Definition
A numerical value that expresses the ratio of the force resisting motion between two surfaces in contact to the force pressing those surfaces together. It indicates how readily one surface will slide against another, with lower values representing slicker contact and higher values representing more grip.
Plain English
A number that tells you how slippery or grippy two surfaces are when they touch. A low number means they slide easily; a high number means they resist sliding.
Context Anchor
Seen in runway braking, runway surface condition, tire grip, and aircraft stopping-distance discussions.
Derivation
From Latin coefficiens, meaning 'working together,' combined with friction, from Latin fricare, 'to rub.' The term literally describes a working number that represents the rubbing resistance between surfaces.
Why Pilots Care
Directly affects calculated landing distance on wet, snow-covered, or slippery runways and influences go/no-go decisions.
Analogy
Shoes on dry pavement have more grip than shoes on ice. The coefficient of friction is the number that describes that difference in grip.
Intuition Check
Do not read “coefficient of friction” as the amount of brake pressure being applied. It is a number describing the available grip between two surfaces.
Example Sentence 1
The pilot reviewed the runway condition report and noted the low coefficient of friction caused by the recent rain before calculating landing distance.
Example Sentence 2
Maintenance used a friction tester to measure the coefficient of friction after the rain and updated the ATIS.