This paper investigates the concept of call sign confusion, and particularly the use of alphanumeric call signs. It develops the ideas, research and recommendations from WP98 2011 presented at the 50th Annual IFATCA conference.

A revised policy is recommended.

1.1  At the 50th Annual IFATCA Conference in Amman, Jordan, The Netherlands presented agenda item B.5.12, WP98, regarding the use of alphanumeric RTF call signs and their potential for confusion with other similar call signs, and presentation on a radar display.

1.2  This paper generated much debate and discussion, and therefore the IFATCA EB suggested to the audience that the topic was added to the TOC work programme 2011/12. This motion was carried. This paper is an extension of the work initially carried out by The Netherlands’ MA last year.

1.3  WP98 2011, researched the issue to some considerable depth, and some of the findings from that paper are encompassed into this one.

2.1 Current IFATCA Policy as adopted in Amman is:

2.2  Prior to Amman, IFATCA policy encompassed some of the ideas mentioned above, albeit in a more general format:

The correct use of laid down RTF procedures by both pilots and controllers would eliminate much call sign confusion, and the limited national use of certain alpha-numeric call sign systems may further reduce call sign ambiguity problems. However, available evidence shows that no system is operationally suitable for universal application and consideration should therefore be given to the possibility of having separate call sign systems for domestic, international short haul, and international flights, provided that in respect of international flights ICAO Annex 10 be specific as to the construction of these call signs and must permit their use in both full and abbreviated form.

Any alpha-numeric system considered for use must :

a)  be capable of abbreviation;

b)  not require a large proportion of the call signs to be used in any one geographical area to end in the same character;

c)  be suitable for use on electronic data display;

d)  use more letters than numbers;

e)  be capable of showing amendments from a previously filed flight plan.”

2.3  The risk of an aircraft acting on a clearance intended for another is obvious:

• The aircraft changes from the appropriate frequency;
• The aircraft deviates from a previous level assignment to one to which it is not cleared;
• The workload of controllers and pilots is increased because of the necessity to resolve the confusion;
• The controller misunderstands the intentions of aircraft and their control;
• Whilst responding to a message, the aircraft blocks a transmission from the intended recipient.

2.4  Any of the situations mentioned above (not an exhaustive list), may result in a level bust at the least, or even a loss of separation.

2.5  ICAO

2.5.1 Annex 10 (Aeronautical Telecommunications), Volume II Communication Procedures including those with PANS status, states the following on radiotelephony call signs for aircraft:

“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.”

2.5.2  The following is written on abbreviating call signs:

“5.2.1.7.2.2 Abbreviated call signs

5.2.1.7.2.2.1 The aircraft radiotelephony call signs shown in 5.2.1.7.2.1.1, with the exception of Type c), may be abbreviated in the circumstances prescribed in 5.2.1.7.3.3.1. Abbreviated call signs shall be in the following form:

Type a) — the first character of the registration and at least the last two characters of the
call sign;

Type b) — the telephony designator of the aircraft operating agency, followed by at least
the last two characters of the call sign;

Type c) — no abbreviated form.”

2.5.3  Alphanumeric call signs are not specifically mentioned in Annex 10, nor in any other ICAO document. This however does not imply that ICAO does not endorse the use of alphanumeric call signs. The term “flight identification” used in 5.2.1.7.2.1.1. type c) of Annex 10 Volume II leaves sufficient freedom for Airline Operators (AOs) to use alphanumeric call signs. It however does not give any guidance on how those call signs should be composed.

2.6 European Action Plan for Air Ground Communications Safety

2.6.1  In 2006 a European Action Plan for Air Ground Communications Safety was finalized, which was produced by Eurocontrol in close cooperation with various organizations like IATA, European Cockpit Association (ECA), Flight Safety Foundation (FSF) and IFATCA. The plan addresses the subject of air-ground communications and provides recommendations that should assist in reducing the number of incidents, including level busts and runway incursions, where communication problems are a contributory factor. A separate briefing note on call sign confusion is included in this plan.

2.6.2  The briefing note on call sign confusion has a section on numeric vs. alphanumeric call signs. It states:

4.1. “Many airlines continue to use their IATA commercial flight numbers as call sign suffixes. However, because they tend to be allocated in batches of sequential and very similar numbers, call sign confusion occurs.

