FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsHard
A transport aircraft is certificated with a maximum cabin pressure differential of 8.60 psi. While cruising at FL410, ambient pressure is approximately 2.18 psi and the cabin is maintained at an 8,000-foot cabin altitude. If the outflow valve malfunctioned closed and the cabin pressure rose to sea-level pressure (14.70 psi) while the airplane remained at FL410, the resulting cabin differential pressure would be approximately:
- A14.7 psi, exceeding the limit by the full sea-level pressure value
- B12.5 psi, which exceeds the structural limit and risks structural damage
- C6.5 psi, which remains within the structural limit
- D8.6 psi, exactly equal to the structural limit
Show answer & explanationAnswer & explanation
Correct answer: B. 12.5 psi, which exceeds the structural limit and risks structural damage
Differential pressure equals cabin pressure minus ambient pressure. With cabin at sea-level pressure (14.70 psi) and ambient at FL410 (≈2.18 psi), differential = 14.70 − 2.18 ≈ 12.52 psi. This exceeds the airplane's 8.60 psi structural limit, which is why pressurization systems and safety/relief valves are designed to prevent the cabin from ever reaching sea-level pressure at such high altitudes.
Why the other options are wrong
- A. 14.7 psi would only be correct if ambient pressure were zero, which is not the case at FL410.
- C. 6.5 psi understates the actual differential produced by this malfunction.
- D. 8.6 psi is the structural limit, not the value that would actually occur in this scenario.
Cabin Pressure Differential Limit
The maximum allowable difference between cabin pressure and outside ambient pressure, set by fuselage structural strength; exceeding it risks structural damage.
- Differential = cabin pressure − ambient pressure
- Typical transport limit is around 8.0–9.0 psi
- Safety/relief valves prevent exceeding structural limits
- Ambient pressure decreases sharply with altitude
Memory trick: Squeeze too hard on a soda can (fuselage) and it bursts