If cabin pressure is higher than the atmosphere, what is wrong?

If cabin pressure is higher than the atmosphere, what is wrong?

• A. The safety valve isn't opening, allowing air to escape.
• B. The outflow valve is stuck.
• C. The cabin pressure controller is failing.
• D. The air intake is blocked.

Answer: (A)

Cabin pressurization relies on balancing air in with air vented out. The main way to control pressure is through the outflow valve, which lets conditioned air escape to hold the cabin at the desired level. There’s also a safety relief valve that opens automatically if the differential pressure gets too high to protect the structure. If the cabin pressure is higher than the outside, the system isn’t venting enough air. The primary protection for this situation is the safety relief valve. When it fails to open, there’s no extra path for excess cabin air to escape, so the cabin pressure climbs relative to the outside. That’s why this scenario points to the safety valve not opening as the likely problem. Blocking the air intake wouldn’t raise cabin pressure; it would more likely reduce cabin pressure by limiting the incoming air. A stuck outflow valve could cause high pressure as well, but the safety valve failing to open is the direct overpressure protection failure that explains the observed condition.

Cabin pressurization relies on balancing air in with air vented out. The main way to control pressure is through the outflow valve, which lets conditioned air escape to hold the cabin at the desired level. There’s also a safety relief valve that opens automatically if the differential pressure gets too high to protect the structure.

If the cabin pressure is higher than the outside, the system isn’t venting enough air. The primary protection for this situation is the safety relief valve. When it fails to open, there’s no extra path for excess cabin air to escape, so the cabin pressure climbs relative to the outside. That’s why this scenario points to the safety valve not opening as the likely problem.

Blocking the air intake wouldn’t raise cabin pressure; it would more likely reduce cabin pressure by limiting the incoming air. A stuck outflow valve could cause high pressure as well, but the safety valve failing to open is the direct overpressure protection failure that explains the observed condition.