Summary

IFATCA Policy on alpha-numeric call signs has been around for more than 20 years and therefore urgently needs to be reviewed. As part of this review, ICAO documentation has been checked and other relevant information such as the European Action Plan for Air Ground Communications Safety, IFALPA Policy and an Aeronautical Information Circular from the United Kingdom have been considered. Finally, operational experience from the Netherlands, where the use of alpha-numeric call signs has become widespread, has also been considered. The outcome of this review is that IFATCA Policy needs to be amended and expanded, which is reflected in the Draft Recommendations in this working paper.

Introduction

1.1 The Netherlands Guild of Air Traffic Controllers has adopted IFATCA Policy as its own, endeavouring to be in full compliance with it. The Dutch Guild tries to make use of experience and knowledge of IFATCA whenever possible.

1.2 IFATCA Policy on alpha-numeric call signs has been accepted in 1984, and amended, deleted and newly adopted in the years thereafter. The last working paper with Policy recommendations on this subject was presented in 1987.

1.3 The Netherlands Guild believes that IFATCA Policy should be reviewed regularly, preferably every 10 to 15 years or when developments are of such nature that review is required.

1.4 Alpha-numeric call signs have been introduced to alleviate the problems associated with call sign confusion This working paper will not address the whole subject of call sign confusion, but will specifically review IFATCA Policy on alpha-numeric call signs.

1.5 An alpha numeric call sign is one where the suffix consists of number(s) followed by one or more letters or of number(s) followed by a combination of letters and numbers.

Discussion

2.1 IFATCA Policy

2.1.1 IFATCA Policy on alpha-numeric call signs (Com 4.1 page 3241 IFATCA Technical and Professional Manual) is:

2.2 ICAO

2.2.1 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.”

2.2.2 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.”

2.2.3 Alpha-numeric 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 alpha-numeric 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.

2.3 European Action Plan for Air Ground Communications Safety

2.3.1 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 bursts and runway incursions, where communication problems are a contributory factor. A separate briefing note on call sign confusion is included in this plan.

2.3.2 The briefing note on call sign confusion has a section on numeric vs. alpha-numeric 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 deconflict any similar numeric call signs. Where there is no solution to those call signs that have a potential for numeric confusion, alpha-numeric call signs can be adopted.”

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 to a larger variation in call signs. It is true that the potential for confusion remains, but the likelihood of occurrence is certainly much smaller.

2.3.3 Under Recommendations for Aircraft Operators the following is mentioned:

6.6. “If alpha-numeric 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).”

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

2.4 United Kingdom Aeronautical Information Circular (AIC)

2.4.1 In the United Kingdom an AIC on radiotelephony (RTF) call sign confusion has been published, ref. AIC: P 054/2009 – 16-JUL-2009.

2.4.2 In this AIC the following definitions of numeric- and alpha numeric call signs are used:

“A numeric call sign is one where the suffix consists of numbers only. An alpha numeric call sign is one where the suffix consists of number(s) followed by one or more letters.”

This definition is not complete, but nevertheless a good basis for a possible IFATCA definition of alpha-numeric call signs. The definition above does not consider call signs where the prefix consists of number(s) followed by a combination of letters and numbers (e.g. BTI6C4) as an alpha numeric call sign, while it is the authors believe that these call signs should also be considered as an alpha-numeric call sign. An amended definition is recommended for inclusion in the IFATCA Technical and Professional Manual.

2.4.3 The AIC also reports on call sign confusion occurrences and provides a division per type of call sign. The result is that the majority (84%) of the incidents involved numeric only call signs, while only 10% involved alpha numeric only call signs. This difference is extensive, but it must be said that there are no numbers presented together with the percentages.

