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.) 

## APB- 

(See AUTOMATED PROBLEM DETECTION BOUNDARY.) 

## APD- 

(See AUTOMATED PROBLEM DETECTION.) 

## APDIA- 

(See AUTOMATED PROBLEM DETECTION INHIBITED AREA.) 

APPROACH CLEARANCE- Authorization by ATC for a pilot to conduct an instrument approach. The type of instrument approach for which a 

