Definition
Mu (μ) meters are units used to report runway surface friction. A mu value is a decimal number between 0 and 100 (or 0.00 to 1.00 depending on the reporting system) that represents how much grip a runway surface offers to aircraft tires. Higher mu values indicate better friction; lower values indicate slippery conditions caused by water, snow, ice, or slush. Mu values are measured by ground friction-testing equipment and reported in NOTAMs and ATIS broadcasts to help pilots assess braking action on contaminated runways.
Plain English
A number that tells pilots how slippery a runway is. Big number means good grip, small number means slippery.
Context Anchor
Seen in NOTAMs and runway condition information, especially when a runway may be wet, icy, snowy, or otherwise slippery.
Derivation
Mu is the Greek letter μ, which engineers and physicists use as the standard symbol for the coefficient of friction — the technical measure of how much one surface grips another. Aviation borrowed the symbol directly, so a 'mu reading' is literally a friction reading.
Why Pilots Care
The reported MU value directly affects calculated landing distances and whether a runway is safe to use in wet or contaminated conditions.
Intuition Check
Do not read “meters” here as a distance unit. In this context, MU is about measured runway grip, not length.
Example Sentence 1
The tower reported mu values of 42, 38, and 35 for the touchdown, midpoint, and rollout zones, indicating medium braking action.
Example Sentence 2
Pilots reviewed the latest mu-meter readings before accepting a landing clearance on the contaminated surface.