Air Conditioning & Cabin Pressurisation

Air supply, the air-cycle machine, pressurisation control by the outflow valve, and safety/warning devices.

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Main ideas

Conditioned air normally comes from engine BLEED AIR, supplemented by the APU or a ground cart; it is cooled and temperature-controlled before distribution to the cabin and flight deck.
Most jets cool with an AIR-CYCLE MACHINE (the 'pack'): bleed air is compressed, cooled in heat exchangers and then EXPANDED through a turbine, which 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; the cabin-to-outside pressure difference is the differential pressure, structurally limited.
Cabin pressure is set by the OUTFLOW VALVE, which modulates air LEAVING the cabin (inflow being roughly constant); positive- and negative-pressure relief valves and a cabin-altitude warning protect the structure and occupants.
Temperature is trimmed per zone by mixing hot trim (bleed) air with cold pack air.
⚠ Exam trap: cabin pressure is controlled by the OUTFLOW valve regulating air LEAVING the cabin (not the inflow); the air-cycle machine cools mainly by EXPANDING air across a turbine.

Key formulas

Solved examples

What controls the cabin pressure, and what happens to it as the aircraft climbs?
The outflow valve controls cabin pressure by regulating how much air escapes the cabin, while the packs deliver a roughly constant inflow. As the aircraft climbs, the pressure controller progressively closes the outflow valve so less air escapes, maintaining the scheduled cabin altitude; relief valves cap the maximum differential pressure to protect the fuselage.

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