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
- 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.
- Differential pressure
- ΔP = cabin pressure − ambient pressure
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.
Cabin pressurisation & differential pressure
Air con & pressurisation concept map
Air Conditioning & Pressurisation
Air conditioning & pressurisation quiz
Air Conditioning & Pressurisation, quiz
1. In the graph shown the cabin altitude climbs far more slowly than the aircraft altitude. The gap marked as the difference between the two is the:
Cabin differential pressureCabin rate of climbAircraft true altitude2. The main source of conditioned air on a jet is:
Engine bleed airThe batteryThe fuel tanks3. An air-cycle machine cools the air mainly by:
Expanding it across a turbine (doing work)Adding a chemical refrigerantBurning fuel4. Cabin pressure is controlled primarily by the:
Outflow valve (air leaving the cabin)Inflow from the packs aloneLanding gear5. Cabin differential pressure is the difference between:
Cabin pressure and ambient pressureTwo engine bleedsFuel and oil pressure6. A typical maximum cabin altitude at cruise is about:
8000 ft25 000 ftSea level always7. Water is removed from pack air to:
Prevent fogging and icing in the system/cabinAdd weightIncrease humidity8. A positive-pressure relief valve protects against:
Excess cabin differential pressureLoss of fuelLow oil pressure