Air Regulations DGCA pilots. Definitions of aviation terms

In this chapter, we should familiarize with the commonly used terms of aviation.

Note 1.— Throughout the text of this document the term “service” is used as an abstract noun to designate functions, or
service rendered; the term “unit” is used to designate a collective body performing a service.
Note 2.— The designation (RR) in these definitions indicates a definition which has been extracted from the Radio
Regulations of the International Telecommunication Union (ITU) (see Handbook on Radio Frequency Spectrum
Requirements for Civil Aviation including statement of approved ICAO policies (Doc 9718)).
When the following terms are used in the International Standards for Rules of the Air, they have the following meanings:

Acrobatic flight.  Manoeuvres intentionally performed by an aircraft involving an abrupt change in its attitude, an abnormal
attitude, or an abnormal variation in speed.

ADS-C agreement. A reporting plan which establishes the conditions of ADS-C data reporting (i.e. data required by the air
traffic services unit and frequency of ADS-C reports which have to be agreed to prior to using ADS-C in the provision of
air traffic services).
Note.— The terms of the agreement will be exchanged between the ground system and the aircraft by means of a
contract, or a series of contracts.

Advisory airspace. An airspace of defined dimensions, or designated route, within which air traffic advisory service is
available.

Advisory route. A designated route along which air traffic advisory service is available.

Aerodrome. A defined area on land or water (including any buildings, installations and equipment) intended to be used either
wholly or in part for the arrival, departure and surface movement of aircraft.

Aerodrome control service. Air traffic control service for aerodrome traffic.

Aerodrome control tower. A unit established to provide air traffic control service to aerodrome traffic.

Aerodrome traffic. All traffic on the manoeuvring area of an aerodrome and all aircraft flying in the vicinity of an aerodrome.
Note.— An aircraft is in the vicinity of an aerodrome when it is in, entering or leaving an aerodrome traffic circuit.
Aerodrome traffic zone. An airspace of defined dimensions established around an aerodrome for the protection of aerodrome
traffic.
Aeronautical Information Publication (AIP). A publication issued by or with the authority of a State and containing
aeronautical information of a lasting character essential to air navigation.

Aeronautical station (RR S1.81). A land station in the aeronautical mobile service. In certain instances, an aeronautical
station may be located, for example, on board ship or on a platform at sea.

Aeroplane. A power-driven heavier-than-air aircraft, deriving its lift in flight chiefly from aerodynamic reactions on surfaces
which remain fixed under given conditions of flight.

Airborne collision avoidance system (ACAS). An aircraft system based on secondary surveillance radar (SSR) transponder
signals which operates independently of ground-based equipment to provide advice to the pilot on potential conflicting
aircraft that are equipped with SSR transponders.

Aircraft. Any machine that can derive support in the atmosphere from the reactions of the air other than the reactions of the
air against the earth’s surface.

Air-ground control radio station. An aeronautical telecommunication station having primary responsibility for handling
communications pertaining to the operation and control of aircraft in a given area.

Air-taxiing. Movement of a helicopter/VTOL above the surface of an aerodrome, normally in ground effect and at a ground
speed normally less than 37 km/h (20 kt).
Note.— The actual height may vary, and some helicopters may require air-taxiing above 8 m (25 ft) AGL to reduce
ground effect turbulence or provide clearance for cargo slingloads.

Air traffic. All aircraft in flight or operating on the manoeuvring area of an aerodrome.

Air traffic advisory service. A service provided within advisory airspace to ensure separation, in so far as practical, between
aircraft which are operating on IFR flight plans.

Air traffic control clearance. Authorization for an aircraft to proceed under conditions specified by an air traffic control unit.
Note 1.— For convenience, the term “air traffic control clearance” is frequently abbreviated to “clearance” when used
in appropriate contexts.
Note 2.— The abbreviated term “clearance” may be prefixed by the words “taxi”, “take-off”, “departure”, “en route”,
“approach” or “landing” to indicate the particular portion of flight to which the air traffic control clearance relates.

Air traffic control service. A service provided for the purpose of:
a) preventing collisions:
1) between aircraft, and
2) on the manoeuvring area between aircraft and obstructions, and
b) expediting and maintaining an orderly flow of air traffic.

Air traffic control unit. A generic term meaning variously, area control centre, approach control unit or aerodrome control
tower.
Air traffic service. A generic term meaning variously, flight information service, alerting service, air traffic advisory service,
air traffic control service (area control service, approach control service or aerodrome control service).

