Pneumatic, Bleed Air and Pressurisation Systems
Bleed air sources and uses
Bleed air is taken from engine compressor stages (low and high pressure ports, with a pressure-regulating and shut-off valve), from the APU, or from a ground cart. It supplies air-conditioning packs, pressurisation, engine and wing anti-icing, hydraulic reservoir pressurisation, and engine starting. Know the protections: bleed leak detection loops, overheat and overpressure shut-off valves, and the fact that a bleed trip on one side may be isolated automatically.
Air conditioning and pressurisation
Air-cycle machines cool bleed air by compressing it, passing it through a heat exchanger and expanding it through a turbine; water separation controls humidity. Cabin pressure is set by the outflow valve, modulated by the cabin pressure controller in response to selected landing field elevation and cruise altitude. Key definitions: cabin altitude, differential pressure, and the maximum differential limited by structural relief valves.
Traps and worked example
Traps: confusing cabin altitude with aircraft altitude; assuming the outflow valve opens to increase pressure (it closes); and forgetting that a single pack can normally maintain pressurisation at reduced altitude capability.
Worked example: at FL350 with a cabin altitude of 8,000 ft and a differential of about 8.5 psi, a question asks what happens if one pack fails. Reasoning: remaining pack continues; cabin altitude may rise slowly; the crew descends to a level the remaining pack can hold. The answer is the drift-down, not an immediate emergency descent unless cabin altitude exceeds warning levels.