Definition
A pressurization system operating mode in which the cabin pressure is held at a fixed amount higher than the outside (ambient) air pressure, regardless of the aircraft's altitude. As the aircraft climbs and outside pressure drops, cabin pressure drops by the same amount, keeping the difference between inside and outside constant.
Plain English
The cabin is always kept at a set amount more pressurized than the air outside. If the outside pressure falls, the cabin pressure falls by the same amount, so the gap between them stays the same.
Context Anchor
Seen in aircraft pressurization system descriptions, especially for high-altitude aircraft.
Derivation
‘Differential’ comes from Latin differentia, meaning ‘difference.’ Here it refers to the pressure difference between the cabin and the outside air. ‘Constant’ means that difference is held steady rather than allowed to vary.
Why Pilots Care
Prevents ear discomfort and structural stress by holding a safe, unchanging pressure difference during cruise.
Analogy
Think of keeping two cars a fixed distance apart on a road. Both cars may move, but the gap between them stays the same. In constant differential mode, the cabin pressure and outside pressure may both change, but the pressure gap stays fixed.
Grounding Statement
As the airplane climbs and outside pressure drops, the system controls cabin pressure so the set pressure gap is not exceeded.
Intuition Check
Do not read “constant” as meaning the cabin pressure never changes. Here, “constant” means the pressure difference between inside and outside stays fixed.
Example Sentence 1
In constant differential mode, the cabin pressure dropped steadily as the aircraft climbed, keeping a fixed pressure difference across the fuselage.
Example Sentence 2
The technician tested the outflow valve in constant differential mode to confirm the scheduled pressure schedule was followed.