Cabin Pressurization Systems

Pressurization lets a transport climb into efficient thin air while keeping occupants in a comfortable, survivable cabin altitude. Mastery of the pressurization system — and its failure modes — is core ATP knowledge.

Most jets bleed high-pressure air from the engine compressor (bleed air), cool and condition it through the air-conditioning packs, and feed it into the cabin. Cabin pressure is regulated not by how much air comes in, but by how much is allowed out through the outflow valve. A pressurization controller modulates the outflow valve to hold a scheduled cabin altitude and rate of change.

• Cabin altitude — the pressure altitude the occupants actually experience inside (e.g., 8,000 ft when the airplane is at FL390).
• Cabin differential pressure — the difference between cabin pressure and outside ambient pressure, limited by structural design (often around 8–9 psi). The maximum differential sets how low a cabin altitude can be maintained at a given flight level.
• Cabin rate of climb/descent — how fast cabin altitude changes; kept gentle (often a few hundred fpm) for ear comfort.

A typical controller offers auto, standby/manual, and a dump function. Safety devices include positive-pressure relief valves (prevent over-pressurization beyond max differential), negative-pressure relief valves (prevent outside pressure exceeding cabin pressure, e.g., in a rapid descent), and a cabin altitude warning (often a horn around 10,000 ft cabin altitude). Passenger oxygen masks typically deploy automatically near 14,000 ft cabin altitude.

• Explosive decompression occurs faster than the lungs can vent (under ~0.5 sec) — risk of lung damage.
• Rapid decompression is fast but the lungs can keep up.
• Gradual/slow decompression is insidious; it may be detected only by the cabin altitude warning or creeping symptoms of hypoxia.

Any significant decompression demands the memory items: oxygen masks on, crew communication established, and an emergency descent to a safe altitude (typically 10,000 ft or the minimum safe altitude, whichever is higher). The combination of pressurization plus a quick-don mask is what makes high cruise altitudes survivable.