Why are some ATC call signs different than the flight number?

Question

When listening to air control traffic, or watching flights on flightradar24.com, some flights show up with ATC call signs (as transmitted on ADS-B) that do not match the flight number as they usually do, some examples:

TK1880 / THY9QT
TOM6347 / TOM1FT
LH1660 / DLH5KK

It seems to be more frequent on flight codes that have 4 digits, but this isn't exclusive. Who assigns these call signs, and what is the pattern for the naming?

Answer 1

It's a solution to the Call-sign Confusion.

Many large airlines operate call sign de-confliction programmes. These involve reviewing company call signs to ensure that aircraft with similar call signs are not likely to be routinely in the same airspace at the same time, and a process to systematically resolve ongoing issues arising from reports of similar call signs from their flight crew, ANSPs or other operators.

In busy airports, it's very common to have many flights with similar [sounding] call-signs (ATC perspective). You'd often hear the ATC stress the intended target.

Potential effects are:

Loss of communication, loss of separation, level bust, AIRPROX, or midair collision.

However, passengers expect their bookings to be numbers, hence the split between ICAO call-sign (air traffic control) and IATA flight code (booking). From a marketing perspective, a changing IATA flight number isn't good for the customers and booking systems.

Taking TK656 (THY3YE) as an example, we also find TK65 and TK56. If one of those three were to return (emergency) or be delayed, it will add unnecessary confusion if it meets one of the others. There could very well be other operators with similar numbers in and out of Istanbul as well.

Solution:

Large operators should routinely consider using a combination of numeric and alphanumeric call sign formats.

Guidelines:

• Avoid the use of similar call signs within the company
• Where practicable, proactively co-ordinate with other operators to minimize similar numeric and alphanumeric elements of call signs
• Avoid call signs with a four-number sequence; all-numeric call signs should be limited to a maximum of three digits
• Do not use the same digit repeated more than once (e.g. RUSHAIR 555)
• If letter suffixes are to be used with a preceding number sequence, limit the full string to a maximum of four alphanumeric components and, to the extent possible, coordinate letter combinations with other airspace and airport users
• Do not use alphanumeric call signs which have their last two letters as the destination’s ICAO location indicator (e.g. RUSHAIR 25LL for a flight inbound to London Heathrow)
• Where the total number of flights operated is large, it is likely to be best to use a combination of wholly some numeric and some alphanumeric call signs rather than all numeric or all alphanumeric
• If similarly-numbered call signs are unavoidable within a company, allow a significant time (at least 3 hours at any shared-use vicinity) and/or geographical split between aircraft using them
• If it is considered that useful capacity in the allocation of call signs has been reached, then consider applying for and using a second company call sign designator
• Do not use similar/reversed digits/letters in alphanumeric call-signs (e.g. RUSHAIR 87MB and RUSHAIR 78BM)
• For short haul flights, avoid using number sequences for particular routes which begin the day with ..01 and then continue sequentially through the day.

—skybrary.aero [edited for proofreading]

Note: unlike the US, in Europe and elsewhere pronouncing numbers in a call sign requires saying each number individually, e.g. four-three-one-one, and not forty-three-eleven.

Answer 2

Who assigns these callsigns, and what is the pattern for the naming?

Callsigns are allocated by ICAO. Their Annex 10 Radiotelephony Procedures says

5.2.1.7.2 Radiotelephony call signs for aircraft

5.2.1.7.2.1 Full call signs

5.2.1.7.2.1.1 An aircraft radiotelephony call sign shall be one of the following types:

Type a) — the characters corresponding to the registration marking of the aircraft; or

Type b) — the telephony designator of the aircraft operating agency, followed by the last four characters of the registration marking of the aircraft;

Type c) — the telephony designator of the aircraft operating agency, followed by the flight identification.

Note 1.— The name of aircraft manufacturer or name of aircraft model may be used as a radiotelephony prefix to the Type a) call sign above (see Table 5-1).

Note 2.— The telephony designators referred to in b) and c) above are contained in ICAO Doc 8585 — Designators for Aircraft Operating Agencies, Aeronautical Authorities and Services.

In the case of TOM6347 / TOM1FT,

Flight Number:         TOM6347 
Aircraft Registration: G-CPEU
Aircraft type:         B752
Callsign:              TOM1FT

As Jamiec commented, TOM1FT 'matches "C" - 1FT is the carrier-allocated "flight identification"'

I guess the use of alphanumeric flight identifications may be due to the 7-character limit on ICAO callsigns, the use of 3-character ICAO prefixes, the use of single-character suffixes (e.g. for delayed flights) and the need by large airlines for more than 1000 flight numbers - Reference

Flight Identifications can be changed when two or more flights with similar flight numbers fly close to each other.

Answer 3

IATA and ICAO has different methods of specifying flight designators. For IATA:

A flight designator is the concatenation of the airline designator, xx(a), and the numeric flight number, n(n)(n)(n), plus an optional one-letter "operational suffix" (a). Therefore, the full format of a flight designator is xx(a)n(n)(n)(n)(a).

ICAO on the other hand, calls for:

the ICAO designator for the aircraft operating agency followed by the flight identification... when in radio telephony the call sign to be used by the aircraft will consist of the ICAO telephony designator for the operating agency followed by the flight identification

There is no rule that I'm aware of that says that their identifiers should be the same. Flight plans use ICAO codes for flight identification, while IATA codes are mostly used for ticketing and other commercial purposes. It is entirely possible that the same aircraft has different codes, usually to avoid callsign confusion, as the ICAO callsign is sued for ATC purposes.

That being said, this seems to be mostly used in Eurpoe, rather than US (all the flights you've given as examples are from Europe). From this forum:

... In North America the airlines only add a letter designator to a flight number if a flight with the same number is in the air at the same time; which is a rare occournance.

On the other hand; if you heard this in Europe then its a different set of rules. I hear from a friend in London the regulatory authorities (such as National Air Traffic Services in the UK) allow the airlines to only use certian numbers at any one time and they often add a bunch of letters to the end just so they can't get the flights mixed up.

And also,

Using different IATA (passenger/luggage) and ICAO (flightplans/atc) flight numbers rarely happens in north america, but is a common occurrence with European airlines....

From this book:

In order to reduce the possibility of two callsigns on one frequency at any time sounding too similar, a number of airlines, particularly in Europe, have started using alphanumeric callsigns that are not based on flight numbers.

Also see the answers here, here and here

Answer 4

Differing flight numbers and ATC call signs are to avoid having two flights with similar call signs in the area at the same time. In theory.

In practice, it doesn't really work as well as it does on paper. Many similar flights are missed and fly together anyway, and the different call signs are confusing in themselves.

We usually brief a different ATC call sign as a threat in our preflight briefing. Most pilots I've flown with are of the opinion that they cause more problems than they solve.

Answer 5

IATA is used in the ticketing agent world. ICAO is used in air traffic control. The IATA flight number is the ticketing and marketing flight number. When in Air traffic control, there might be through flight, with different aircraft or some other reason the airline can't have the flight number active at the same time, so the airline will then assign a letter or some other reason to make that flight number unique in the ATC system. British Airways does it a lot as well for their international flights.