Alternate aerodrome. 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. Alternate aerodromes include the following:
Take-off alternate. An alternate aerodrome at which an aircraft can land should this become necessary shortly after take-off and it is not possible to use the aerodrome of departure.
En-route alternate. An aerodrome at which an aircraft would be able to land after experiencing an abnormal or emergency condition while en route.
Destination alternate. An alternate aerodrome to which an aircraft may proceed should it become either impossible or inadvisable 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 that flight.

Altitude. The vertical distance of a level, a point or an object considered as a point, measured from mean sea level.
Approach control office. A unit established to provide air traffic control service to controlled flights arriving at, or departing from, one or more aerodromes.
Approach control service. Air traffic control service for arriving or departing controlled flights.
Approach funnel. A specified airspace around a nominal approach path within which an aircraft approaching to land is considered to be making a normal approach.
Approach sequence. The order in which two or more aircraft are cleared to approach to land at the aerodrome.
Appropriate ATS authority. The relevant authority designated by the State responsible for providing air traffic services in the airspace concerned.
Appropriate authority.
a)  Regarding flight over the high seas: the relevant authority of the State of Registry.
b)  Regarding flight other than over the high seas: the relevant authority of the State having sovereignty over the territory being overflown.
Rules of the Air and Air Traffic Services (PANS-RAC)
Apron. A defined area, on a land aerodrome, intended to accommodate aircraft for purposes of loading or unloading passengers, mail or cargo, fuelling, parking or maintenance.
Area control centre. A unit established to provide air traffic control service to controlled flights in control areas under its jurisdiction.
Area control service. Air traffic control service for controlled flights in control areas.
Area navigation (RNAV). A method of navigation which permits aircraft operation on any desired flight path within the coverage of station-referenced navigation aids or within the limits of the capability of self-contained aids, or a combination of these.
Area navigation route. An ATS route established for the use of aircraft capable of employing area navigation.
Assignment, assign. Distribution of frequencies to stations. Distribution of SSR Codes or 24-bit aircraft addresses to aircraft.
ATIS. The symbol used to designate automatic terminal information service.
ATS route. A specified route designed for channelling the flow of traffic as necessary for the provision of air traffic services.
Note 1.— The term “ATS route” is used to mean variously, airway, advisory route, controlled or uncontrolled route, arrival or departure route, etc.
Note 2.— An ATS route is defined by route specifications which include an ATS route designator, the track to or from significant points (way-points), distance between significant points, reporting requirements and, as determined by the appropriate ATS authority, the lowest safe altitude.
Automatic dependent surveillance (ADS). A surveillance technique in which aircraft automatically provide, via a data link, data derived from on-board navigation and position-fixing systems, including aircraft identification, four-dimensional position and additional data as appropriate.
　
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