Module 11A.4 — Air conditioning and cabin pressurisation (ATA 21)

Levels

For turbine aeroplanes, 11.4 (a)–(d) — pressurisation, air supply, air conditioning, safety and warning devices — are level 1 for A1 and level 3 for B1.1 (detailed knowledge, able to diagnose). One of the largest topic weightings in Module 11.

Why pressurise?

At altitude the partial pressure of oxygen is too low for normal physiology (hypoxia). Pressurising the cabin keeps the cabin altitude low enough for comfort and safety while the aircraft flies high for efficiency.

Parameter	Typical value / rule
Maximum normal cabin altitude (certification)	8 000 ft at maximum operating altitude (≈ 10.9 psi absolute)
Cabin altitude warning	About 10 000 ft (aural and visual)
Passenger oxygen masks auto-deploy	About 14 000 ft cabin altitude (typically 13 500–14 000 ft)
Maximum differential pressure (jets)	About 8–9 psi (e.g. 8.6 psi; newer composite fuselages higher)
Comfortable cabin rate of change	Climb about 500 ft/min, descent about 300 ft/min
Differential pressure	Cabin pressure − ambient pressure (psid)

11.4(b) Air supply

• Engine bleed air from intermediate/high-pressure compressor stages: low-pressure port used normally; HP valve opens at low engine power. Bleed passes a pre-cooler (cooled by fan air) and a pressure-regulating and shut-off valve (PRSOV) to the pneumatic manifold (about 40–45 psi).
• APU bleed air — on the ground and as backup in flight (altitude limited).
• Ground conditioned air cart (low-pressure connector direct to the mix manifold) or high-pressure ground cart (into the pneumatic manifold to run packs or start engines).
• Electric compressors (bleedless designs, e.g. B787) take ram air and compress it electrically.
• Distribution: mix manifold combines pack air and filtered recirculated cabin air (about 50/50 on many jets; recirculation fans with HEPA filters), then ducts to flight-deck and cabin zones; trim air (hot bleed air) added per zone to fine-tune temperature; gasper and individual outlets; avionics cooling (extract/blower fans).

Key fact: Bleed air for air conditioning is pre-cooled by fan air in the pre-cooler; zone temperature is fine-tuned by adding hot trim air downstream of the packs.