Definition
A numerical value that expresses how much grip exists between a tire and the runway surface. A higher friction coefficient means more grip and shorter stopping distance; a lower value means less grip and longer stopping distance. Runway condition reports use this value to describe braking effectiveness, especially when the surface is wet, contaminated, or affected by hydroplaning.
Plain English
A number that tells you how grippy or slippery the runway is. Higher number, better grip. Lower number, more slippery.
Context Anchor
Seen in runway braking, hydroplaning, and wet-runway landing discussions, especially when describing how water can reduce tire grip.
Derivation
From Latin frictio meaning 'rubbing,' and coefficient, meaning a number that measures a property. Together: a number that measures how much rubbing resistance exists between two surfaces.
Why Pilots Care
A low friction coefficient on wet runways can cause loss of braking effectiveness and directional control, leading to hydroplaning and runway excursions.
Grounding Statement
On a smooth wet runway, a thin film of water can keep the tire from gripping firmly, so the friction coefficient drops.
Intuition Check
Do not think of friction coefficient as the total braking power of the airplane. It is the grip available at the tire-runway contact surface.
Example Sentence 1
After the rain, the tower reported a reduced friction coefficient on Runway 27, so the crew planned for a longer landing roll.
Example Sentence 2
When the friction coefficient drops below 0.3 due to standing water, the risk of viscous hydroplaning increases sharply.