UHF
See ultra-high frequency.

Ultimate load factor.
In stress analysis, the load that causes physical breakdown in an aircraft or aircraft component during a strength test, or the load that according to computations, should cause such a breakdown.

ULTRAHIGH FREQUENCY
The frequency band between 300 and 3,000 MHz. The bank of radio frequencies used for military air/ground voice communications. In some instances this may go as low as 225 MHz and still be referred to as UHF.

ULTRALIGHT VEHICLE
A single-occupant aeronautical vehicle operated for sport or recreational purposes which does not require FAA registration, an airworthiness certificate, nor pilot certification. Operation of an ultralight vehicle in certain airspace requires authorization from ATC (Refer to 14 CFR Part 103.)

Ultralight
A single person only vehicle as defined by 14 CFR section 103.1.

UNABLE
Indicates inability to comply with a specific instruction, request, or clearance.

UNASSOCIATED
A radar target that does not display a data block with flight identification and altitude information. (See ASSOCIATED.)

Uncaging
Unlocking the gimbals of a gyroscopic instrument, making it susceptible to damage by abrupt flight maneuvers or rough handling.

Uncontrolled airspace
Class G airspace that has not been
designated as Class A, B, C, D, or E. It is airspace in which air traffic control has no authority or responsibility to control air traffic; however, pilots should remember there are VFR minimums which apply to this airspace.

Uncontrolled lift
Lift that occurs without specific action by the pilot. Often referred to as false lift.

UNDER THE HOOD
Indicates that the pilot is using a hood to restrict visibility outside the cockpit while simulating instrument flight. An appropriately rated pilot is required in the other control seat while this operation is being conducted. (Refer to 14 CFR Part 91.)

Underpower
Using less power than required for the purpose of achieving a faster rate of airspeed change.

UNDERSLUNG
A rotor hub that rotates below the top of the mast, as on semirigid rotor systems.

Understanding.
A basic level of learning at which a student comprehends or grasps the nature or meaning of something.

UNFROZEN
The Scheduled Time of Arrival (STA) tags, which are still being rescheduled by traffic management advisor (TMA) calculations. The aircraft will remain unfrozen until the time the corresponding estimated time of arrival (ETA) tag passes the preset freeze horizon for that aircrafts stream class. At this point the automatic rescheduling will stop, and the STA becomes frozen.

UNICOM
A nongovernment communication facility which may provide airport information at certain airports. Locations and frequencies of UNICOMs are shown on aeronautical charts and publications. (See AIRPORT/FACILITY DIRECTORY.) (Refer to AIM.)

United States Terminal Procedures Publication (TPP
Booklets published in regional format by the NACO that include DPs, STARs, IAPs, and other information pertinent
to IFR flight.

UNLOADED ROTOR
The state of a rotor when rotor force has been removed, or when the rotor is operating under a low or negative G condition.

UNPUBLISHED ROUTE
A route for which no minimum altitude is published or charted for pilot use. It may include a direct route between NAVAIDs, a radial, a radar vector, or a final approach course beyond the segments of an instrument approach procedure. (See PUBLISHED ROUTE.) (See ROUTE.)

UNRELIABLE (GPS/WAAS)
An advisory to pilots indicating the expected level of service of the GPS and/or WAAS may not be available. Pilots must then determine the adequacy of the signal for desired use.

Unstabilized approach
The final approach of an aircraft that has not achieved a stable rate of descent or controlled flight track by a predetermined altitude, usually 500 feet AGL.

Unstable atmosphere
A state of the atmosphere in which the lapse rate of temperature is great enough that a vertically displaced parcel will be warmer than its surroundings and will rise because of buoyancy without need for an external lifting force.

Unstable wave
A wave whose amplitude grows with time.

Unusable fuel
Fuel that cannot be consumed by the engine. This fuel is considered part of the empty weight of the aircraft.

Unusual attitude
An unintentional, unanticipated, or extreme aircraft attitude.

UPSWELL
Motion opposite the direction the swell is moving. If the swell is moving from north to south, a seaplane going from south
to north is moving upswell.

Upwind Leg
A flight path parallel to the landing runway in the direction of landing.

UPWIND LEG
(See TRAFFIC PATTERN.)

Upwind Leg
A flight path parallel to the landing runway in the direction of landing.

URET
(See USER REQUEST EVALUATION TOOL.)

URGENCY
A condition of being concerned about safety and of requiring timely but not immediate assistance; a potential distress condition. (See ICAO term URGENCY.)

URGENCY [ICAO]
A condition concerning the safety of an aircraft or other vehicle, or of person on board or in sight, but which does not require immediate assistance.

Usability.
The functionality of tests.

USAFIB
(See ARMY AVIATION FLIGHT INFORMATION BULLETIN.)

Useful lift (load)
The potential weight of the pilot, passengers, equipment and fuel. It is the basic empty weight of the aircraft subtracted from the maximum allowable gross weight.

Useful load
The weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil. It is the basic empty weight subtracted from the maximum allowable gross weight. This term applies to general aviation aircraft only.

USER REQUEST EVALUATION TOOL (URET)
User Request Evaluation Tool is an automated tool provided at each Radar Associate position in selected En Route facilities. This tool utilizes flight and radar data to determine present and future trajectories for all active and proposal aircraft and provides enhanced, automated flight data management.

User-defined waypoints
Waypoint location and other data which may be input by the user, this is the only GPS database information that may be altered (edited) by the user.

User-defined waypoints
Waypoint location and other data which may be input by the user, this is the only GPS database information that may be altered (edited) by the user.