call-sign Confusion

Feature Desk, Aviation Gurukul, GOLN

3 years ago

Call-sign Confusion

Description

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 of 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 that correspond to the last two letters of the destination’s ICAO location indicator (e.g. RUSHAIR 25LL for a flight inbound to London Heathrow);

Effects

Loss of communication, loss of separation, level bust, AIRPROX, or mid-air collision.

Defenses

Many larger airlines operate call sign de-confliction programs. 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.

Typical Scenarios

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 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.

Contributory Factors

Failure of an operator to give sufficient consideration in the allocation of RTF call signs.
Pilot workload.
Controller workload.
Interruption or distraction.
High rates of climb or descent.
Level bust in holding patterns.
Airspace and procedure design.
The density of traffic.

Solutions

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 a 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 minimize 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 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 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 program (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) requires 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 numbers: MDSTRl, RDDL172, ABX91, AAL351A, and SWA2604.

Examples of FAA unacceptable call sign designators with flight numbers: RDDL1720 (more than seven characters), NEWS42G (additional alphabet letter used for the non-scheduled operator), BKA16CH (two alphabet letters), and LBQ17523 (more than seven characters).