4.2. Several airlines have switched to alpha-numeric call signs reasonably successfully in recent years. However, if every operator adopts alpha-numeric call signs, the limited choices available within the maximum of 4 elements allowed within a call sign suffix means that call sign confusion, similar to the existing numeric system, is likely to result.

4.3. Before changing to an effective all-alpha-numeric call sign system, which involves a significant amount of work, it is recommended that operators review their existing numeric call sign system to de-conflict any similar numeric call signs. Where there is no solution to those call signs that have a potential for numeric confusion, alphanumeric call signs can be adopted.”

2.6.3  The statement on the limited choices available because of the maximum of four elements within a call sign suffix, and the fact that the potential of call sign confusion is similar to the existing systems is remarkable. From a mathematical perspective this can be challenged, as having a choice from 26 letters in stead of ten numbers leads toa larger variation in call signs. It is true that the potential for confusion remains, but the likelihood of occurrence is certainly much smaller.

2.6.4 Under Recommendations for Aircraft Operators the following is mentioned:

6.6. “If alphanumeric suffixes are to be used, coordinate letter combinations with other airspace and airport users.

6.7. Do not use alpha-numeric call signs 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).

[…]

6.11. Do not use similar/reversed digits/letters in alpha-numeric call signs (e.g. RUSHAIR 87MB and RUSHAIR 78BM).”

2.6.4 The conclusion on the above is that there is no method of application available for universal use. However, the experience with the use of alphanumeric call signs has lead to a number of important and valuable recommendations.

2.7 DGAC (France) research

2.7.1 Approximately 800 safety occurrences related to similar call signs were collected by the French regulator (DGAC) in 2003. These included almost 100 that had a direct safety impact leading to STCA alerts, level busts, clearance misunderstandings and AIRPROXs.

2.8 UK CAA research and publications

2.8.1  The increasing use of alphanumeric call signs in ATM has contributed to a possible reduction in the frequency of call sign confusion by both ATC and pilots.

2.8.2  Although an older study, the UK CAA, together with partner bodies such as the airlines and ANSPs created the ACCESS (Aircraft Call-sign Confusion Evaluation Safety Study) initiative in 1997. A total of 482 reports regarding call sign similarity were collated from both ATCOs and pilots. Of the 72 that were deemed to be of higher severity;

• 405 (84%) involved numeric only call signs,
• 51 (10%) involved alpha-numeric only call signs,
• 17 (4%) involved a combination of the above,
• 9 (2%) did not contain sufficient information.

2.8.3  The UK CAA in their publication AIC 107-2000 recommend that an important factor in reducing the number of confusion events was the correct use of standard phraseology and RTF techniques;

• use caution when language difficulties may exist,
• advise adjacent sectors if it is felt that potential confusion may exist between aircraft likely to enter their airspace,
• do not clip transmissions,
• do not use readback time to execute other tasks,
• ensure clearances are readback correctly,
• monitor flight crew compliance with RTF call sign use.

2.9 NLR (The Netherlands) research

2.9.1 A wider study conducted by the Dutch regulator (NLR) regarding 444 occurrences, found that the contributory factors cited in communication problems involving similar call signs were related to human factors;

• controller accent (34%),
• controller speech rates (28%),
• pilot distraction (25%),
• pilot expectation (22%),
• pilot fatigue (20%).

They also found two common factors that include frequency congestion (28%) and blocked transmissions (30%).

2.10 Eurocontrol Call Sign Similarity Tool

2.10.1  Eurocontrol have recognised the safety implications of call sign similarity and created the “Call sign similarity risk reduction program” in April 2008. Their results have been published and implemented as part of the CSS Tool (Call Sign Similarity) in September 2011. The following paragraphs give an insight into this project.

2.10.2  The project was split into 3 phases, called service levels:

Service Level 0

• Establishing a Call Sign Management Cell (CSMC)
• Developing the CSS Tool
• Publishing Similarity Rules

Service Level 1

• Single Aircraft Operator De-confliction
• Supported by the CSS Tool
• Reduction rate equalled 74%

Service Level 2

• Multi Aircraft Operator De-confliction (over 20, incl., Air France, Swiss, Lufthansa, easyJet, and Thomson) between different schedules
• The combined reduction between level 1 and 2 = 80%

2.10.3 The similarity rules that were published to individual aircraft operators enabled a quick win scenario, before the use of the CSS Tool was required.

