Reducing “Initial Call” Frequency Congestion

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Reducing “Initial Call” Frequency Congestion

55TH ANNUAL CONFERENCE, Las Vegas, USA, 14-18 March 2016

WP No. 165

Reducing “Initial Call” Frequency Congestion

Presented by PLC and TOC


The paper investigates the possible reduction in frequency congestion using the introduction of alternative procedures for aircraft initial calls to ATC units.


1.1  Frequency congestion is a significant problem for many ATC units and can reduce the efficiency of the airspace, or in some cases present a safety issue.

1.2  For units with a high ‘throughput’ of traffic, (i.e. many aircraft each handled for short periods), such as approach, director, ground, or tower, the initial call by aircraft has a significant impact on the congestion of the frequency. Evidence suggests it is reasonably common to have an aircraft making an initial call either interrupt a dialogue between a controller and pilot, or have a transmission ‘blocked’ (simultaneous transmissions) – requiring the repetition of the transmissions (European Action Plan for Air Ground Communications Safety, Eurocontrol 2006. Page 6).

1.3  There are existing ICAO procedures, and various alternative implementations globally that provide for the abbreviation or omission of the initial call under certain circumstances, to reduce frequency congestion.

1.4  During the Second Working Group meeting of the ICAO Air Traffic Management Operations Panel (ATMOPSP), in October 2015, the member from the United States presented a paper highlighting the discrepancies with various implementations of the use of MONITOR compared to the ICAO definition of the phrase, including datalink communications. After some discussion it was suggested the Panel could seek the view of the IFATCA membership on the matter and thus the outcomes of this paper will be presented at the next ATMOPSP meeting in April 2016 (Meeting Report, Second Working Group Meeting, ICAO Air Traffic Management Operations Panel, Dubai, UAE, 4-8 October 2015).

1.5  This paper will analyse all the available options, review any discrepancies in globally harmonised implementation, and recommend further action where appropriate.


2.1 Logically, an ideal initial step is to analyse the existing requirements for an aircraft when establishing initial contact with any ATC unit. International communications procedures are contained in several ICAO documents, primarily Annex 10 — Aeronautical Telecommunications, Volume II — Communication Procedures including those with PANS status, Annex 11 — Air Traffic Services, and PANS-ATM (ICAO Doc 4444), Procedures for Air Navigation Services — Air Traffic Management.

2.2 Paragraph of Annex 10, Vol II defines the meaning for certain terms to be used in communications, the relevant terms are:

CONTACT: “Establish communications with…” …

MONITOR: “Listen out on (frequency).” …

STANDBY: “Wait and I will call you.”


2.3 Similarly, PANS-ATM Chapter 12 contains the phraseologies to be used in Air Traffic Services and declares the following: TRANSFER OF CONTROL AND/OR FREQUENCY CHANGE

a) CONTACT (unit call sign) (frequency) [NOW]; …

d) STAND BY FOR (unit call sign) (frequency); …

g) MONITOR (unit call sign) (frequency); …

Note.— An aircraft may be requested to “STAND BY” on a frequency when it is intended that the ATS unit will initiate communications soon and to “MONITOR” a frequency when information is being broadcast thereon.


2.4 Referring back to Annex 10, paragraph provides for the contents of an initial transmission to be prescribed by authorities, usually this is local or national, but in some cases this could be regional authorities. When establishing initial contact on, or when leaving, a VHF frequency, an aircraft station shall transmit such information as may be prescribed by the appropriate Authority.


2.5 And shortly thereafter, paragraph explains actions to be taken in the event that communications cannot be established with an ATC unit, which although not directly relevant, it will need to be taken into consideration. When an aircraft station fails to establish contact with the appropriate aeronautical station on the designated channel, it shall attempt to establish contact on the previous channel used and, if not successful, on another channel appropriate to the route. …


2.6 Returning to PANS-ATM, there are several more applicable references; paragraphs and introduce specific items to be mentioned in an initial call. Element d), flight level or altitude, shall, however, be included in the initial call after a change of air-ground voice communication channel. When assigned a speed to maintain, the flight crew shall include this speed in their position reports. The assigned speed shall also be included in the initial call after a change of air-ground voice communication channel, whether or not a full position report is required.

