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ATC Unit Coordination

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Category: Loss of Separation Loss of Separation
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This article describes the coordination procedures between different ATC units. It describes the types of coordination used (verbal and non-verbal), the procedures used and provides ideas for best practices to be used during coordination.

The article does not cover the coordination procedures:

  • within a planner-executive team;
  • between different sectors of the same ATC unit;
  • between ground and tower operating at the same airfield (described in a dedicated article).

Transfer of Control Stages

The coordination and transfer of control of a flight between successive ATC units is done by a dialogue comprising the following stages:

  • notification of the flight in order to prepare for coordination, as necessary;
  • coordination of conditions of transfer of control by the transferring ATC unit;
  • coordination, if necessary, and acceptance of conditions of transfer of control by the accepting ATC unit;
  • the transfer of control to the accepting ATC unit or control sector.

Coordination between ATC Units

The procedures for coordination between ATC units are described in various documents, the most notable being:

  • Letters of Agreement (LoA), especially when the two units belong to different ATC centres or ANSPs.
  • Manuals of ATS, most often when the units are wihtin the same ATC centre or ANSP.

These documents define, inter alia, the standard procedures for transfer of flights between the units. These procedures may explicitly require a coordination for each flight or they may define situations where such coordination is not necessary. An example of the latter is the silent transfer of control procedure which allows the transfer of aircraft to be executed without coordination if a specified set of conditions is met (e.g. if two successive aircraft cross the boundary at the same flight planned fix, maintaining the same level, the separation between them does not fall below 10 NM and they are diverging after crossing the boundary then there is no need for the transferring controller to call the accepting and coordinate the transfer. The estimate still needs to be provided, though, either in the form of a verbal or an OLDI message).

A popular phrase in LoAs is "...unless otherwise verbally coordinated". This generally means that although there are standard procedures established, it is possible to deviate from most of them if everyone involved agrees.

Verbal and Non-verbal Coordination

Verbal coordination is usually done using telephone lines but in some cases a dedicated hotline connection can be established (usually for coordination between tower and approach). Face-to-face communication may also be used sometimes if different units are located in the same room (e.g. approach and tower, approach and area). The use of this method however is usually discouraged by the local procedures.

Verbal coordination is used in complex situations or where electronic coordinations have not been implemented. They can be used for any situation. The disadvantage of verbal coordination is that it requires both controllers to be free from other tasks for a relatively long period of time. A typical verbal coordination lasts about 30 seconds but can take more than a minute for complex situations (e.g. involving multiple aircraft or in case of an emergency or unusual situation) or if additional information is required (e.g. full flight plan messages, weather reports, etc.).

Non-verbal (electronic) coordination may include OLDI messages (preliminary information(ABI), estimate (ACT), revision (REV), request on frequency (ROF), etc.). If the ATM systems of the two units are closely integrated, various electronic coordinations may be available, including release procedure, coordination for a direct route or transferring an aircraft on a heading, the ability to counter-propose (a heading, level, fix, etc.), the ability to highlight a particular aircraft to support the phone coordination and many others.

Electronic coordinations are generally used in routine and less complex situations. They require less time to perform (usually a few seconds) and generally lower controller workload. Unlike verbal coordinations, these do not require both controllers' attendance at the same time.

The main limitation of electronic coordinations is that they are only applicable to pre-defined scenarios. Also, they are not suitable for time critical situations where a timely response is essential.

Coordination Procedures

The most common coordination and coordination-related procedures include:

  • Exchange of flight information (e.g. estimates, revisions, basic and extedned flight data as well as entry clearances)
  • Approval requests. These are defined in local documents, e.g. Letters of Agreement or Manuals of Operations. An approval request may be a part of the initial flight information exchange. Examples of situations that may require this procedure:
    • A late change of the coordinated flight level (e.g. several minutes before the boundary);
    • The level an aircraft is to be cleared to climb if departing from an aerodrome that is close to the boundary;
    • The use of cpecific levels or level bands;
    • Clearing an aircraft to fly on a direct route to a point in the next ATS unit airspace (or further), etc.
  • Transfer of control and communication. The transfer of control is usually effected at the moment the aircraft passes the boundary between the two ATS units. Transfer of communication, however, usually takes place earlier (1-2 minutes in most cases). The receiving controller is not supposed to modify the ATC clearance of the aircraft prior the boundary crossing unless the release procedure has been coordinated (see next point).
  • Release. This procedure is a part of the transfer of control and communication. The transferring controller delegates control of an aircraft to the receiving controller whilst the aircraft is still within the transferring controller's airspace. The responsibility for separation provision lies with the transferring controller. The release may be restricted at the transferring controller's discretion, e.g. the aircraft may only be released for descent to a specified level.
  • Notification. These procedures include informing the neighbouring unit of certain situations, e.g.:
    • radart or other equipment failure;
    • aircraft emergency;
    • aircraft not contacting the receiving unit;
    • activation of a special use area (danger, prohibited, segregated, etc.);
    • change of the operating frequency (e.g. due to change of the sector configuration or an issue with a freqency).
  • Rejection and counter-proposal. If a controller cannot accept an aircraft under the conditions specified, they can reject the coordination and offer terms under which they would.

