Radiotelephony Call Signs

A call sign is the name a station uses on the aeronautical radio. A call opens with the call sign of the station addressed, and every readback ends with the aircraft's call sign, because the controller must know which aircraft has accepted a clearance. Two families exist: aircraft call signs, built from registrations and operators' designators, and the call signs of ground stations, built from a place name and a suffix that says what kind of service is on the other end.

ICAO Annex 10, Volume II, defines the forms and the rules for shortening them; the operators' designators are listed in ICAO Doc 8585, and the four-letter location indicators, which identify aerodromes and units in writing rather than by voice, in Doc 7910. The rules are much the same everywhere, and exam questions on them appear from PPL communications to ATPL.

Aircraft call sign types

Every aircraft carries nationality and registration marks. The nationality mark, such as G for the United Kingdom, comes from the series of radio call signs that the ITU allocates to the State of Registry; the State then assigns the registration mark, separated from it by a hyphen when it begins with a letter. Combinations that could be confused with distress, urgency or safety signals, such as SOS, PAN, XXX and TTT, or with Q codes, are not allowed.

From these marks and from the operator's designator ICAO builds three types of call sign, known as aircraft call sign types (Type A, B, C):

Type A is typical of private and general aviation flights, while airlines normally use Type C, the telephony designator followed by the flight number, spoken digit by digit like every number in a call sign: SCANDINAVIAN NINER TREE SEVEN.

Aircraft of the heavy wake turbulence category, with a maximum certificated take-off mass of 136,000 kg or more, add the word HEAVY after the call sign in the initial call to each ATS unit. The FAA's heavy category is based on a take-off weight capability of 300,000 lb or more, and the Airbus A380 and Antonov An-225 use SUPER. See wake turbulence.

Telephony designators

An operator receives two designators through ICAO, both published in Doc 8585: a three-letter designator, used in flight plans, in air traffic service messages and in the aircraft identification that ATC systems display, and a telephony designator, the word spoken on the radio. The two can look quite unlike each other: British Airways files and displays as BAW, but speaks as SPEEDBIRD.

The difference matters in the cockpit. The aircraft identification entered in a Mode S transponder must match item 7 of the flight plan exactly, in its written form, for example BAW123, not the spoken form SPEEDBIRD ONE TWO THREE; the registration goes there only when the registration is the call sign. A wrong entry can leave the flight uncorrelated or mislabelled on the controller's display (see transponders and SSR).

Abbreviating call signs

Call sign abbreviation is allowed only when three conditions are met: satisfactory communication has been established, no confusion is likely, and the ground station has used the abbreviated form first. The pilot never shortens the call sign on their own initiative, because only the controller can see whether a similar call sign is on the frequency. On first contact with each new station the full call sign is used again.

The abbreviated forms are:

• Type A: the first character of the registration, or the manufacturer's name or model, followed by at least the last two characters. G-ABCD becomes G-CD, and CHEROKEE XY-ABC becomes CHEROKEE BC or CHEROKEE ABC.
• Type B: the telephony designator followed by at least the last two characters of the registration.
• Type C: none; the full call sign is always used.

A typical sequence runs like this. The pilot calls "(station) Approach, Golf Alfa Bravo Charlie Delta" and is answered with the full call sign. Later the controller transmits "Golf Charlie Delta, descend to altitude three thousand feet", and from then on the pilot may reply "Descend to altitude three thousand feet, Golf Charlie Delta". Had the controller kept using the full call sign, perhaps because a G-ABCE was also on the frequency, the pilot would have kept it too.

Ground stations shorten too: once satisfactory communication is established and no confusion is likely, the station's suffix may be omitted. A pilot unsure whether a message was meant for their aircraft asks, rather than acting on it. Every readback ends with the call sign, in full or in the abbreviated form now in use.

Changes of call sign

When two aircraft with the same or confusingly similar call signs are on the same frequency, the controller may instruct one of them to use another call sign for the time being:

• "CHANGE CALL SIGN TO (new call sign) [UNTIL FURTHER ADVISED]";
• "REVERT TO FLIGHT PLAN CALL SIGN (call sign)" when the risk has passed.

Only the ground station can order a change of call sign; an aircraft cannot rename itself. The change is temporary and is normally cancelled once the two aircraft are no longer on the same frequency.

Similar call signs are a persistent hazard: a crew accepts a clearance meant for another aircraft, and the error surfaces only when the aircraft moves. The defences are straightforward. Pilots listen carefully when a similar call sign is on the frequency, include their call sign in every readback so that the controller can hear who accepted the clearance, and question any instruction that seems to be meant for someone else. The FAA's AIM makes the same points for line up and wait operations on several runways: listen for similar call signs, acknowledge every instruction with the call sign and, if unsure whether a take-off clearance was meant for you, ask before starting the take-off roll.

In the United States a controller's reply establishes two-way communication for entry into Class C or D airspace only if it uses the aircraft's call sign: "(call sign), standby" does, "aircraft calling, standby" does not.

View full schematic

Ground station call sign suffixes

An aeronautical station's call sign is the name of its location followed by an ATS unit call sign suffix that shows the unit or type of service:

Examples are "Shannon Control", the call sign of the Shannon area control centre in the Shannon Oceanic Transition Area, "London Information" and "Paris Control". In the United States an air route traffic control centre is called "Center", as in "Memphis Center". Some States add further suffixes, which are listed in their AIPs.

The suffix tells the pilot what the station can and cannot do, which is why it matters even when the place name is familiar. Only air traffic control units, such as TOWER, GROUND, APPROACH and CONTROL, issue clearances and instructions. A station using INFORMATION or RADIO passes information and the decisions stay with the pilot; much of what a RADIO operator passes is traffic reported by other pilots. INFORMATION is used both by en-route flight information units and by an aerodrome flight information service, so the context shows which is meant. At busy aerodromes clearance delivery is split from ground control so that long route clearances do not block the taxi frequency. The structure of these units is described in air traffic control units.

ICAO location indicators

An ICAO location indicator is the four-letter code that identifies an aerodrome, or an ATS unit, in writing. The indicators are published in ICAO Doc 7910. The first two letters usually identify the State: EG for the United Kingdom, LF for France, ED for Germany, so EGLL is London Heathrow, LFPG Paris Charles de Gaulle and EDDM Munich. In the contiguous United States many indicators are a K followed by the three-letter identifier, as in KJFK.

Location indicators are not spoken as call signs; on the radio the station is called by its place name. They appear instead wherever text is exchanged:

• in the flight plan, where items 13 and 16 give the departure and destination aerodromes, with ZZZZ and the name in item 18 when an aerodrome has no indicator;
• at the head of METAR and TAF reports;
• as the logon address for CPDLC, where the crew enters the indicator of the ATS unit it wants to connect to.

They are also distinct from the three-letter airport codes that IATA assigns, such as JFK.

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.