Note.— Omission of element d) may be possible when flight level or altitude, as appropriate, derived from pressure-altitude information can be made continuously available to controllers in labels associated with the position indication of aircraft and when adequate procedures have been developed to guarantee the safe and efficient use of this altitude information.


2.7 Particular attention should be paid to an explanatory note attached to the above paragraphs which allows for the omission of level information where appropriate.

2.8 And finally, procedures specifically related to frequency changes:

4.11.3 Radiotelephony procedures for air-ground voice communication channel changeover

When so prescribed by the appropriate ATS authority, the initial call to an ATC unit after a change of air-ground voice communication channel shall contain the following elements:

a) designation of the station being called;

b) call sign and, for aircraft in the heavy wake turbulence category, the word “Heavy”;

c) level, including passing and cleared levels if not maintaining the cleared level;

d) speed, if assigned by ATC; and

e) additional elements, as required by the appropriate ATS authority.


2.9 So to paraphrase all the above requirements:

2.9.1 CONTACT means for the aircraft to change frequency and initiate communications with another unit. In this case the aircraft must specify:

a)  Station callsign

b)  Aircraft callsign (incl. Heavy if applicable)

c)  Level (and passing level)

d)  Speed (if assigned)

Additional elements may be introduced by the ATS authority, and in some cases elements may be omitted (such as level) provided that appropriate safety work has been conducted and the necessary alternative procedures are in place.

2.9.2 MONITOR means to listen on the advised frequency for a broadcast; normally ATIS or weather information. It is implied that this is in addition to the primary frequency, and does not appear to be intended for regular communication channels.

2.9.3 STAND BY FOR means to change frequency, but do not initiate contact; wait and the ATS unit will contact you shortly.

2.10. Depending on the unit in question, some of the information that is required to be included in the initial call is usually redundant.

2.10.1.  For instance, an approach sector will generally issue a lower level on first contact, therefore the requirement to include a ‘cleared level’ in the transmission (as a cross check) is questionable, as it will usually immediately change. In cases where a new level is not assigned, local procedures could require that the controller reissue the level to the pilot rather than the opposite – this would seem a more efficient option.

2.10.2.  Other information such as HEAVY/SUPER could be considered redundant where the controller display clearly highlights this to the controller, however this would have to be a consideration depending on local implementation. Similarly, Mode S downlink information has the capacity (where equipped) to provide information in a more efficient manner than verbally from the pilot, in this particular case; level and speed information. Although existing procedures in most places to for allow for this.

2.11.  In any case, there is clearly an opportunity to reduce the content of the initial call in certain circumstances. There are some existing ICAO procedures to achieve this, and also variations of such that have been implemented at a local level.

2.12.  There are known alternative implementations of CONTACT, specifically the use of CONTACT WITH CALLSIGN ONLY in several locations (London, Amsterdam, Dubai, Madrid, Frankfurt, Munich, Zurich (All except Amsterdam are reports from local controllers, January 2016. Details available is required)) which attempts to minimise the disruption of the CONTACT call, but retains the confirmation that the frequency transfer has successfully been conducted. However, as there is no internationally recognised version of this procedure some confusion is known to exist in the pilot community (Professional Pilots Rumour Network-, forum threads 240735, 340899, 368254, 518524, 552171. 7 January 2016). Transfer to Schiphol Approach

… While being transferred from Amsterdam Radar to Schiphol Approach, initial contact shall be restricted to SCHIPHOL APPROACH + CALL SIGN only in order to avoid channel congestion. In specific situations, Amsterdam Radar may request pilots to report additional information to Schiphol Approach in the initial contact

(The Netherlands AIP – Implementation of Callsign Only, AIP, The Netherlands, Eff. 7 January 2016. Part 3 Aerodromes (AD) AD 2.22 para


2.13.  The other options (MONITOR and STAND BY FOR) do not immediately provide confirmation that the aircraft has successfully selected and switched to the correct frequency. In the event that this has not occurred, there are may be delays encountered, and in some cases safety implications where aircraft are operating in high density airspace. While there are existing procedures to cater for the instance when communication transfer has failed, in some circumstances it is preferable to have confirmation built into the procedure.

