47TH ANNUAL CONFERENCE, Arusha, Tanzania, 10-14 March 2008
WP No. 85
Study Numbering of Aircraft in the Aerodrome Traffic Circuit
Presented by TOC
The IFATCA Technical and Operations Committee (TOC) was asked to investigate numbering of aircraft in the aerodrome traffic circuit. One of the reasons for this request was an incident in Israel, whereby the use of the term ‘number’ in radiotelephony was considered to be one of the contributory factors of this incident. After reviewing the relevant ICAO publications, TOC concludes that the use of ‘number’ in phraseology appears to be insufficiently supported by ICAO rules and guidelines. As a result, controllers world-wide use the term frequently, and often also refer to other phases of flight than the aerodrome traffic circuit. Besides this, ICAO does not provide the desired clarity when it comes to the ending of the aerodrome traffic circuit. For these reasons, TOC has decided to propose draft recommendations, to provide the much needed clarity with the use of ‘number’.
1.1 At the Istanbul conference (2007), Italy requested to add to the Work Programme a study on numbering of aircraft in the circuit. Italy was under the impression that ICAO Standards and Recommended Practices (SARPs) were not giving sufficient guidance on the correct procedure with this numbering and how it should be applied. The simple question was asked; “when does number two become number one?”
1.2 Following an incident at Ben Gurion Airport (LLBG) in Israel, the Air Traffic Controllers’ Association of Israel also asked the Technical and Operations Committee (TOC) to investigate the use of numbering of aircraft in the aerodrome traffic circuit.
1.3 The purpose of this working paper is not to formulate a solution, but will merely list the relevant publications and will propose a way forward.
2.1.1 ICAO Doc 4444 Air Traffic Management, definitions:
“Aerodrome traffic. All traffic on the manoeuvring area of an aerodrome and all aircraft flying in the vicinity of an aerodrome.
Note.— An aircraft is in the vicinity of an aerodrome when it is in, entering or leaving an aerodrome traffic circuit.
Aerodrome traffic circuit. The specified path to be flown by aircraft operating in the vicinity of an aerodrome.”
The definition of an aerodrome traffic circuit is not completely clear, which automatically leads to difficulties with the numbering of aircraft in that aerodrome traffic circuit. When does ‘the specified path to be flown’ end in case of an aircraft going for a landing? Figure 7-1 on page 7-5 of ICAO Doc 4444 Air Traffic Management shows the following:
Based on the figure above, it must be concluded that the traffic circuit ends over the threshold of the runway-in-use. Even though this is merely a derivative and an interpretation of the figure above, it nevertheless is an answer to the question where the aerodrome traffic circuit, and the ‘specified path to be flown’, ends. After discussing this, TOC concluded that this answer to the question is acceptable and that aircraft in the aerodrome traffic circuit should lose their number in the aerodrome traffic circuit when passing the threshold.
2.1.2 ICAO Doc 4444 Air Traffic Management:
“220.127.116.11.3 Prior to entering the traffic circuit or commencing its approach to land, an aircraft shall be provided with the following elements of information, in the order listed, with the exception of such elements which it is known the aircraft has already received:
a) the runway to be used;
b) the surface wind direction and speed, including significant variations there from;
c) the QNH altimeter setting and, either on a regular basis in accordance with local arrangements or, if so requested by the aircraft, the QFE altimeter setting.
Note.— The meteorological information listed above is to follow the criteria used for meteorological local routine and special reports, in accordance with Chapter 11, 18.104.22.168.2 to 22.214.171.124.3.9.”
According to the text above, the number in sequence or number to land is not part of the items that must be provided to the aircraft prior to entering the traffic circuit, but it also does not rule out the use of it.
2.1.3 ICAO Doc 4444 Air Traffic Management:
“126.96.36.199 ESSENTIAL LOCAL TRAFFIC INFORMATION
188.8.131.52.1 Information on essential local traffic shall be issued in a timely manner, either directly or through the unit providing approach control service when, in the judgment of the aerodrome controller, such information is necessary in the interests of safety, or when requested by aircraft.
184.108.40.206.2 Essential local traffic shall be considered to consist of any aircraft, vehicle or personnel on or near the manoeuvring area or traffic operating in the vicinity of the aerodrome, which may constitute a hazard to the aircraft concerned.”
We should ask ourselves the question if the proceeding aircraft in the circuit should be classified as ‘essential local traffic’ in general. Fact however is that, if a pilot is expected to follow a certain aircraft (either to e.g. cross behind or simply in the aerodrome traffic circuit), then this pilot will need to be able to identify the traffic. It is left up to the judgement of the controller to establish when local traffic becomes ‘essential local traffic’.
2.1.4 ICAO Doc 4444 Air Traffic Management:
“220.127.116.11.3 Essential local traffic shall be described so as to be easily identified.”
If traffic is classified as ‘essential local traffic’, then the controller must describe the traffic in an understandable manner and a comprehensive way, with the purpose of identification by the pilot involved. No mention is made of using the number in the aerodrome traffic circuit as part of this, but the use of it is also not ruled out. The controller must be well aware that there are situations imaginable in which the use of ‘number’ in isolation is not enough to ensure identification. This could result in situations in which both the pilot and the controller are not aware of any mis-identification, with the possible safety implications.
