What cabin differential should be observed before shutdown?

• A. Approximately 0 PSID
• B. Approximately 0.7 PSID
• C. Approximately 6.0 PSID
• D. Approximately -0.25 PSID

Answer: (A)

The key idea is understanding what the cabin differential pressure represents: the difference between the cabin pressure and the outside air pressure. In flight, the cabin is pressurized to a positive differential, so the hull is experiencing a load. On the ground, with engines off and the system depressurizing, you expect the cabin to be at ambient pressure, so the differential should be essentially zero.

Therefore, observing approximately zero PSID before shutdown shows that the cabin is at ambient pressure and no residual pressurization is present, which is the normal state on the ground and a safe condition to shut down the systems. Values indicating any positive differential would mean the cabin is still pressurized on the ground, which isn’t expected, and a negative differential isn’t typical for this procedure.