Pilot/Controller Glossary

ALERFA (Alert Phase) [ICAO]- A situation wherein
apprehension exists as to the safety of an aircraft and
its occupants.
ALERT- A notification to a position that there
is an aircraft-to-aircraft or aircraft-to-airspace
conflict, as detected by Automated Problem
Detection (APD).

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.)

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 communication 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 horizontal 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 numerals 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 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- 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

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.

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(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 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.)