2.1.5 ICAO Doc 4444 Air Traffic Management:
This is the first, and only, time that ‘NUMBER’ is used. It is understood that this refers to the number in the aerodrome traffic circuit. Besides this, no mention is made of numbering in the circuit anywhere but in the phraseology part of ICAO Doc 4444 Air Traffic Management. It is important to note the combined use of ‘FOLLOW’ with ‘NUMBER’ The use of ‘FOLLOW’ in this case assists the controller in describing essential local traffic with the objective of clear and unambiguous identification. If the information of aircraft type and position is not expected to be sufficient to meet this objective, then the controller is expected to pass additional instructions, and it is the controllers responsibility to use best judgement to assess what information is required.
2.1.6 According the relevant parts of ICAO Doc 4444 Air Traffic Management, a controller must describe ‘essential local traffic’. Only in that case information must be passed to the aircraft with the intention of enabling the pilot to identify this ‘essential local traffic’. In all chapters but Chapter 12 (Phraseologies), no mention is made of the term ‘NUMBER’. However, it is recognized that the use of ‘NUMBER’ is nowadays an often used method of describing essential local traffic.
2.1.7 In Chapter 12 of ICAO Doc 4444 Air Traffic Management, the term ‘NUMBER’ is introduced. The added value of using this term appears to be twofold;
– it can assist both the controller and the pilot in the visual identification process; and
– it can assist in acquiring or maintaining situational awareness for the pilot.
The term ‘NUMBER’ should therefore not be seen as a separate item to be used to identify relevant traffic, as it always needs to be used in combination with the term ‘FOLLOW’.
2.2 Ben Gurion (LLBG) incident
2.2.1 On the 8th of February 2007, at Ben Gurion airport in Israel, two aircraft got in close proximity. The incident occurred in the afternoon when a sequence of aircraft was approaching from the west for landing on runway 30. One of the two aircraft involved was executing an Area Navigation (RNAV) visual approach and came in close proximity of the other aircraft (1300 meters /0.7NM) at same altitude. One of the aircraft was on the end of base leg, and the other aircraft involved was on final. Both aircraft turned to avoid collision. One of the contributory factors of this incident was the fact that essential aerodrome traffic was pointed out with the use of the term ‘number’, which resulted in misunderstandings with the pilots involved as both the controller and the pilots used a different reference for the numbering.
2.2.2 One of the recommendations of the incident report is:
“To address ICAO in order to define the phase in which the landing aircraft stops to be number 1 or a factor in the numbering and to publish it in the DOC 4444” (Original text translated from report).
2.3 Controllers globally regularly use the term ‘NUMBER in phraseology. Combinations like: ‘number to land’, ‘number for departure’, ‘number in sequence’ and ‘number in approach’ have all been heard. This information is generally used as additional information to the pilot. Pilots in general appreciate any information that assists them in acquiring or maintaining situational awareness. The use of the term ‘NUMBER’, in all possible combinations, can therefore be very useful to pilots. However, it is important to note that the current unregulated use of ‘NUMBER’ in radiotelephony can easily lead to misunderstandings. An example of this is the possible difference between ‘number in approach’ and ‘number to land’ at aerodromes with a mix of VFR and IFR movements. It is of the utmost importance that phraseology is introduced to avoid any confusion between numbering of traffic in the aerodrome control circuit and other phases of flight.
3.1 The use of ‘number’ in phraseology appears to be insufficiently supported by ICAO rules and guidelines.
3.2 ICAO does not give sufficient guidance on the ending of an aerodrome traffic circuit. Only one figure in ICAO Doc 4444 Air Traffic Management appears to provide the desired and required clarity, namely over the threshold of the runway-in-use. This interpretation provides an acceptable answer and an interim solution, but further work is required by ICAO on the production of supporting rules and guidelines.
3.3 Numbering of aircraft is only used in ICAO Doc 4444 Air Traffic Management in the Phraseology chapter on traffic in the circuit. The use of the term ‘NUMBER’ in Radiotelephony should, when used in another phase of flight, be clearly differentiated.
3.4 If a controller uses ‘NUMBER … FOLLOW’ in Radiotelephony in relation to an aircraft in the aerodrome traffic circuit, then he or she needs to be aware that the use of that phraseology in isolation might not be sufficient to ensure identification of the correct aircraft.
3.5 The use of the term ‘NUMBER’ must be retained as it supports the pilot to acquire or maintain situational awareness.
It is recommended that;
4.1 IFATCA Provisional Policy is:
If ‘number’ is used in other than the aerodrome traffic circuit, i.e. for situational awareness, then the controller should use the phraseology:
‘NUMBER … IN [approach, taxi] SEQUENCE’.
and is included in the IFATCA Manual on page 3 2 3 17.
4.2 The IFATCA Executive Board brings the following issues to the attention of ICAO Montreal:
- Other uses of the term ‘number’
- How to number in the circuit
ICAO Doc 4444 Air Traffic Management, 14th Edition 2001 amendment 5.
Report on Ben Gurion incident, severe incident case number 3-07, part 1, published by the State of Israel, Ministry of Transportation by Chief Investigator Office Mr. Y. Raz.
Last Update: September 29, 2020