Glossary - Letter U

If you wish to find a term by name, please click the relevant letter below to be taken to a list.


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UAR |abbreviation| upper air route


UAS |abbreviation| upper air space


UHF |abbreviation| ultra high frequency


UIR |abbreviation| upper air region


UK |abbreviation| United Kingdom

ULD |abbreviation| Unit Load Device

ultimate |adjective| final, from which no further advance can be made / To determine the ultimate load which a structure must be capable of withstanding, a multiplier, called the ultimate factor of safety is used. The ultimate responsibility for safety rests with the crew. /


ultra- |prefix| beyond


ultra high frequency, ultra high frequency band |noun| a radio frequency range between 300 MHz and 3000 MHz. Abbreviation: UHF


ultralight |noun| a small single-seat or two-seat aircraft constructed of light materials and powered by a small motor, flown mainly for recreation


ultrasonic |adjective| referring to frequencies in the range of 20,000 Hz which cannot be heard by the human ear


ultrasonic inspection |noun| a non-destructive inspection of materials using extremely high frequency vibrations. Also called: ultrasonic detection


ultraviolet |adjective| referring to or occurring in the invisible part of the light spectrum beyond violet. Abbreviation: UV - ultraviolet radiation > the invisible part of the light spectrum beyond violet


unaccompanied |adjective| - unaccompanied baggage > baggage that travels on a different flight from the passenger who owns it. 

unambiguous |adjective| having only one possible meaning. If a word or sentence is ambiguous, it may be interpreted in several ways.

unaware |adjective| not knowing, not realizing / The cockpit crew were unaware of the situation in the cabin. /

unconsciousness |noun| the state of being without normal sensory awareness

uncontrolled airspace |noun| airspace in which air traffic control does not provide a service and in which an ATC clearance is not required to fly / While first learning to handle an aircraft, student pilots fly in uncontrolled airspace. / (NOTE: Pilots must still follow certain rules when flying through uncontrolled airspace.)


uncoordinated flight |noun| flight, especially during turns, in which the horizontal and vertical forces acting on the aircraft are out of balance. This can result in the aircraft going into a slip or a skid.


undercarriage |noun| the landing gear of an aircraft / To reduce the effect of drag by fixed undercarriages a retractable type of undercarriage was introduced. / (NOTE: The undercarriage is often called the landing gear or simply gear. The main landing gear are nearest the aircraft’s center of gravity. Main landing gear are designed to withstand a greater landing shock than the nose wheel or tail wheel and consequently should make contact with the surface first when landing.)


undercarriage assembly |noun| wheels, struts and linkages which make up the complete unit


undercarriage down and locked |noun| confirmation that the undercarriage is secure in preparation for landing


undergo |verb| to experience, to pass through a process / When water changes from vapor to liquid, energy is released into the atmosphere which is thus warmed, although the water itself does not undergo a change of temperature. / (NOTE: undergoing – underwent – has undergone)


underlying |adjective|  1. being under / Thermal modifications occur when the temperature of the underlying surface differs from that of the source region. /  2. forming the basis of a theory or principle / The principle underlying the construction of a mercury barometer has not changed since 1643, when Torricelli first demonstrated that the atmosphere has weight. /


undershoot |verb| to land before, or in front of the intended target / Because of the strong wind, the student pilot undershot the runway and landed before the runway threshold. /

undershoot wind-shear |noun| a wind-shear characterized by a decrease in aircraft airspeed

underside |noun| the surface underneath something / The underside of the wing should be carefully inspected for damage or leaks. /


undertake |verb| to do / In light aircraft, pilot/passenger communication can be satisfactorily undertaken verbally on a one to one basis. / (NOTE: undertaking – undertook – has undertaken)


undulating |adjective| rising and falling in gentle slopes / Flight over undulating terrain will result in changing indications of aircraft height on the indicator of the radio altimeter. /


uniform |adjective| the same, not varying in quality, dimensions, etc. / An engine should be run at low r.p.m. (revolutions per minute) after flight to allow engine components to cool to a uniform temperature. /


unique |adjective| the one and only of its sort, having no like or equal / The pulse coded message contains a unique 4-number identification. /


unit |noun|  1. a quantity or amount used as a standard, an accepted measurement / The internationally agreed unit of pressure is the millibar. The higher the sun is in the sky, the more intense is the radiation per unit area. /  2. a person, group or device, complete in itself / The operation of flying controls is by means of self-contained power flying control units (PFCUs). /

