021 Airframe, Systems and Powerplant topic guide
Cabin Pressurisation
Cabin pressurisation exists because the human body cannot function reliably at the altitudes a modern aircraft cruises at. Rather than pressurising the cabin to sea-level pressure throughout the flight, the system holds a cabin altitude that is typically kept around 6000 to 8000 ft at cruise, a compromise that keeps occupants comfortable without demanding a fuselage strong enough to withstand sea-level differential pressure at the aircraft's full operating altitude.
The whole system is really a controlled leak. Conditioned air is continuously pumped into the cabin, and the outflow valve decides how much of it is allowed to leave. Everything else, the safety valve, the negative relief valve, the controllers, exists to make that one controlling element safe and to keep working if it does not behave as commanded.
The outflow valve as the controlling element
The outflow valve sits in the fuselage skin and modulates its opening continuously to hold whatever cabin altitude the controller has scheduled for the current flight level. Close the valve slightly and less air escapes, so cabin pressure rises and cabin altitude falls. Open it and the reverse happens. Because it is the only element that is actively driven throughout the flight, almost every pressurisation question is really a question about what the outflow valve is doing and why.
During climb the controller schedules the cabin to climb more slowly than the aircraft, so the valve progressively closes to keep the cabin behind the aircraft's rate of ascent. During descent the logic reverses: the cabin must be back down near ambient pressure before touchdown, so the valve opens to let the cabin descend ahead of the aircraft in places, always subject to the maximum rate the system is designed to apply comfortably to the ears and sinuses of everyone aboard.
Differential pressure and its limits
Differential pressure is simply the difference between cabin pressure and outside ambient pressure, and it is the quantity the fuselage structure is actually designed against, not cabin altitude itself. Every pressurised aircraft has a maximum differential pressure built into its structural limits, and the outflow valve's scheduling exists specifically to keep the aircraft inside that limit at every cruise altitude it is certified for.
Because the limit is structural, the system needs a backstop that does not depend on the same controller that sets the schedule. That backstop is the safety valve.
Safety valves, negative relief, and what failures look like
The safety valve is a separate, purely mechanical relief valve that opens automatically once differential pressure reaches a set value above the normal maximum, venting cabin air overboard regardless of what the primary controller is doing. It is there for the case where the outflow valve fails closed or the controller commands an unsafe schedule, and it protects the structure rather than participating in day-to-day control.
The negative relief valve guards the opposite extreme: it opens if cabin pressure ever falls below outside ambient pressure, for instance during a rapid descent after a high cruise altitude, because a fuselage built to resist being pushed outward is not necessarily built to resist being crushed inward. A failure that leaves the outflow valve stuck open produces a rising cabin altitude and falling differential pressure, a stuck-closed valve produces the opposite, rising differential pressure, and recognising which symptom belongs to which fault is the practical skill the exam is testing.
- Outflow valve: continuously modulated, sets cabin altitude schedule
- Safety valve: relieves an overpressure differential automatically
- Negative relief valve: prevents cabin pressure dropping below ambient
- Stuck-closed outflow valve: differential pressure rises
- Stuck-open outflow valve: cabin altitude rises towards aircraft altitude
Worked example
Worked example: identifying the failed component from its symptom
Not quite. The correct answer is A.
Correct answer: A
- A. Correct. An outflow valve that will not close lets conditioned air escape faster than scheduled, so cabin pressure falls and cabin altitude rises even though nothing else has changed.
- B. A safety valve that cannot open only matters during an overpressure event; a shut safety valve does not cause cabin altitude to rise, it removes a protection that was not needed here.
- C. This valve only opens to admit outside air when cabin pressure is below ambient. A rising cabin altitude in cruise is not the scenario this valve responds to.
- D. A valve stuck closed traps air in the cabin and drives differential pressure up, which lowers cabin altitude, the opposite of the symptom described.
Step by step
- Note the symptom: cabin altitude is rising, meaning cabin pressure is falling relative to what was scheduled.
- Ask which component actively controls how much air leaves the cabin: the outflow valve.
- A valve that has failed open allows more air out than commanded, so pressure falls and cabin altitude rises, matching the symptom.
- Check the alternatives against the same symptom: both relief valves are backstops for different failure directions and a closed outflow valve would produce the opposite trend.
Common mistakes
Treating the safety valve as the normal control element
The safety valve only relieves an overpressure condition and takes no part in the continuous scheduling of cabin altitude. Answering a normal-operations question with the safety valve's function scores zero even if the arithmetic reasoning elsewhere is sound.
Mixing up which failure direction each relief valve protects against
The safety valve and the negative relief valve protect opposite extremes. Assigning an overpressure scenario to the negative relief valve, or vice versa, is a guaranteed wrong answer regardless of how well the rest of the question is understood.
Assuming cabin altitude and aircraft altitude must move together
The entire purpose of the system is to decouple the two during climb and descent. A question describing the cabin lagging or leading the aircraft's altitude change is describing normal, correct operation, not a fault.
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Last reviewed July 2026