Air traffic services airspaces. Airspaces of defined dimensions, alphabetically designated, within which specific types of
flights may operate and for which air traffic services and rules of operation are specified.
Note.— ATS airspaces are classified as Class A to G

Air traffic services reporting office. A unit established for the purpose of receiving reports concerning air traffic services and
flight plans submitted before departure.
Note.— An air traffic services reporting office may be established as a separate unit or combined with an existing unit,
such as another air traffic services unit, or a unit of the aeronautical information service.

Air traffic services unit. A generic term meaning variously, air traffic control unit, flight information centre or air traffic
services reporting office.

Airway. A control area or portion thereof established in the form of a corridor.

Alerting service. A service provided to notify appropriate organizations regarding aircraft in need of search and rescue aid,
and assist such organizations as required.

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 where the necessary services and facilities are available, where aircraft performance requirements can be met and which is operational at the expected time of use. Alternate aerodromes include the following:

Take-off alternate. An alternate aerodrome at which an aircraft would be able to land should this become necessary shortly after take-off and it is not possible to use the aerodrome of departure.

En-route alternate. An alternate aerodrome at which an aircraft would be able to land in the event that a diversion
becomes necessary while en route.

Destination alternate. An alternate aerodrome at which an aircraft would be able to land 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 (MSL).

Approach control service. Air traffic control service for arriving or departing controlled flights.

Approach control unit. A unit established to provide air traffic control service to controlled flights arriving at, or departing
from, one or more aerodromes.

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.
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 ground- or space-based navigation aids or within the limits of the capability of self-contained aids, or a combination of these.
Note.— Area navigation includes performance-based navigation as well as other operations that do not meet the
definition of performance-based navigation.

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 (waypoints), distance between significant points, reporting requirements and, as determined by the appropriate ATS authority, the lowest safe altitude.
Automatic dependent surveillance — broadcast (ADS-B). A means by which aircraft, aerodrome vehicles and other objects can automatically transmit and/or receive data such as identification, position and additional data, as appropriate, in a broadcast mode via a data link.
Automatic dependent surveillance — contract (ADS-C). A means by which the terms of an ADS-C agreement will be exchanged between the ground system and the aircraft, via a data link, specifying under what conditions ADS-C reports would be initiated, and what data would be contained in the reports.
Note.— The abbreviated term “ADS contract” is commonly used to refer to ADS event contract, ADS demand contract, ADS periodic contract or an emergency mode.

Ceiling. The height above the ground or water of the base of the lowest layer of cloud below 6 000 metres (20 000 feet)
covering more than half the sky.

Changeover point. The point at which an aircraft navigating on an ATS route segment defined by reference to very high frequency omnidirectional radio ranges is expected to transfer its primary navigational reference from the facility behind the aircraft to the next facility ahead of the aircraft.
Note.— Changeover points are established to provide the optimum balance in respect of signal strength and quality between facilities at all levels to be used and to ensure a common source of azimuth guidance for all aircraft operating along the same portion of a route segment.

Clearance limit. The point to which an aircraft is granted an air traffic control clearance.

Command and control (C2) link. The data link between the remotely piloted aircraft and the remote pilot station for the purposes of managing the flight.

Control area. A controlled airspace extending upwards from a specified limit above the earth.

Controlled aerodrome. An aerodrome at which air traffic control service is provided to aerodrome traffic.
Note.— The term “controlled aerodrome” indicates that air traffic control service is provided to aerodrome traffic but does not necessarily imply that a control zone exists.

Controlled airspace. An airspace of defined dimensions within which air traffic control service is provided in accordance with the airspace classification.
Note.— Controlled airspace is a generic term which covers ATS airspace Classes A, B, C, D and E as described in
Annex 11, 2.6.

Controlled flight. Any flight which is subject to an air traffic control clearance.

Controller-pilot data link communications (CPDLC). A means of communication between controller and pilot, using data
link for ATC communications.

