The use of similar call signs by aircraft operating in the same area and especially on the same RTF frequency often causes potential and actual flight safety incidents. This hazard is usually referred to as “call sign confusion”.
Impact of Callsign Confusion on Air Traffic
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.
Causes of Callsign 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 differ in only a single digit, which might not be pronounced or broadcast if the transmission is clipped (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 contain the same alphanumeric characters in a different order (e.g. AB1234 and BA 2314).
- Call signs contain repeated digits (e.g. RUSHAIR 322, RUSHAIR 232).
- 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);
Accidents and Incidents Involving Callsign Confusion
- loss of communication - 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.
- loss of separation - 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.
- level bust - 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.
- AIRPROX - 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.
- midair collision.
Defences Against Callsign Confusion
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.
Of course awareness of the impact of callsign confusion is the greatest defence, but the effect of language is considered to be an important factor for both native and non-native English speaking pilots and ATCOs. Aviation English Asia Ltd courses include exercises on detecting and clarifying discrepancies in callsigns, in addition to increasing awareness of the idiosyncrasies of different pronunciations of callsigns. This is particularly important for pilots who are flying within a new and unfamiliar region.
Adapted from an article on skybrary.