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
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).