine — the expansion does work and drops the temperature sharply; water is removed to prevent fogging/icing.
- PRESSURISATION keeps the cabin altitude low (typically ≤8000 ft) despite high aircraft altitude, so occupants stay comfortable and conscious; the difference between cabin and outside pressure is the cabin differential pressure, structurally limited.
- Cabin pressure is controlled by the OUTFLOW VALVE, which modulates how much air LEAVES the cabin (inflow from the packs is roughly constant); positive-pressure relief, negative-pressure relief and a cabin-altitude warning protect the structure and occupants.
- Temperature is trimmed by mixing hot bleed (trim) air with cold pack air per zone.
- ⚠ Exam trap: cabin pressure is set by the OUTFLOW valve controlling air LEAVING the cabin, not by varying the inflow; the air-cycle machine cools mainly by EXPANDING air across a turbine (doing work), not by a refrigerant.
Key formulas
- Differential pressure
- ΔP = cabin pressure − ambient pressure
Solved examples
As an aircraft climbs at constant cabin altitude, how does the pressurisation system keep the cabin pressure steady while the packs deliver roughly constant airflow?
The cabin controller gradually CLOSES the outflow valve as the aircraft climbs, letting less air escape so the roughly constant pack inflow maintains the target cabin pressure/altitude. The outflow valve is the primary control element; relief valves cap the maximum differential pressure to protect the fuselage.
Air con & pressurisation concept map
FreeAir Conditioning & Pressurisation
Air conditioning & pressurisation quiz
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