2.14.  The MONITOR instruction, while it appears to be intended for broadcast information, is known to be used in multiple locations by towers as a transfer instruction, and by oceanic units when referring to HF frequencies for SELCAL watch.

2.15.  In reference to the use on the ground, the known implementations consist of either the verbal instruction to MONITOR GROUND or aerodrome signage instructing aircraft to MONITOR TOWER (freq). While the intention is clear, and the potential for any misunderstanding is minimal, it appears that the correct phraseology that should be used in this case is in fact STAND BY FOR (unit) ON (freq). As these implementations include the use of fixed aerodrome signage, any consideration on amending the phraseology will necessitate the expense of amending signage.

2.16 With regard to the use in oceanic control, the term MONITOR is generally used in conjunction with CPDLC as an instruction to switch to the advised HF radio frequencies and maintain a SELCAL watch. In this case the term STAND BY FOR initially appears to be more appropriate, however in many cases communications may never actually be conducted on the HF channel. Other than for purposes of performing a SELCAL check (the check is not mandatory in all remote airspace), in normal circumstances communications will continue by CPDLC for the extent of the flight and the use of HF will not be needed.

2.17 As the CPDLC message set is hard coded and fixed for at least the next decade (ICAO State Letter SP 52/4-15/44 (12 June 2015) Proposals for the amendment of Annexes 4, 6, Parts I, II and III, 10, Volumes II and III, 11, 15, PANS-ABC (Doc 8400) and PANS-ATM (Doc 4444) relating to DLIC, CPDLC, ADS-C, PBCS and SATVOICE arising from OPLINKP/2), there is no real possibility to replace MONITOR with the STAND BY FOR message and the continued use of MONITOR in this circumstance should not introduce any misunderstanding.

Maastricht UAC is known to have implemented use of the STAND BY FOR procedure in high density enroute airspace (“Stand by for Maastricht” Accessed 7 January 2016). However, the interpretation of the procedure, which has altered it into effectively a CALLSIGN ONLY procedure and the subsequent clarification has increased confusion (Professional Pilots Rumour Network-, forum thread 558646. Accessed 7 January 2016).


3.1.1 Where the standard CONTACT transmission is considered too verbose for the receiving unit, the existing alternative STAND BY FOR should provide an improvement in frequency and workload management. It does not however provide confirmation that the transfer has occurred. Where this confirmation is desirable, the CALLSIGN ONLY procedure has been introduced in several regions. Locations that utilise this procedure seem to be pleased with the result. It would be beneficial for global harmonisation if there was an ICAO procedure defining CALLSIGN ONLY to be used in lieu of local procedures.

3.2 The existing ICAO definition for MONITOR is appropriate, and should generally be used to refer to broadcast information or other channels where an actual dialogue of communication will not normally take place. Such examples are ATIS, AWIS, 121.5.

3.2.1 There are implementations of MONITOR in some locations – generally towers – that are inconsistent with the ICAO definition, and would be more appropriate as STAND BY FOR. It would simplify global harmonisation if these implementations were changed to STAND BY FOR, however it is recognised that in some locations this would require the replacement of existing aerodrome signage at a cost.

3.2.2 The use of CPDLC had encouraged the use of MONITOR in the oceanic environment, usually when referring to HF frequencies. In consideration of the limitations of amending the CPDLC message set, and the nuances of HF SELCAL watch, it is not considered feasible or practical to correct the use of the term in this environment.

3.3 The ICAO definition for STAND BY FOR seems appropriate, however it does not provide confirmation that the transfer had taken place. In some circumstances this may be appropriate, especially for ground control purposes, however there are no known implementations of this version of the procedure.

3.4 Some locations have implemented an amended version of STAND BY FOR which is effectively CALLSIGN ONLY. This has created significant confusion and these locations should amend their implementation to either conform with the ICAO version of STAND BY FOR, or align as close as possible with existing implementations of CALLSIGN ONLY in lieu of an ICAO version of this procedure.


It is recommended that:

4.1. The Executive Board urge ICAO to consider:

a)  A globally standardised “CALLSIGN ONLY” procedure.

b)  Clarification and global harmonisation of MONITOR and STAND BY FOR.

Last Update: October 1, 2020  

February 11, 2020   821   Jean-Francois Lepage    2016    

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