Control zone. A controlled airspace extending upwards from the surface of the earth to a specified upper limit.
Cruise climb. An aeroplane cruising technique resulting in a net increase in altitude as the aeroplane mass decreases.
Cruising level. A level maintained during a significant portion of a flight.
Current flight plan. The flight plan, including changes, if any, brought about by subsequent clearances.
Danger area. An airspace of defined dimensions within which activities dangerous to the flight of aircraft may exist atspecified times.
Data link communications. A form of communication intended for the exchange of messages via a data link.
Detect and avoid. The capability to see, sense or detect conflicting traffic or other hazards and take the appropriate action.
Estimated off-block time. The estimated time at which the aircraft will commence movement associated with departure.
Estimated time of arrival. For IFR flights, the time at which it is estimated that the aircraft will arrive over that designated point, defined by reference to navigation aids, from which it is intended that an instrument approach procedure will be commenced, or, if no navigation aid is associated with the aerodrome, the time at which the aircraft will arrive over the aerodrome. For VFR flights, the time at which it is estimated that the aircraft will arrive over the aerodrome.
Expected approach time. The time at which ATC expects that an arriving aircraft, following a delay, will leave the holding fix to complete its approach for a landing.
Note.— The actual time of leaving the holding fix will depend upon the approach clearance.
Filed flight plan. The flight plan as filed with an ATS unit by the pilot or a designated representative, without any subsequent changes.
Flight crew member. A licensed crew member charged with duties essential to the operation of an aircraft during a flight duty period.
Flight information centre. A unit established to provide flight information service and alerting service.
Flight information region. An airspace of defined dimensions within which flight information service and alerting service are provided.
Flight information service. A service provided for the purpose of giving advice and information useful for the safe and
efficient conduct of flights.
Flight level. A surface of constant atmospheric pressure which is related to a specific pressure datum, 1 013.2 hectopascals (hPa), and is separated from other such surfaces by specific pressure intervals.
Note 1.— A pressure type altimeter calibrated in accordance with the Standard Atmosphere:
a) when set to a QNH altimeter setting, will indicate altitude;
b) when set to a QFE altimeter setting, will indicate height above the QFE reference datum;
c) when set to a pressure of 1 013.2 hPa, may be used to indicate flight levels.
Note 2.— The terms “height” and “altitude”, used in Note 1 above, indicate altimetric rather than geometric heights
and altitudes.
Flight plan. Specified information provided to air traffic services units, relative to an intended flight or portion of a flight of an aircraft.
Flight visibility. The visibility forward from the cockpit of an aircraft in flight.
Ground visibility. The visibility at an aerodrome as reported by an accredited observer or by automatic systems.
Heading. The direction in which the longitudinal axis of an aircraft is pointed, usually expressed in degrees from North (true, magnetic, compass or grid).
Height. The vertical distance of a level, a point or an object considered as a point, measured from a specified datum.
IFR. The symbol used to designate the instrument flight rules.
IFR flight. A flight conducted in accordance with the instrument flight rules.
IMC. The symbol used to designate instrument meteorological conditions.
Instrument approach procedure. A series of predetermined manoeuvres by reference to flight instruments with specified protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if a landing is not completed, to a position at which holding or en-route obstacle clearance criteria apply. Instrument approach procedures are classified as follows:
Non-precision approach (NPA) procedure. An instrument approach procedure which utilizes lateral guidance but does not utilize vertical guidance.
Approach procedure with vertical guidance (APV). An instrument approach procedure which utilizes lateral and vertical guidance but does not meet the requirements established for precision approach and landing operations. Precision approach (PA) procedure. An instrument approach procedure using precision lateral and vertical guidance with minima as determined by the category of operation.
Note.— Lateral and vertical guidance refers to the guidance provided either by:
a) a ground-based navigation aid; or
b) computer-generated navigation data.
Instrument meteorological conditions. Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling, less than the minima specified for visual meteorological conditions.
Note.— The specified minima for visual meteorological conditions are contained in Chapter 4.
Landing area. That part of a movement area intended for the landing or take-off of aircraft.
Level. A generic term relating to the vertical position of an aircraft in flight and meaning variously, height, altitude or flight level.
Manoeuvering area. That part of an aerodrome to be used for the take-off, landing and taxiing of aircraft, excluding aprons.
Movement area. That part of an aerodrome to be used for the take-off, landing and taxiing of aircraft, consisting of the maneuvering area and the apron(s).
Operator. A person, organization or enterprise engaged in or offering to engage in an aircraft operation.
Note.— In the context of remotely piloted aircraft, an aircraft operation includes the remotely piloted aircraft system.
Pilot-in-command. The pilot designated by the operator, or in the case of general aviation, the owner, as being in command and charged with the safe conduct of a flight.
Pressure-altitude. An atmospheric pressure expressed in terms of altitude which corresponds to that pressure in the Standard Atmosphere.*
Problematic use of substances. The use of one or more psychoactive substances by aviation personnel in a way that:
a) constitutes a direct hazard to the user or endangers the lives, health or welfare of others; and/or
b) causes or worsens an occupational, social, mental or physical problem or disorder.
Prohibited area. An airspace of defined dimensions, above the land areas or territorial waters of a State, within which the flight of aircraft is prohibited.
Psychoactive substances. Alcohol, opioids, cannabinoids, sedatives and hypnotics, cocaine, other psychostimulants, hallucinogens, and volatile solvents, whereas coffee and tobacco are excluded.
Radiotelephony. A form of radiocommunication primarily intended for the exchange of information in the form of speech.
Remote pilot. A person charged by the operator with duties essential to the operation of a remotely piloted aircraft and who manipulates the flight controls, as appropriate, during flight time.
Remote pilot station. The component of the remotely piloted aircraft system containing the equipment used to pilot the remotely piloted aircraft.
Remotely piloted aircraft (RPA). An unmanned aircraft which is piloted from a remote pilot station.
Remotely piloted aircraft system (RPAS). A remotely piloted aircraft, its associated remote pilot station(s), the required command and control links and any other components as specified in the type design.

