Definition
The lowest measured friction value on a runway surface, expressed as a friction coefficient (Mu), at or below which the runway is considered slippery and corrective action by the airport operator is required. Reported friction values below the minimum friction level indicate degraded braking performance and are issued to pilots through NOTAMs or ATC.
Plain English
A set threshold for how much grip a runway has. If the runway's measured grip falls to or below this number, it is officially treated as slippery, and pilots are warned.
Context Anchor
You may see this term in airport pavement condition, runway maintenance, and wet-runway braking discussions.
Derivation
‘Friction’ comes from the Latin ‘fricare,’ meaning ‘to rub.’ Here it refers to the rubbing grip between tires and the runway surface — how well the tires can ‘bite’ rather than slide.
Why Pilots Care
Determines whether a runway remains open for operations; values below this threshold can require runway closure, diversion, or weight restrictions to maintain safe stopping margins.
Analogy
Think of a car tire on a wet road. If the road surface no longer gives the tire enough grip, stopping takes longer and steering feels less certain. A runway has a minimum acceptable grip level for the same reason.
Intuition Check
Minimum Friction Level does not mean the runway is automatically safe for every airplane in every condition. It means the measured surface grip has reached the FAA’s lowest acceptable maintenance threshold.
Example Sentence 1
The tower advised that runway 27's friction reading had dropped below the minimum friction level after the freezing rain, so we elected to hold for treatment.
Example Sentence 2
When friction testing showed readings below the minimum friction level after heavy rain, the runway was closed until it could be swept and retested.