If maximum cabin differential pressure is reached and the aircraft climbs, what happens to cabin differential and cabin altitude?

• A. Cabin differential remains the same; cabin altitude increases
• B. Cabin differential increases; cabin altitude increases
• C. Cabin differential decreases; cabin altitude decreases
• D. Cabin differential remains; cabin altitude remains

Answer: (A)

When the cabin is pressurized, the system maintains a fixed pressure difference between inside and outside, called the cabin differential. It can only go up to a maximum value. If you climb further after hitting that limit, the outside pressure falls, but the system cannot increase the differential beyond its maximum. To keep within the limit, the cabin pressure is reduced along with the outside pressure, so the difference stays at the maximum.

Since cabin pressure decreases as you climb, the cabin’s equivalent altitude (cabin altitude) rises. So the cabin differential stays the same (at its maximum) while the cabin altitude increases.