6/17/21 

Pilot/Controller Glossary 

U 

UHF

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(See ULTRAHIGH FREQUENCY.) 

ULTRAHIGH FREQUENCY (UHF)

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 The frequen-

cy 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

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 A single-occupant 

aeronautical vehicle operated for sport or recreational 
purposes which does not require FAA registration, an 
airworthiness certificate, or pilot certification. 
Operation of an ultralight vehicle in certain airspace 
requires authorization from ATC. 

(Refer to 14 CFR Part 103.) 

UNABLE

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 Indicates inability to comply with a 

specific instruction, request, or clearance. 

UNASSOCIATED

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 A radar target that does not 

display a data block with flight identification and 
altitude information. 

(See ASSOCIATED.) 

UNCONTROLLED AIRSPACE

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 Airspace in which 

aircraft are not subject to controlled airspace (Class 
A, B, C, D, or E) separation criteria. 

UNDER THE HOOD

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 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

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 The Scheduled Time of Arrival (STA) 

tags, which are still being rescheduled by the 
time

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based flow management (TBFM) 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

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 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. 

(See CHART SUPPLEMENT U.S.) 
(Refer to AIM.) 

UNMANNED AIRCRAFT (UA)- A device used or 
intended to be used for flight that has no onboard 
pilot.  This device can be any type of airplane, 
helicopter, airship, or powered-lift aircraft. 
Unmanned free balloons, moored balloons, tethered 
aircraft, gliders, and unmanned rockets are not 
considered to be a UA. 

UNMANNED AIRCRAFT SYSTEM (UAS)- An 
unmanned aircraft and its associated elements related 
to safe operations, which may include control 
stations (ground, ship, or air based), control links, 
support equipment, payloads, flight termination 
systems, and launch/recovery equipment.  It consists 
of three elements: unmanned aircraft, control station, 
and data link. 

UNPUBLISHED ROUTE

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 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)

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 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. 

UNSERVICEABLE (U/S) 

(See OUT OF SERVICE/UNSERVICEABLE.) 

UPWIND LEG

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(See TRAFFIC PATTERN.) 

URGENCY

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 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]

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 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. 

USAFIB

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(See ARMY AVIATION FLIGHT INFORMATION 

BULLETIN.) 

PCG U

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