FAA Aviation Mechanic Airframe (AMA)Fuel, Cabin and Ice Control SystemsMedium
A technician is inspecting the pressurization system of a transport-category aircraft and locates a valve that is separate from the primary cabin pressure outflow valve. This valve opens automatically only if cabin differential pressure exceeds a preset maximum limit, protecting the fuselage from structural damage. This valve is the:
- ANegative pressure relief valve
- BPositive pressure relief (safety) valve
- CBleed air check valve
- DCabin pressure controller
Show answer & explanationAnswer & explanation
Correct answer: B. Positive pressure relief (safety) valve
The positive pressure relief valve (safety valve) is a backup to the outflow valve; it opens automatically if cabin differential pressure exceeds the structural limit, protecting the fuselage from overpressurization even if the outflow valve and its controller fail.
Why the other options are wrong
- A. The negative pressure relief valve protects against cabin pressure being lower than outside, the opposite condition.
- C. A bleed air check valve prevents reverse airflow in the bleed air ducting, unrelated to cabin pressure relief.
- D. The cabin pressure controller schedules the outflow valve but is not itself a relief valve.
Positive Pressure Relief (Safety) Valve
A backup valve independent of the outflow valve that automatically opens to prevent cabin differential pressure from exceeding structural limits.
- Purely mechanical, spring-loaded backup
- Protects fuselage from overpressure
- Separate from primary outflow valve
Memory trick: Safety valve = the fuselage's seatbelt against too much push