U Uniform

UHF (Ultra High Frequency) - Radio frequencies between 300 and 3000 MHz.

undercarriage - The part of an aircraft that provides support while the aircraft is on the ground. It includes wheels, shock absorbers and support struts. There is an undercarriage unit under the nose of the aircraft as well as approximately midway back, under the fuselage. Undercarriage normally includes rubber tires, but may have skis for landing on snow or floats for landing on water.

Uniform - Designator for the letter "U" in the International Phonetic Alphabet.

unimproved airport - An airport with runways made of grass, dirt, or gravel, instead of concrete or asphalt.

unstable air - Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own.

upwind leg - A flight path parallel to the runway in the direction of landing.