Unit Load Device |noun| a pallet or container which can be loaded onto a plane as a single unit. Abbreviation: ULD

universal |adjective| affecting all or everybody / The use of fly-by-wire systems in airliners was delayed to allow thorough development and encourage universal acceptance of the new technology. /

Universal Time Coordinated |noun| for most purposes, the same as GMT (Greenwich Mean Time), also referred to as Zulu in radio-telephony. Abbreviation: UTC

unlawful interference |noun| a situation that occurs when hijackers or terrorists are attempting to take control of or threaten the safety of the aircraft. There is a specific code (7500) to alert the ground of any attempted hijacking as the flight crew may not be able or wish to communicate orally. There may be unexplained and unscheduled changes to the aircraft’s course if the crew is threatened and is complying with the hijacker(s). Threatened in this way, the crew may not reply, or may not reply normally, to ATC and may not follow ATC instructions.

unload |verb| to remove a load from an aircraft / It took three hours to unload the aircraft. /


unloading point |noun| the place where an aircraft is unloaded / After taxiing, a marshaller marshals the aircraft to the disembarkation and unloading point. /

unruly |adjective| aggressive, belligerent, badly behaved / We have a group of unruly football fans on board. /

unsaturated |adjective| - unsaturated air > air that does not contain the maximum amount of water vapor for its temperature


unserviceable |adjective| not operative / The aircraft cannot be flown because the radio is unserviceable. / (NOTE: It is often abbreviated in spoken English as U (you) S (ess).) Abbreviation: U/S


unstick |verb| to cause an aircraft to take off, or take off in an aircraft - |noun| a take-off in an aircraft


update |verb| to bring up to date, to add the latest information to something / Forecasts are updated and reissued every four hours. /


updraft |noun| US same as updraught


updraught |noun| a rising current of air / In cumulonimbus clouds, there are updrafts of tremendous force. / Opposite: downdraft (NOTE: It is written updraft in US English.)


uplift |noun| the lifting of air by surface features / Thunderstorms are triggered off by convection and/or orographic uplift. /


upper |adjective|  1. at high altitude / In modern meteorological practice, upper air analysis and the construction of contour charts is carried out by computer. /  2. top. Opposite: lower - the upper surface of the wing > the surface of the wing facing upwards, as opposed to the underside


upper air chart |noun| a chart showing airflow pattern and distribution of temperatures at specific altitudes above about 10,000 feet


upper air route |noun| a route above FL245, approximately 24,500 ft. Abbreviation: UAR


upper airspace |noun| the airspace above FL245, approximately 24,500 ft. Abbreviation: UAS


upper information region |noun| airspace which covers the same geographical area as a flight information region  but above 24,500 ft. Abbreviation: UIR

upper level winds |noun| winds blowing at altitudes typically between 23,000 and 39,000 feet for the polar jet streams  and at higher levels for the subtropical jet streams. They blow from west to east and, as a result, make eastbound flying times across the North Atlantic approximately one hour shorter than the westbound ones.

upward |adjective| moving or directed up / As the aircraft accelerates down the runway, the forces on the wing tips and wing surfaces start reversing direction and instead of being only downward forces of weight, they become upward forces of lift. / (NOTE: In US English, upward is used as an adjective and as an adverb.)


upwards |adverb| towards the top / Heat is transferred from the Earth’s surface upwards by convection. / Opposite: downwards


upwind |adverb| against the wind / The glider was released from the aero-tow 3 miles upwind of the airfield. / Opposite: downwind

upwind end |noun| the end of a runway which is the opposite end from where an aircraft starts its take-off roll

urgency |noun| importance or need for prompt or fast action / Warnings, cautions and advisory messages are displayed only when necessary and are color coded to communicate the urgency of the fault to the flight crew. /


USA, US |abbreviation| United States of America

U/S |abbreviation| unserviceable, not operational, out of order

usable |adjective| capable of being used / On receiving the evacuate order, cabin crew must assess if their exits are usable. /


usage |noun| the act of using something, consumption / Fuel flight planning combines navigation data with fuel usage. /


use |noun| the act of using something, or the state of being used / It must be ensured that smoke masks are available for use by employees within the aircraft. / - runway in use > runway currently being used for take-offs and landings - |verb| to put something to work for a purpose / Gas turbine engines use low viscosity synthetic oil. /


UTC |abbreviation| Universal Time Coordinated


utilization |noun| the act of making use of / Integral tanks are now favored for aircraft owing to the high utilization of space and reduction in weight. /


utilize |verb| to make use of / The most common type of barograph is one which utilizes an aneroid capsule mechanically connected to a pen. /

UV |abbreviation| ultraviolet

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UTC | Zulu Time
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