Definition
An operating mode of an aircraft cabin pressurization system in which the pressure controller maintains a fixed pressure difference between the cabin interior and the outside air, regardless of how high the aircraft climbs. As the airplane climbs and outside pressure drops, cabin pressure is allowed to drop with it, but only enough to keep the inside-to-outside difference constant.
Plain English
The system holds the cabin pressure a set amount higher than the air pressure outside the airplane. As the airplane climbs, the cabin pressure drops too, but the gap between inside and outside stays the same.
Context Anchor
Seen in cabin pressurization system descriptions, especially when explaining how the system controls cabin pressure at higher aircraft altitudes.
Derivation
‘Differential’ comes from the Latin differentia, meaning ‘a difference.’ In this context the differential is the difference between cabin pressure and outside (ambient) pressure. ‘Constant’ simply means the system keeps that difference fixed.
Why Pilots Care
This mode protects passengers and crew from hypoxia while preventing over-pressurization damage to the airframe at typical jet cruising altitudes.
Grounding Statement
Picture the aircraft climbing into thinner air while the system works to keep the pressure gap between inside and outside from getting larger.
Intuition Check
Do not read “constant” as meaning the cabin pressure stays fixed. In this term, “constant” means the pressure difference between cabin air and outside air stays fixed.
Example Sentence 1
Once the airplane climbed above the isobaric range, the pressurization system switched to constant differential mode to protect the fuselage.
Example Sentence 2
Maintenance verified that the outflow valve modulated correctly in constant differential mode during the climb test.