Repetitive flight plan (RPL). A flight plan related to a series of frequently recurring, regularly operated individual flights with identical basic features, submitted by an operator for retention and repetitive use by ATS units.
Reporting point. A specified geographical location in relation to which the position of an aircraft can be reported.
Restricted area. An airspace of defined dimensions, above the land areas or territorial waters of a State, within which the flight of aircraft is restricted in accordance with certain specified conditions.
RPA observer. A trained and competent person designated by the operator who, by visual observation of the remotely piloted aircraft, assists the remote pilot in the safe conduct of the flight.
Runway. A defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft.
Runway-holding position. A designated position intended to protect a runway, an obstacle limitation surface, or an ILS/ MLS critical/sensitive area at which taxiing aircraft and vehicles shall stop and hold, unless otherwise authorized by the aerodrome control tower.
Note.— In radiotelephony phraseologies, the expression “holding point” is used to designate the runway-holding position.
Safety-sensitive personnel. Persons who might endanger aviation safety if they perform their duties and functions improperly including, but not limited to, crew members, aircraft maintenance personnel and air traffic controllers.
Signal area. An area on an aerodrome used for the display of ground signals.
Special VFR flight. A VFR flight cleared by air traffic control to operate within a control zone in meteorological conditions below VMC.
Taxiing. Movement of an aircraft on the surface of an aerodrome under its own power, excluding take-off and landing.
Taxiway. A defined path on a land aerodrome established for the taxiing of aircraft and intended to provide a link between one part of the aerodrome and another, including:
a) Aircraft stand taxilane. A portion of an apron designated as a taxiway and intended to provide access to aircraft stands only.
b) Apron taxiway. A portion of a taxiway system located on an apron and intended to provide a through taxi route across the apron.
c) Rapid exit taxiway. A taxiway connected to a runway at an acute angle and designed to allow landing aeroplanes to turn off at higher speeds than are achieved on other exit taxiways thereby minimizing runway occupancy times.
Terminal control area. A control area normally established at the confluence of ATS routes in the vicinity of one or more major aerodromes.
Total estimated elapsed time. For IFR flights, the estimated time required from take-off to arrive over that designated point, defined by reference to navigation aids, from which it is intended that an instrument approach procedure will be commenced, or, if no navigation aid is associated with the destination aerodrome, to arrive over the destination aerodrome. For VFR flights, the estimated time required from take-off to arrive over the destination aerodrome.
Track. The projection on the earth’s surface of the path of an aircraft, the direction of which path at any point is usually expressed in degrees from North (true, magnetic or grid).

Traffic avoidance advice. Advice provided by an air traffic services unit specifying maneuveres to assist a pilot to avoid a collision.
Traffic information. Information issued by an air traffic services unit to alert a pilot to other known or observed air traffic which may be in proximity to the position or intended route of flight and to help the pilot avoid a collision.
Transition altitude. The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes.
Unmanned free balloon. A non-power-driven, unmanned, lighter-than-air aircraft in free flight.
Note.— Unmanned free balloons are classified as heavy, medium or light in accordance with specifications contained in
Appendix 5.
VFR. The symbol used to designate the visual flight rules.
VFR flight. A flight conducted in accordance with the visual flight rules.
Visibility. Visibility for aeronautical purposes is the greater of:
a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background;
b) the greatest distance at which lights in the vicinity of 1 000 candelas can be seen and identified against an unlit background.
Note 1.— The two distances have different values in air of a given extinction coefficient, and the latter b) varies
with the background illumination. The former a) is represented by the meteorological optical range (MOR).
Note. 2.— The definition applies to the observations of visibility in local routine and special reports, to the
observations of prevailing and minimum visibility reported in METAR and SPECI and to the observations of ground visibility.
Visual line-of-sight (VLOS) operation. An operation in which the remote pilot or RPA observer maintains direct unaided visual contact with the remotely piloted aircraft.
Visual meteorological conditions. Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling, equal to or better than specified minima.
Note.— The specified minima are contained in Chapter 4.
VMC. The symbol used to designate visual meteorological conditions