13.11 and 13.17 Air conditioning, cabin pressurisation (ATA 21) and oxygen (ATA 35)

Scope

B2 and B2L Airframe Systems rating.

• 13.11(a) Pressurisation (level 3): systems; cabin pressure controllers; control and safety valves; control and indication.
• 13.11(b) Air supply (level 1): engine bleed, APU and ground cart; distribution.
• 13.11(c) Air conditioning (level 3).
• 13.11(d) Safety and warning devices (level 3).
• 13.17 Oxygen (ATA 35) (level 3): cockpit and cabin layout; sources, storage, charging and distribution; supply regulation; indications and warnings.

Why pressurise?

At high altitude the partial pressure of oxygen is too low for normal human performance. A pressurised fuselage keeps the cabin altitude low (typically no higher than 8,000 ft at maximum operating altitude for transport aeroplanes) while the aircraft flies far higher.

Term	Meaning
Cabin altitude	Pressure altitude equivalent to the cabin pressure
Differential pressure (ΔP)	Cabin pressure minus outside pressure — limited by structure (around 8–9 psi on large jets)
Cabin rate of climb/descent	Rate of change of cabin altitude, kept low for comfort (about 500 ft/min up, 300 ft/min down are common design values)
Isobaric	Constant cabin altitude while aircraft altitude changes
Constant differential	Cabin pressure follows outside pressure at max ΔP

Pressurisation control

• Cabin pressure is controlled by regulating the outflow of air, with inflow from the packs roughly constant.
• Outflow valve(s) — motor-driven (electrical, often two motors/channels) or pneumatically controlled; usually at the rear lower fuselage; may provide a small thrust recovery.
• Cabin pressure controller (CPC) — automatic: uses FMS/selected landing-field elevation, aircraft altitude and air-data to schedule cabin altitude and rate. Two automatic channels with automatic transfer, plus manual mode where the crew drives the outflow valve directly.
• Ground function: outflow valve fully open on the ground (no pressurisation); on take-off a small pre-pressurisation avoids a pressure bump at rotation.