TRIAL PLAN- A proposed amendment which utilizes automation to analyze and display potential conflicts along the predicted trajectory of the selected aircraft.
TRSA-
(See TERMINAL RADAR SERVICE AREA.)
TSD-
(See TRAFFIC SITUATION DISPLAY.)
TURBOJET AIRCRAFT- An aircraft having a jet engine in which the energy of the jet operates a turbine which in turn operates the air compressor.
TURBOPROP AIRCRAFT- An aircraft having a jet engine in which the energy of the jet operates a turbine which drives the propeller.
TURN ANTICIPATION- (maneuver anticipation).
TVOR-
(See TERMINAL‐VERY HIGH FREQUENCY
OMNIDIRECTIONAL RANGE STATION.)

TWEB-
(See TRANSCRIBED WEATHER BROADCAST.)
TWO‐WAY RADIO COMMUNICATIONS FAIL-URE-
(See LOST COMMUNICATIONS.)

U

UDF-
(See DIRECTION FINDER.)
UHF-
(See ULTRAHIGH FREQUENCY.)
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- An aeronautical vehicle operated for sport or recreational purposes which does not require FAA registration, an airworthiness certificate, nor pilot certification. They are primarily single occupant vehicles, although some two‐place vehicles are authorized for training purposes. Operation of an ultralight vehicle in certain airspace requires authorization from ATC.
(Refer to 14 CFR Part 103.)
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.)
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.)
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 aircraft's stream class. At this point the automatic rescheduling will stop, and the STA becomes “frozen.”
UNICOM- A nongovernment communication facil-ity which may provide airport information at certain airports. Locations and frequencies of UNICOMs are shown on aeronautical charts and publications.