u/s Unservicable
UAS University Air Squadron (UK)
also:   Unmanned aerial system
UAV Unmanned aerial vehicle
UCAV Unmanned combat air vehicle
UFD Up-front display
UFN Until further notice
UHB Ultra-high bypass
UHF Ultra high frequency
UL Ultralight (aircraft); re: microlight
ULM Ultra light motorised; re: microlight
UN United Nations
UNPROFOR United Nations Protection Force
unstick Speed at which aircraft leaves surface
UPT Undergraduate pilot training (US)
usable fuel Total amount of fuel which can be used in flight (= total capacity of fuel system minus residual fuel in pumps, lines, tanks sumps etc.) 
USAF United States Air Force
USAFE US Air Forces in Europe (USAF)
USB Upper surface blowing; exhaust air blown over top of wing to increase lift
USCG United States Coast Guard
useful load Total sum of payload and usable fuel; MTOW minus empty weight
US gallon = 3.785 litres = 0.833 Imperial gallon
USMC United States Marine Corps
USN United States Navy
UTC Co-ordinated universal time; previously: Greenwich Mean Time, Zulu
UV Ultraviolet
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