2.4.4 The following specific guide lines to help reduce call sign confusion are mentioned:

g. “exhaust numerical possibilities first, before using alpha-numeric call sign systems. If alpha-numeric call signs are inevitable, co- ordinate letter combination with existing operators, taking into account all other airspace and airport users;

h. try to avoid using alpha-numeric call signs which correspond to the last two letters of the destination’s ICAO location indicator eg ABC 96LL for flight inbound to London Heathrow where the ICAO indicator is EGLL; …

n. avoid, whenever practicable, flight numbers ending in a zero or five eg 5 may be confused visually with s and zero when combined with two digits, ie 150, may be confused with a heading/level; …

p. in alpha-numeric call signs avoid phonetic letters that can be confused with another operator designator prefix eg D – Delta (the airline).”

Again, operational experience has lead to important recommendations for a safe application of alpha numeric call signs. Zero and five are specifically mentioned as numbers that might lead to visual confusion, experience from an ATC perspective is that confusion is not only limited to these numbers. As letters and numbers are often used at random, the controller can be confused when a letter looks like a number on an ATC display. This is specifically the case when the letters B, I, O, S and Z are used as these look much like respectively numbers 8, 1, 0, 5 and 2.

2.5 IFALPA Policy

2.5.1 IFALPA does not have Policy specifically on alpha-numeric call signs. The only relevant Policy in IFALPA Annex 10 (Aeronautical Telecommunication) is the following:

“5.2.1.7 Calling

5.2.1.7.2 Radiotelephony call signs for aircraft

Where airline flight numbers are used as call signs, they should be so selected that pairs of flight number call signs likely to cause confusion on the lines indicated in i) and ii) above do not occur for aircraft in flight on the same ATC sectors. All callsign systems should be tested in a simulation of airline traffic with regard to possible misidentification of flights or callsign confusion by pilots or controllers. Possible causes of callsign confusion found should be eliminated by changing the callsigns of the flights concerned.

The computer studies should encompass flights of all airlines operating in a given area and be repeated at least twice a year to consider the changes in flight plans.”

2.5.2 IFALPA had Policy on an alternative methodology for call signs composition. This Policy has recently been deleted, the reasons for this decision are unknown.

2.6 Alpha numeric call signs in Europe and specifically the Netherlands

2.6.1 On average, approximately 30% of all flights to Amsterdam hold an alpha-numeric call sign. This gives a clear indication of the extent to which these call signs have found their way into present day ATM.

2.6.2 A large number of airlines in Europe use alpha numeric call signs, including KLM, Ryanair, Austrian Airlines, FlyBe, Lufthansa, British Airways, Finnair, Air Berlin, Brussels Airlines, Wizzair, German Wings, Swiss, Iberia, Air Lingus, and easy Jet.

2.6.3 Even though the use of alpha numeric call signs are widespread in Europe, all airlines use different compositions of letters and numbers. It is difficult to identify a single method, as it appears that all airlines have different composition methods and/or all compose the call signs with random use of letters and numbers. KLM call signs are always two numbers followed by one letter, e.g. KLM27Y, KLM40M and KLM53W. Easy Jet uses one, two or three numbers and one or two letters, e.g. EZY12GW, EZY7FT and EZY621T. Ryanair uses one or two numbers followed by two letters, e.g. RYR7BK, RYR18KH and RYR2PG. British Airways appears to use letters that are associated with the destination aerodrome (e.g. use of AM in the call sign for flights to Amsterdam [EHAM], and use of DL in the call sign for flights to Dusseldorf [EDDL]). Air Lingus as a last example uses different combinations and thereby does not always group numbers and letters, e.g. EIN6G8.

All these different methods and compositions could still be placed under one single method of application or system. This system could be based on random use of letters and numbers, but should prescribe that all start with a number as part of the prefix. The reason for starting with a number is to avoid confusion with the three letter ICAO designator of the Operator.

2.6.4 It is not always easy to differentiate certain letters from numbers on the ATC display, especially because airlines use letters and numbers randomly. This issue can easily lead to an increase in controller workload as the controller would have to spend more time at identifying the correct call sign, or with the use/readback of wrongly used call signs. Even though the design of the HMI could alleviate this problem, it is still desirable to avoid any issues with this potential confusion by simply not using the letters involved.

