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Pilot/Controller Glossary 

6/17/21 

conflict, as detected by Automated Problem 
Detection (APD). 

ALERT AREA

 

(See SPECIAL USE AIRSPACE.) 

ALERT NOTICE (ALNOT)

 A request originated 

by a flight service station (FSS) or an air route traffic 
control center (ARTCC) for an extensive commu-
nication search for overdue, unreported, or missing 
aircraft. 

ALERTING SERVICE

 A service provided to notify 

appropriate organizations regarding aircraft in need 
of search and rescue aid and assist such organizations 
as required. 

ALNOT

 

(See ALERT NOTICE.) 

ALONG

TRACK DISTANCE (ATD)

 The hori-

zontal distance between the aircraft’s current position 
and a fix measured by an area navigation system that 
is not subject to slant range errors. 

ALPHANUMERIC DISPLAY

 Letters and numer-

als used to show identification, altitude, beacon code, 
and other information concerning a target on a radar 
display. 

(See AUTOMATED RADAR TERMINAL 

SYSTEMS.) 

ALTERNATE AERODROME [ICAO]

 An aero-

drome to which an aircraft may proceed when it 
becomes either impossible or inadvisable to proceed 
to or to land at the aerodrome of intended landing. 

Note: The aerodrome from which a flight departs 

may also be an en-route or a destination alternate 
aerodrome for the flight. 

ALTERNATE AIRPORT

 An airport at which an 

aircraft may land if a landing at the intended airport 
becomes inadvisable. 

(See ICAO term ALTERNATE AERODROME.) 

ALTIMETER SETTING

 The barometric pressure 

reading used to adjust a pressure altimeter for 
variations in existing atmospheric pressure or to the 
standard altimeter setting (29.92). 

(Refer to 14 CFR Part 91.) 

(Refer to AIM.) 

ALTITUDE

 The height of a level, point, or object 

measured in feet Above Ground Level (AGL) or from 
Mean Sea Level (MSL). 

(See FLIGHT LEVEL.) 

a. 

MSL Altitude

 Altitude expressed in feet 

measured from mean sea level. 

b. 

AGL Altitude

 Altitude expressed in feet 

measured above ground level. 

c. 

Indicated Altitude

 The altitude as shown by an 

altimeter. On a pressure or barometric altimeter it is 
altitude as shown uncorrected for instrument error 
and uncompensated for variation from standard 
atmospheric conditions. 

(See ICAO term ALTITUDE.) 

ALTITUDE [ICAO]

 The vertical distance of a level, 

a point or an object considered as a point, measured 
from mean sea level (MSL). 

ALTITUDE READOUT

  An aircraft’s altitude, 

transmitted via the Mode C transponder feature, that 
is visually displayed in 100-foot increments on a 
radar scope having readout capability. 

(See ALPHANUMERIC DISPLAY.) 
(See AUTOMATED RADAR TERMINAL 

SYSTEMS.) 

(Refer to AIM.) 

ALTITUDE RESERVATION (ALTRV)

 Airspace 

utilization under prescribed conditions normally 
employed for the mass movement of aircraft or other 
special user requirements which cannot otherwise be 
accomplished. ALTRVs are approved by the 
appropriate FAA facility. 

(See AIR TRAFFIC CONTROL SYSTEM 

COMMAND CENTER.) 

ALTITUDE RESTRICTION

 An altitude or alti-

tudes, stated in the order flown, which are to be 
maintained until reaching a specific point or time. 
Altitude restrictions may be issued by ATC due to 
traffic, terrain, or other airspace considerations. 

ALTITUDE RESTRICTIONS ARE CANCELED

 

Adherence to previously imposed altitude restric-
tions is no longer required during a climb or descent. 

ALTRV

 

(See ALTITUDE RESERVATION.) 

AMVER

 

(See AUTOMATED MUTUAL-ASSISTANCE 

VESSEL RESCUE SYSTEM.) 

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