Definition
A formal status applied to an aircraft, component, system, or facility indicating that it is not in a condition fit for use and must not be operated or relied upon until it has been inspected, repaired, or otherwise restored to an airworthy or working state.
Plain English
Marked as not usable. The item or facility is broken, expired, or otherwise not fit for use, and must not be used until it has been fixed or cleared for service.
Context Anchor
Seen in maintenance records, defect reports, equipment labels, and preflight decisions when something on or around the aircraft may not be safe or allowed to use.
Derivation
From the prefix 'un-' (not) and 'serviceable,' which comes from the Latin 'servire' (to serve). Literally 'not able to be put into service.' The aviation use carries the formal sense: the item cannot be put to its intended use.
Why Pilots Care
An unserviceable item cannot be used for flight operations and must be resolved before dispatch or continued flight.
Intuition Check
Unserviceable does not just mean old, worn, or inconvenient. It means the item is not fit to be used for its intended aviation job until the problem is corrected or the flight is specifically allowed under the rules.
Example Sentence 1
The mechanic placed a red unserviceable tag on the attitude indicator after it failed during the run-up.
Example Sentence 2
Maintenance tagged the hydraulic pump unserviceable after the pressure test failed.