Aviation Abbreviations and Definitions

Aviation runs on abbreviations. A single line of an AIP, a NOTAM or a METAR can hold more short forms than words, and the radio adds its own: a controller who says "CAVOK" or "contact Control" relies on both sides knowing exactly what is meant. Most of these short forms are standardized by ICAO, so that ACC, CTR or H24 mean the same from one State to the next.

Behind many abbreviations sits a defined term. The ICAO Annexes and procedures documents open with definitions, and in Europe the Standardised European Rules of the Air (SERA) carry them into law. A maneuvering area, a clearance limit or a control zone has one precise meaning, and exam questions test the wording: a definition read loosely is a common reason for a wrong answer. This article groups the abbreviations and definitions met most often by subject; the subjects themselves are covered in the linked articles.

How ICAO abbreviations work

An ICAO abbreviation is one listed in Doc 8400, Procedures for Air Navigation Services, Abbreviations and Codes (PANS-ABC). France, for example, applies the ninth edition, with amendments up to No. 34 applicable since 28 November 2024, and notifies no difference from it. Each State lists the abbreviations used in its own publications in section GEN 2.2 of its AIP, and a national list holds more than ICAO's: the French one gives every entry in French and English and includes French forms, such as ATI for the international standard atmosphere and AESA for EASA, and national ones, such as SIV for a flight information sector.

Three conventions help when decoding:

• Abbreviations and codes. Most entries shorten English words (RWY, TWY, ACFT for aircraft). Some come from codes: the Q code survives in QNH, QFE and QDM, and weather reports are written in the aeronautical meteorological code.
• One form, several meanings. The same letters can stand for different things, so context decides: French GEN 2.2 lists OCA as both oceanic control area and obstacle clearance altitude, and APP as both approach control and a visual approach chart.
• Spoken or spelled. On the radio common abbreviations such as ILS, VOR, NDB, QNH and QFE are said as letters, not spelled with the phonetic alphabet. Some are pronounced as words: ATIS, AFIS, AIRAC, CAVOK, NOTAM, PAPI, SELCAL, SID, STAR, TACAN, VOLMET and VORTAC. Proper names, service abbreviations and words of doubtful spelling are spelled out (see radiotelephony transmitting technique).

Air traffic services units and airspace

ATS (air traffic services) is the generic term for flight information service, alerting service, air traffic advisory service and air traffic control (ATC) service, the last divided into area, approach and aerodrome control. An air traffic services unit (ATSU) is the generic term for an ATC unit, a flight information center or an ATS reporting office (see air traffic services).

The FIR is the basic unit of airspace: all of a State's airspace lies in one or more FIRs, and controlled airspace is built inside them. Where a State divides its airspace vertically, the upper part is a UIR; European exam material places the division at FL195, as France does, while the United Kingdom uses FL245 (see airspace structure). An airway is a control area, or part of one, established in the form of a corridor.

A CTR protects instrument arrivals and departures and normally extends at least 5 NM from the aerodrome in the directions from which approaches are made; a TMA stacks control areas above the control zones. An ATZ protects the traffic of a single aerodrome, and at an aerodrome with AFIS it is the zone in which that service is given; the AFIS officer passes information but issues no clearances.

View full schematic

The FAA's names differ: an air route traffic control center (ARTCC, call sign CENTER) does the work of an ACC, a terminal radar approach control (TRACON) that of approach control, and the nearest equivalents of control zones are the surface areas of Class B, C, D and E airspace.

Aerodrome and runway terms

ICAO Annex 14 divides an aerodrome by use (see aerodrome physical characteristics):

• the maneuvering area is the part used for takeoff, landing and taxiing of aircraft, excluding the aprons;
• the movement area is the maneuvering area plus the aprons;
• the apron is where aircraft park, load and unload passengers, mail or cargo, refuel and are maintained.

Aerodrome traffic is all traffic on the maneuvering area plus all aircraft flying in the vicinity of the aerodrome, which means in, entering or leaving the traffic circuit. The threshold (THR) is the beginning of the part of the runway usable for landing.

