Definition
A designation indicating that a piece of equipment, navigation aid, runway, taxiway, light, or facility is not available for use because it is broken, out of service, under maintenance, or otherwise not functioning to required standards.
Plain English
It's not working or not available right now. Don't plan on using it.
Context Anchor
Seen in airport notices, flight planning information, maintenance notes, and reports about equipment or services that are not available for normal use.
Derivation
From 'serviceable' (fit for service or use) with the prefix 'un-' meaning 'not.' If something is serviceable, it can be used; if it is unserviceable, it cannot.
Why Pilots Care
Using unserviceable equipment can lead to equipment failure, flight delays, or safety incidents, so pilots must verify status before every operation.
Intuition Check
Unserviceable does not just mean old, inconvenient, or low quality. It means not usable for its intended aviation purpose right now.
Example Sentence 1
The NOTAM stated that runway 27 edge lights were U/S, so we planned to land before sunset.
Example Sentence 2
Before takeoff, confirm that the navigation aid is not marked U/S on the approach plate.