4 examples of alpha numeric and numeric call signs
as presented on the ATC display

2.6.5 Operational experience with the use of letters instead of numbers is that it can lead to an increase in workload. This increase is a logical consequence of the fact that many letters comprise of multiple syllables, while the majority of the numbers do not. The use of letters instead of numbers will lead to more syllables and more syllables lead to longer transmission times. Longer transmission times lead to a higher workload for both controllers and pilots. Sector capacity values are not always re-established based on the change from numeric call signs to a combination of numeric and alpha numeric call signs.

2.7 Review of IFATCA Policy

IFATCA Policy on alpha numeric call signs is very extensive and addresses several items in one Policy statement. For this reason the Policy is reviewed in parts.

2.7.1

This statement is addressing more than alpha-numeric call signs alone. The first part of the sentence addresses the need for controllers and pilots to adhere to RTF procedures in order to eliminate part of the call sign confusion. Even though it is correct to state that adherence to procedures will eliminate much of the issues, it is the authors believe that there are more issues that play a role when it comes to call sign confusion, especially since the alpha numeric call sign has been introduced. Besides those issues, it is also the author’s believe that IFATCA Policy should not be written to stress the need to adhere to existing procedures. After all, if the procedures are not good, then the correct use of those procedures will not alleviate the problem. The second part of the sentence touches on a need to have a limited national use of certain alpha-numeric call sign systems to further reduce ambiguity. KLM uses alphanumeric call signs for their European operation, and numeric call signs for their intercontinental flights. A high number of airlines are operating to Amsterdam using alpha-numeric call signs, so this demonstrates that there are several airlines using those call signs for other than national use. This also shows that there is no limited use of alpha-numeric call signs, but that it has become widespread and common use in ATM. Basically, reality has overtaken IFATCA Policy. Finally there is no proof available that demonstrates that the use of alpha-numeric call signs reduce ambiguity problems, as it also introduces new and other ambiguity problems. Based on the above arguments, the first part of the Policy statement is recommended for deletion.

2.7.2

The first part of the statement refers to alpha-numeric call sign systems. It is true that there is no one single agreed system, there are however several composition methods as shown in para 2.5.3. Each of the methods composes the call signs based on different rules, but fact is that these methods are all, and some more than others, operationally suitable as these are in use today. All these methods should be grouped under one single system and that system can be based on random use of letters and numbers, but always starting with a number as prefix.

The solution that is mentioned in IFATCA Policy, different separate call sign systems, has not found its way to ICAO documentation. This is logical, as ICAO normally does not list detailed requirements for application of a Standard. The alpha-numeric call signs in use are in full compliance with ICAO Annex 10 Standards, including the rules for abbreviation. It is the authors believe that these Standards are not the issue, but the operational application of those Standards are. Therefore the amended Policy recommendation will still ask for one single system, will make reference to ICAO Standards, and at the same time will put emphasis on the operational issues.

2.7.3

It has become practice that IFATCA lists requirements in its Policy. This is a proactive way to initiate developments and at the same time influence it. The list above is very detailed and difficult to make suitable for all regions of the globe. Besides this, operational experience with the use of alpha-numeric call signs has taken us one step further. IFATCA should still list requirements, but some of the above should be deleted as other safeguards have been implemented. Other requirements should be made more generic and yet again others should be made in line with ICAO Annex 10. Some of these requirements will find their way to the draft recommendation as an amendment to Policy.

2.8 This working paper has, as written in paragraph 1.4, not addressed the principle of call sign confusion but only and specifically alpha-numeric call signs. There are nevertheless issues that are equally relevant to alpha-numeric call signs as to numeric call signs. Amongst these are;

• Possible confusion when two airlines are within one ATC sector at a single moment in time, both with a call sign that end on the same numbers and/or letters (e.g. KLM12D and EZS452D); and
• Confusion with headings, levels and speeds when a call sign ends on a 0 or a 5 (e.g. KLM4E50). It would be beneficial to IFATCA if these issues were also investigated when TOC performs a study on call sign confusion.

