Definition
A status marker indicating that a piece of equipment, navigation aid, runway, taxiway, light, or facility is not available for operational use. When something is marked U/S, it must not be relied upon for flight planning or in-flight use until it is restored to service and that restoration is officially published or communicated.
Plain English
U/S means broken, switched off, or otherwise not working — pilots should treat it as if it isn't there.
Context Anchor
Seen in airport notices, flight planning information, maintenance write-ups, and status reports for lights, equipment, runways, or services.
Derivation
A military and engineering shorthand carried into aviation. 'Serviceable' means fit for service or use; the prefix 'un-' reverses it. The slash form 'U/S' became standard radio and written shorthand because it is quick to say, write, and read.
Why Pilots Care
Pilots must know when aids or services are unavailable so they can choose safe alternatives and avoid reliance on inoperative equipment.
Intuition Check
Do not read U/S as a minor inconvenience or a casual note. In aviation use, it means the item is not available for normal use and should not be relied on.
Example Sentence 1
The NOTAM showed that the ILS for Runway 27 was U/S, so the pilot planned to fly the RNAV approach instead.
Example Sentence 2
Before takeoff the pilot confirmed no runways or lighting were U/S.