FAA Commercial Pilot Airplane (CAX)Aircraft SystemsMedium
An airplane's pressurization controller operates in an 'isobaric' control mode during a climb below the maximum differential pressure limit. What does this mean for the cabin environment?
- ACabin altitude is maintained at a constant value while the aircraft continues to climb
- BThe outflow valve remains fully closed for the entire climb
- CCabin rate of climb is always equal to the aircraft's rate of climb
- DCabin differential pressure is held constant regardless of aircraft altitude
Show answer & explanationAnswer & explanation
Correct answer: A. Cabin altitude is maintained at a constant value while the aircraft continues to climb
In isobaric mode, the controller schedules the outflow valve to hold the cabin at a selected constant altitude (constant absolute pressure) while the aircraft climbs, until the differential pressure limit is reached; after that, the system switches to differential mode. This keeps cabin altitude steady rather than cabin differential.
Why the other options are wrong
- B. The outflow valve modulates continuously; it isn't simply closed throughout.
- C. Cabin rate of climb is deliberately much slower than aircraft rate of climb.
- D. That describes differential mode, which takes over after isobaric mode's limit is reached.
Isobaric Pressurization Mode
A pressurization control mode that maintains a constant cabin altitude (pressure) as the aircraft climbs, until the maximum differential limit is reached.
- Named for constant absolute pressure ('iso-bar')
- Switches to differential mode once max ΔP is reached
- Outflow valve modulates to hold selected cabin altitude
Memory trick: Isobaric = one bar held steady, no matter how high you climb.