Definition
A unit of pressure expressing the force, in pounds, applied to one square inch of surface area. In aviation it is used to specify tire inflation pressure, hydraulic system pressure, fuel pressure, oil pressure, and pneumatic system pressure.
Plain English
A way of measuring how hard something is pushing on a small patch of surface. One PSI means one pound of push spread over a square inch.
Context Anchor
Seen in aircraft tire-pressure checks and wet-runway discussions, especially when estimating the speed at which a tire may start riding on water instead of the runway.
Derivation
Built from three plain English words: pounds (the unit of force), per (for each), and square inch (the unit of area). The phrase says exactly what the measurement is — pounds of force on each square inch of surface.
Why Pilots Care
Tire pressure measured in PSI directly affects the speed at which dynamic hydroplaning can begin and the overall handling of the aircraft on the ground.
Intuition Check
PSI is not the total weight on the tire. It is the amount of pressure applied to each square inch.
Example Sentence 1
The pilot checked the main tires before flight and confirmed each was inflated to 42 PSI as specified in the POH.
Example Sentence 2
Dynamic hydroplaning becomes more likely when tire pressure in PSI is below the value recommended in the aircraft manual.