RVR is the range over which the pilot of an aircraft on the runway centerline can see the runway surface markings or the lights delineating the runway or identifying its centerline. It is measured for a particular runway and may be more or less than the reported visibility (see visibility and runway visual range).

Navigation and procedures

Level is the generic term for the vertical position of an aircraft in flight and covers height, altitude and flight level. Altitude is measured from mean sea level, height from a specified datum, and a flight level (FL) is a surface of constant pressure related to the 1013.2 hPa datum. AMSL, AGL and AAL mean above mean sea level, ground level and aerodrome level. QNH is the altimeter setting that makes the altimeter read aerodrome elevation on the ground; QFE is the atmospheric pressure at aerodrome elevation, with which it reads zero. TA and TRL are the transition altitude and transition level (see altimeter settings).

An ATC clearance is an authorization for an aircraft to proceed under conditions specified by an air traffic control unit, and the clearance limit is the point to which it is granted. That is often not the destination: in "cleared to Lambourne via flight planned route, climb FL90", the limit is Lambourne, and an aircraft approaching it without a further clearance expects to hold there. A holding point in this sense is a specified location, identified visually or by other means, near which an aircraft in flight is kept in accordance with ATC clearances; a holding procedure is the predetermined maneuver that keeps it within a specified airspace while it awaits further clearance.

The times have exact meanings too. The ETA is the estimated time over a specified point; in a flight plan, the total estimated elapsed time (EET) of an IFR flight runs to the point where the instrument approach is expected to begin, or to the aerodrome where there is no such point. The EAT, expected approach time, is when ATC expects a delayed arriving aircraft to leave the holding fix to complete its approach. Heading is the direction in which the longitudinal axis points; track is the projection on the earth's surface of the path actually flown.

Meteorological abbreviations

A METAR is an aviation routine weather report and a SPECI a special report in the same format, issued when conditions change significantly; a TAF is an aerodrome forecast. A METAR may end with a trend forecast for the next 2 hours using BECMG (becoming), TEMPO (temporary) or NOSIG (no significant change) (see METAR and SPECI).

CAVOK, spoken "cav-oh-kay", replaces the visibility, RVR, weather and cloud groups only when all four conditions hold:

• visibility 10 km or more;
• no cloud below 1,500 m (5,000 ft) or the highest minimum sector altitude, whichever is greater;
• no cumulonimbus (CB) or towering cumulus (TCU);
• no weather of significance to aviation.

CAVOK says nothing about the wind; GEN 2.2 sums it up as visibility, cloud and present weather better than prescribed values or conditions. When CAVOK does not apply, NSC means no significant cloud and NCD that an automatic station detected none. Cloud amounts are FEW (1 to 2 oktas), SCT (3 to 4), BKN (5 to 7) and OVC (8).

Three broadcast and warning terms complete the set. ATIS, the automatic terminal information service, repeats current routine information for arriving and departing aircraft. VOLMET is meteorological information for aircraft in flight. A SIGMET gives information on en-route weather phenomena that may affect the safety of aircraft operations, and is issued for an FIR rather than an aerodrome.

Time, units and message priority

Aviation times are in UTC, coordinated universal time, on the 24-hour clock, marked Z in weather messages; on the radio normally only the minutes are given. Hours of service in AIP entries and NOTAM use these abbreviations (see aeronautical information services):

The radio bands are HF, 3 to 30 MHz, VHF, 30 to 300 MHz, and UHF, 300 to 3,000 MHz; VHF voice communication uses 118.000 to 136.975 MHz. AIRAC, aeronautical information regulation and control, is the system of common effective dates 28 days apart for changes to charts and databases.

Messages in the aeronautical mobile service have a fixed order of priority: distress (MAYDAY), urgency (PAN PAN), communications relating to direction finding, flight safety messages, meteorological messages and flight regularity messages. A request to the company for fuel or steps on arrival is flight regularity traffic and gives way to everything else.

Commonly confused terms

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.