u/s - unserviceable (i.e not working) when applied to an aircraft or its equipment.

UAS - University Air Squadron.

UDF - UHF direction finding.

UFN - until further notice.

UHF - ultra-high frequency. Radio frequencies in the 300-3,000 MHz band.

UHMRA - Upper Heyford Mandatory Radio Area.

UIR - Upper Information Region, covering the same geographic areas as a FIR, but extending vertically upwards from 24,500 feet, within which certain additional operational rules apply. Also UIS, Upper Information Service.

Unicom - privately operated advisory A/G radio service at uncontrolled airfields (USA). UNL-unlimited

UTC - Co-ordinated Universal Time, formerly Greenwich Mean Time (see also Zulu).