The use of similar call signs by aircraft operating in the same area and especially on the same RTF frequency often gives rise to potential and actual flight safety incidents. This hazard is usually referred to as “call sign confusion”.
The following are some examples of the more common causes for call sign confusion:
- Airlines allocate commercial flight numbers as call-signs; these are normally consecutive and therefore similar (e.g. RUSHAIR 1431, RUSHAIR 1432, etc.)
- Airlines schedule flights with similar call signs to be in the same airspace at the same time.
- Call signs coincidentally contain the same alphanumeric characters in a different order (e.g. AB1234 and BA 2314).
- Call signs contain repeated digits (e.g. RUSHAIR 555).
- Alpha-numeric call signs end in two letters which correspond to the last two letters of the destination’s ICAO location indicator (e.g. RUSHAIR 25LL for a flight inbound to London Heathrow);
Loss of communication, loss of separation, level bust, AIRPROX, or midair collision.
Many larger 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 systemmatically resolve ongoing issues arising from reports of similar call signs from their flight crew, ANSPs or other operators.
An aircraft receives and acts on a clearance intended for another aircraft, in consequence of which:
- The aircraft takes up a heading or routing intended for the other; or,
- The aircraft commences a climb or descent to a level to which it has not been cleared; or,
- The aircraft departs the RTF frequency; or,
- In responding to the message, the aircraft blocks a transmission from the intended recipient; or,
- The intended recipient does not receive the clearance, and fails to take up the desired heading or routing, or fails to climb or descent to the desired level; or,
- The workload of ATCOs and pilots is increased due to the necessity to resolve the confusion.
- Failure of operator to give sufficient consideration in allocation of RTF call signs.
- Pilot workload.
- Controller workload.
- Interruption or distraction.
- High rates of climb or descent.
- Level bust in holding patters.
- Airspace and procedure design.
- Density of traffic.
- Larger operators with high flight densities in particular airspace should consider routinely using a combination of numeric and alphanumeric call sign formats.
- Observe the following guidance in selecting call signs (see also link for EUROCONTROL Call Sign Similarity Service in the Further Reading section):
- Avoid the use of similar call signs within the company;
- Where practicable, proactively co-ordinate with other operators to minimise similar numeric and alphanumeric elements of call signs;
- Avoid call signs with a four-number sequence; all-numeric callsigns 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 preceeding 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 callsigns 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.
- Employ a call sign de-confliction programme (see under Defences above).
- Some countries impose additional call sign requirements to reduce the risk of confusion. For instance, the U.S. Federal Aviation Administration (FAA) require that the following criteria are met if a call sign is to be authorized in the NAS (National Airspace System):
- The combination of call sign designator and flight number must not exceed 7 alphanumeric characters;
- To avoid similar or same call sign confusion, the call sign designator letters must immediately be followed in sequence only by the numerals of the flight number;
- No additional letters of the alphabet are permitted after the call sign designator (except for the bullet below);
- For operational purposes, Part 121 and Part 135 scheduled aircraft operators may use a letter as the final character of the aircraft identification as long as it is preceded by a numeral (i.e., AAL351A);
Examples of FAA acceptable call sign designators with flight number: MDSTRl, RDDL172, ABX91, AAL351A and SWA2604.
Examples of FAA unacceptable call sign designators with flight number: RDDL1720 (more than seven characters), NEWS42G (additional alphabet letter used for non-scheduled operator), BKA16CH (two alphabet letters), and LBQ17523 (more than seven characters).
Accidents and Incidents
The following events include Call Sign Confusion as a contributory factor:
On 29 March 2018, a Boeing 737-700 commenced a late go-around from landing at Amsterdam on a runway with an extended centreline which passed over another runway from which a Boeing 737-800 had already been cleared for takeoff. An attempt by the controller responsible for both aircraft to stop the departing aircraft failed because the wrong callsign was used, so low level divergent turns were given to both aircraft and 0.5nm lateral and 300 feet vertical separation was achieved. The Investigation concluded that the ATC procedure involved was potentially hazardous and made a safety recommendation that it should be withdrawn.
On 31 October 2012, a Boeing 737-800 on go around after delaying the breaking off of a fast and high unstable ILS approach at Oslo lost separation in IMC against another aircraft of the same type and Operator which had just taken off from the same runway as the landing was intended to be made on. The situation was aggravated by both aircraft responding to a de-confliction turn given to the aircraft on go around. Minimum separation was 0.2nm horizontally when 500 feet apart vertically, both climbing. Standard missed approach and departure tracks were the same.
On 3 May 2017, an Airbus A330 and an Airbus A319 lost prescribed separation whilst tracking in opposite directions on a radar-controlled ATS route in eastern Myanmar close to the Chinese border. The Investigation found that the response of the A330 crew to a call for another aircraft went undetected and they descended to the same level as the A319 with the lost separation only being mitigated by intervention from the neighbouring Chinese ACC which was able to give the A319 an avoiding action turn. At the time of the conflict, the A330 had disappeared from the controlling ACCs radar.
On 12 April 2013, a Ryanair Boeing 737-800 took a climb clearance intended for another Ryanair aircraft on the same frequency. The aircraft for which the clearance was intended did not respond and the controller did not notice that the clearance readback had come from a different aircraft. Once the wrong aircraft began to climb, from FL360 to FL380, a TCAS RA to descend occurred due to traffic just transferred to a different frequency and at FL370. That traffic received a TCAS RA to climb. STCA was activated at the ATS Unit controlling both Ryanair aircraft.
On 18 June 2010, an ATR 42 began a daylight take off on runway 28 at Zurich without ATC clearance at the same time as an A340 began take off from intersecting runway 16 with an ATC clearance. ATC were unaware of this until alerted to the situation by the crew of another aircraft which was waiting to take off from runway 28, after which the ATR 42 was immediately instructed to stop and did so prior to the runway intersection whilst the A340 continued departure on runway 16 .
On 1 August 2007, the crew of a Beech 1900 aircraft holding on an angled taxiway at Auckland International Airport mistakenly accepted the take-off clearance for another Beech 1900 that was waiting on the runway and which had a somewhat similar call sign. The pilots of both aircraft read back the clearance. The aerodrome controller heard, but did not react to, the crossed transmissions. The holding aircraft entered the runway in front of the cleared aircraft, which had commenced its take-off. The pilots of both aircraft took avoiding action and stopped on the runway without any damage or injury.
- HS1 Analysis of an AIRPROX between a B737 and an A340;
- HS2 Seminole in California;
- HS2 Getting the Message Across;
- HS2 Analysis of an AIRPROX in Japan.
AGC Safety Letters:
EUROCONTROL Action Plan for Air-Ground Communications Safety, including:
The Action Plan for Air-Ground Communications Safety may be viewed as a whole. Alternatively, the Briefing Notes may be viewed separately as follows:
- AGC Briefing Note 1 - General;
- AGC Briefing Note 2 - Call Sign Confusion;
- AGC Briefing Note 4 - Blocked Transmissions;
- AGC Briefing Note 5 - Radio Discipline.
EUROCONTROL Call Sign Similarity (CSS) Service