Definition
Vₚ is the minimum groundspeed at which dynamic hydroplaning will begin on a wet runway, calculated as approximately nine times the square root of the tire pressure (in pounds per square inch) for a rotating tire. Above this speed, a wedge of water can lift the tire off the runway surface, causing loss of braking and directional control.
Plain English
Vₚ is the speed at which a tire on a wet runway can stop touching the pavement and start riding on top of the water instead. Once the airplane reaches that speed, the wheels can no longer grip the surface to brake or steer.
Context Anchor
Seen in discussions of wet-runway landings and dynamic hydroplaning speed calculations.
Derivation
The 'V' follows the standard aviation convention for velocity (speed). The 'p' stands for hydroplaning. Aviation uses 'V-speeds' (V₁, Vᵣ, V₂, etc.) as a shorthand for critical speeds, so Vₚ fits the same family.
Why Pilots Care
Knowing this speed helps a pilot anticipate when hydroplaning may begin and adjust landing or takeoff decisions accordingly to maintain directional control.
Grounding Statement
Picture the tire moving fast enough that water builds up under it and lifts it slightly off the runway surface.
Intuition Check
Do not treat V‚š as an airspeed target or a guaranteed exact number. Here it is an estimate of tire speed over a wet runway where hydroplaning may begin.
Example Sentence 1
With main tire pressure at 36 psi, the pilot calculated a Vₚ of about 54 knots and decided to delay landing until the heavy rain passed.
Example Sentence 2
The pilot reduced landing speed well below the published V for the tire pressure to keep the wheels firmly on the wet runway.