These rules are as follows:

Figure 1: Eurocontrol CSS Rules

2.10.4  These rules are used prior to the publishing of aircraft operator schedules (IATA season) up to 18 months ahead in 4 stages (Use Case UC):

• UC1 – during commercial improvements i.e., a new route or new numbering policy (6+months ahead)
• UC2 – when ATC call signs are attributed in a single aircraft operator
• UC3 – ad-hoc improvements from ATC reports when in service
• UC4 – (actually before UC3) a sanity check performed by the CSMC prior to the new IATA season.

2.10.5  European airspace users when using the Central Flow Management Network Operations Portal (CFMU NOP) to input their schedules are invited to use the CSS Tool to indentify possible similarity problems. The use of the NOP is a requirement for scheduled operators.

2.10.6  The tool works as follows:

1. Build

• The schedule is uploaded into the NOP and a CSS check initiated. The tool associates an airspace profile for each flight in the schedule from a profile database.

Figure 2: Example of CSS Tool user display.

2.11 Design of Radar Displays

2.11.1  The design of radar displays, and the fonts that are used to display aircraft call signs may contribute to some occasions when letters and numbers can be confused (see 2011 WP98 B.5.12 presentation).

2.11.2  This is a local phenomena and one that has not been highlighted by the various work studies that have been completed by agencies. It would, therefore, be reasonable to suggest that this topic is not related to call sign confusion, but individual display design. It would not be reasonable to require all aircraft operators to consider restricting the use of some characters, in order to comply with one ATC sectors’ equipment.

2.11.3  Individual display design should incorporate ATCO involvement from the very beginning and throughout the life of the system through upgrades. No IFATCA policy exists which relates to this process.

3.1  The risk of call sign confusion remains a threat to ATM safety.

3.2  The use of alphanumeric call signs can reduce the number of confusion related safety incidents, however these call signs need to be managed by ICAO, and reviewed by national regulators, aircraft operators, ATCOs and pilots.

3.3  In cases where the type font of a radar label is difficult to read, action should be taken at a local level into the radar display design.

3.4  No evidence has been found that the use of alphanumeric call signs is solely responsible for an increased workload for controllers.

3.5  ICAO should use the recommendations from the reports widely available from individual agencies and regulators to filter the requests for aircraft operators identification suffixes.

3.6  ICAO shall adopt the recommendations of the UK CAA CAP 700 publication, and the Eurocontrol Briefing Note 110 to provide a guide for all parties involved in the allocation of call signs.

3.7  The use of tools such as the Eurocontrol CSS Tool, are expected to reduce call sign similarity events significantly.

3.8  Standard use of correct RTF procedures shall be employed by both ATCOs and pilots in order to mitigate the possible risks associated from call sign confusion.

4.1  It is recommended that:

IFATCA Policy is:

To reduce the possibility of call sign confusion:

• Call signs that correspond to the last two designators of both ICAO and IATA airport designators shall not be used.
• In alphanumeric call signs, aircraft operator designators shall not be chosen that will create confusion with phonetic letters.
• The use of tools that reduce possible call sign similarity shall be encouraged.

And is included in the IFATCA Technical and Professional Manual.

4.2  It is recommended that:

IFATCA Policy on Call sign Confusion

A universally applicable system for the use of alphanumeric call signs should be developed. This system should consider at least the following requirements:

• full compliance with ICAO Annex 10;
• call signs which correspond to the last two letters of the destinations ICAO location indicator (e.g.ABC12AM for a flight bound for Amsterdam (EHAM)), or the IATA airport indicator (e.g. ABC1AMS for a flight bound for Amsterdam (AMS)) shall not be used;
• in alphanumeric call signs, phonetic letters that can be confused with another operator designator prefix (e.g. Delta) shall not be used, and;
• alphanumeric call signs shall not comprise of the letters B, G, I, O, S and Z in the flight identification because of the potential visual confusion with 8, 6, 1, 0, 5 and 2.

The introduction of alphanumeric call signs in an ATM system can lead to an increase in workload for both controllers and pilots. An increase in workload shall be mitigated in order to maintain relevant safety levels.

Is deleted.

DGAC information brief.

Eurocontrol Briefing Note 110.

Eurocontrol Call sign Similarity Tool.

Eurocontrol Action Plan for Air Ground Communications Safety, 2006.

ICAO Annex 10 (Aeronautical Telecommunications), Volume II Communications Procedures IFATCA WP98, 2011.

NLR information brief.

UK CAA CAP 704.

UK AIC 107-2000.