Definition
The rapid acceleration of an airplane's wheels from zero rotation to ground speed at the instant the tires make contact with the runway during landing. Until spin-up is complete, the tires are skidding rather than rolling, and braking and directional control through the wheels is limited.
Plain English
The brief moment at touchdown when the wheels go from not turning at all to turning at the same speed the airplane is moving over the ground.
Context Anchor
Encountered during landing discussions, especially in touchdown and rollout, where the tires first meet the runway.
Derivation
Plain English: 'spin-up' describes the wheel rapidly spinning up to speed. The term is borrowed from general mechanical usage where any rotating part 'spins up' from rest to operating speed.
Why Pilots Care
Recognizing wheel spin-up helps pilots anticipate tire stress, hydroplaning risk on wet runways, and the need for smooth touchdown technique to protect the gear and maintain directional control.
Intuition Check
Wheel spin-up does not mean the airplane is powering the wheels like a car. Airplane wheels are normally free-rolling; the runway surface makes them start turning at touchdown.
Example Sentence 1
After touchdown, the pilot waited for wheel spin-up before applying the brakes to slow the airplane on the rollout.
Example Sentence 2
On a wet runway, delayed wheel spin-up can signal potential hydroplaning if the tires are not firmly contacting the surface.