Conclusions

3.1 There is no agreed definition on alpha-numeric call signs. This definition is needed for IFATCA to provide guidance to its MAs, as it will provide a basis for a universal applicable system but also because ICAO does not have specific provision on alphanumeric call signs. This IFATCA definition should ensure that all combinations that are in use today will fit under it.

3.2 ICAO Annex 10 has Standards on call signs. Alpha-numeric call signs in general are in accordance with ICAO regulations, as the term ‘flight identification’ is not further detailed and AOs can therefore also compose a flight identification with a combination of letters and numbers. However, requirements for the safe application of alphanumeric call signs are lacking from ICAO Standards.

3.3 The use of alpha numeric call signs has become widespread in Europe, and alpha numeric call signs appear to be ‘here to stay’. Operational experience is that this concept is safe and workable, but consideration must be given to workload associated with the use of letters instead of numbers, and with the potential confusion for ATC with certain letters.

3.4 No sound evidence is found to support the statement that the use of alpha-numeric call signs leads to a reduction in call sign confusion, even though operational experience is that it does. On the other hand, an increase in cases of call sign confusion is also not demonstrated.

3.5 There is no agreed universally applicable system for the application of alpha-numeric call signs. Such a global system is needed to ensure a safe use of alpha-numeric call signs and to limit confusion to the maximum extent possible. There are several alphanumeric call sign composition methods in use today, and all these methods should be grouped under one single system and that system can be based on random use of letters and numbers

3.6 Experience has lead to a number of recommendations by various organizations for use of alpha numeric call signs. IFATCA must make use of these recommendations and list requirements for a universal system for the establishment of alpha numeric call signs.

3.7 Specific issues with the potential for visual confusion on ATC displays must be mitigated by a limitation to use the letters I, O, B, S and Z.

3.8 Call sign confusion is a subject that is much broader than alpha-numeric call signs alone. It is beneficial to IFATCA if TOC studies the subject of call sign confusion.

Recommendations

It is recommended that;

4.1 An alpha-numeric call sign is defined as:

“An alpha numeric call sign is one where the suffix consists of:

• number(s) followed by one or more letters; or
• number(s) followed by a combination of letters and numbers.”

and is included on page 3 2 4 1 of the IFATCA Technical and Professional Manual.

4.2 IFATCA Policy on page 3 2 4 1 of the IFATCA Technical and Professional Manual:

“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.”

is replaced by:

“A universally applicable system for the use of alpha-numeric 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 flight bound for Amsterdam (EHAM)) shall not be used;
• in alpha numeric call signs phonetic letters that can be confused with another operator designator prefix (e.g. Delta) shall not be used; and
• alpha numeric call signs shall not comprise of the letters B, I, O, S and Z in the flight identification because of the potential visual confusion with 8, 1, 0, 5 and 2.”

4.3 IFATCA Policy is:

“The introduction of alpha-numeric call signs in an ATM system can lead to an increase in work load for both controllers and pilots. An increase in workload shall be mitigated in order to maintain relevant safety levels.”

and is included on page 3 2 4 1 of the IFATCA Technical and Professional Manual.

4.4 “Study Call Sign Confusion” is included in the 2012 TOC Work Program.

References

IFATCA Technical and Professional Manual 2010.

IFALPA Annex 10 (Aeronautical Telecommunications).

ICAO Annex 10 (Aeronautical Telecommunications), Volume II Communication Procedures including those with PANS status, Sixth Edition October 2001.

European Action Plan for Air Ground Communications Safety, Edition 1, May 2006, https://www.eurocontrol.int/safety/gallery/content/public/library/AGC_action_plan.pdf

United Kingdom Aeronautical Information Circular, AIC: P 054/2009 – 16-JUL-2009, https://www.nats-uk.ead-it.com/aip/current/aic/EG_Circ_2009_P_054_en.pdf