Definition
The difference in pressure between two points or two sides of a barrier. In the vertical speed indicator (VSI), it refers to the pressure difference between the air sealed inside the instrument case (which leaks out slowly through a calibrated orifice) and the current static pressure inside the diaphragm. This difference deflects the diaphragm and drives the VSI needle to indicate rate of climb or descent.
Plain English
The gap between two pressures. Whenever the air on one side is at a different pressure than the air on the other side, that gap is the pressure differential.
Context Anchor
Seen in explanations of how the vertical speed indicator senses climb and descent.
Derivation
Differential comes from the Latin differre, meaning to carry apart or set apart. So a pressure differential is simply pressures set apart from each other -- one compared against the other.
Why Pilots Care
It is the signal the VSI converts into an immediate climb or descent rate shown on the instrument face.
Grounding Statement
When the airplane changes altitude, the air pressure around it changes, and the VSI compares two pressures inside the instrument to sense that change.
Intuition Check
Do not read pressure differential as simply “air pressure.” It means the difference between two pressures being compared.
Example Sentence 1
As the aircraft climbs, the pressure differential between the trapped air in the VSI case and the falling static pressure causes the needle to show a positive rate of climb.
Example Sentence 2
During descent the static pressure increases, creating a pressure differential that drives the needle downward.