Definition
The minimum groundspeed at which a tire, rolling on a wet runway, lifts off the surface and rides on a thin film of water rather than maintaining contact with the pavement. For dynamic hydroplaning, this speed is approximately nine times the square root of the tire pressure in psi (e.g., a tire inflated to 36 psi will hydroplane at about 54 knots).
Plain English
The speed at which a tire stops touching the runway and starts skimming on top of standing water, like a water ski. Once you reach that speed on a wet runway, braking and steering become very poor.
Context Anchor
Seen in wet-runway landing and takeoff discussions, especially when learning why braking and steering may be poor on a runway with standing water.
Derivation
From 'hydro-' (Greek hydor, water) and 'planing' (to glide on a surface, as a boat does when it rises and skims across water). The term was borrowed from boating: a planing hull rides on top of the water rather than pushing through it. A hydroplaning tire does the same thing.
Why Pilots Care
Once hydroplaning begins, wheel brakes, nosewheel steering, and antiskid systems lose effectiveness, greatly increasing the risk of a runway overrun or loss of directional control.
Analogy
It is similar to a water ski: below a certain speed it sinks into the water, but at higher speed it can skim across the top. A tire can do something similar on standing water.
Grounding Statement
On a fast landing into standing water, the tires may not fully touch the pavement at first, so the airplane may not slow or steer the way the pilot expects.
Intuition Check
Hydroplaning speed is not a guaranteed on/off number. It is an estimate of when hydroplaning can begin; runway water depth, tire condition, and aircraft weight can affect what actually happens.
Example Sentence 1
With main tire pressure at 49 psi, the calculated hydroplaning speed is about 63 knots, so the pilot planned a firm touchdown well below that figure.
Example Sentence 2
Landing at a speed above hydroplaning speed on standing water can cause the aircraft to slide without braking or steering response.