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Pilot/Controller Glossary 

ALTERNATE AERODROME [ICAO]

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 An aerodrome 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

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An airport at which an aircraft may land if a landing at the intended airport becomes 

inadvisable. 

(See ICAO term ALTERNATE AERODROME.) 

ALTIMETER SETTING

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 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

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 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

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 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.) 
(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. ALTRVs must be classified as either “moving” or 

“stationary.” 

(See MOVING ALTITUDE RESERVATION.) 
(See STATIONARY ALTITUDE RESERVATION.) 
(See AIR TRAFFIC CONTROL SYSTEM COMMAND CENTER.) 

ALTITUDE RESTRICTION

−

 An altitude or altitudes, 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 restrictions is 

no longer required during a climb or descent. 
ALTRV

−

 

(See ALTITUDE RESERVATION.) 

AMVER

−

 

(See AUTOMATED MUTUAL-ASSISTANCE VESSEL RESCUE SYSTEM.) 

APB

−

 

(See AUTOMATED PROBLEM DETECTION BOUNDARY.) 

APD

−

 

(See AUTOMATED PROBLEM DETECTION.) 

PCG A

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