Poor Coordination

Poor coordination (or lack thereof) may result in:

  • Loss of Separation at Sector Boundaries;
  • Airspace Infringement, e.g. an aircraft entering a prohibited or danger area shortly after crossing the boundary;
  • Loss of Communication, e.g. in case of failure to inform the neighbouring unit of a frequency change or failure to coordinate the proper frequency;
  • Increased workload, especially for the accepting sector. A situation that was supposed to be coordinated but was not is often discovered after some time and has the potential to ruin the controller's plan. A new plan needs to be made in such situation, with less than the usual time available.
  • Runway Incursion which could also be followed by a Missed Approach and subsequent need to rearrange all the traffic.

Best Practices

The advise given in this section is derived from experience and common sense and is not intended to supercede or replace local procedures.

  • Identification of situations that are subject to coordination is critical. Most of these are described in local procedures but some are considered as "similar" and are not explicitly stated. For example (see picture below), a coordination with ATC unit B for a direct route may place the flight in close proximity to ATC unit C. In this case, it is up to the controller from ATC unit A to coordinate with ATC unit C.
ATC unit A should coordinate with both B and C
  • A coordination done twice is usually better than no coordination. If a controller suspects that a coordination has been performed, the best course of action is to verify instead of assume.
  • More than one coordination may be necessary for a particular flight. For instance, if a direct route is negotiated with the next unit and after that the pilot requests climb, the new level should also be coordinated as it may cause a conflict (but may not have caused it had the aircraft been following its flight planned route).
  • Coordination is not about sending a message but about two controllers from different units agreeing to the same plan. Both parties should be comfortable with the common solution.
  • Coordinations should be timely. A late coordination is often not optimal but a call made too early (e.g. more than one hour before boundary crossing) may also be inefficient - the receiving controller will most likely not observe the aircraft in question and may forget about the coordination.
  • Use of Standard Phraseology or simple language reduces the chance of misinterpretation and ambiguity. Also, a simpler plan is less ambiguius, easier to coordinate, implement and monitor and is therefore generally safer.
  • If a controller has any reason to doubt whether they have been correctly understood, they should doublecheck even if their colleague has uttered "approved", "agreed", etc. (which may formally be used as an excuse to conclude the coordination).
  • Electronic coordinations should be used whenever possible. Even if the initiating controller's workload permits a verbal coordination, this may not be the case with the other controller.
  • Coordinations take time that could have been used for other tasks. Therefore controllers should try to stick to the standard procedures and reduce coordinations, except if:
    • safety concerns arise;
    • the workload level is low. Controllers should try to consider the workload of the neighbouring unit.
    • the coordination would reduce traffic complexity (e.g. solve a conflict). Controllers should be careful, though, not to "move" the conflict into the neighbouring unit.
  • The rejection procedure should preferably only be used when there is a good reason to do so (e.g. a safety concern). The rejection should be complemented with an alternative plan rather than being a statement like "I cannot accept the traffic under these conditions.".
  • A "block" coordination may sometimes be used to significantly reduce the workload but this should be done with caution and the time and space parametres should be defined as clearly as possible. An example of block coordination is a situation where a high number of aircraft are avoiding adverse weather using similar routes. Instead of making a phone call to coordinate each flight the sending controller may coordinate with the next unit that all traffic will cross the boundary on its present heading with an instruction to resume onw navigation (when able) to a suitable navigation point in the next airspace. Note that local instructions may forbid or restrict the use of such coordinations.

Accidents and Incidents

This section contains events that have "ATC Unit Co-ordination" as a contributing factor.

  • A306 / B744, vicinity London Heathrow UK, 1996 (On 5 April 1996 a significant loss of separation occurred when a B744, taking off from runway 27R at London Heathrow came into conflict to the west of Heathrow Airport with an A306 which had carried out a missed approach from the parallel runway 27L. Both aircraft were following ATC instructions. Both aircraft received and correctly followed TCAS RAs, the B744 to descend and the A306 to adjust vertical speed, which were received at the same time as corrective ATC clearances.)
  • A319 / PRM1, en-route, near Fribourg Switzerland, 2011 (On 10 June 2011 an ATC error put a German Wings A319 and a Hahn Air Raytheon 390 on conflicting tracks over Switzerland and a co-ordinated TCAS RA followed. The aircraft subsequently passed in very close proximity without either sighting the other after the Hahn Air crew, contrary to Company procedures, followed an ATC descent clearance issued during their TCAS ‘Climb’ RA rather than continuing to fly the RA. The Investigation could find no explanation for this action by the experienced crew - both Hahn Air management pilots. The recorded CPA was 0.6 nm horizontally at 50 feet vertically.)
  • A319, Santiago de Compostela Spain, 2016 (On 10 October 2016, an Airbus 319 was cleared to divert to its first alternate after failing to land off its Cat II ILS approach at Porto and obliged to land at its second alternate with less than final reserve fuel after the first alternate declined acceptance due to lack of parking capacity. The Investigation concluded that adjacent ATC Unit coordination in respect of multiple diversions was inadequate and also found that the crew had failed to adequately appraise ATC of their fuel status. It also noted that the unsuccessful approach at the intended destination had violated approach ban visibility conditions.)
  • A320 / A346, en-route, Eastern Indian Ocean, 2012 (On 18 January 2012, ATC error resulted in two aircraft on procedural clearances in oceanic airspace crossing the same waypoint within an estimated 2 minutes of each other without the prescribed 1000 feet vertical separation when the prescribed minimum separation was 15 minutes unless that vertical separation existed. By the time ATC identified the loss of separation and sent a CPDLC message to the A340 to descend in order to restore separation, the crew advised that such action was already being taken. The Investigation identified various organisational deficiencies relating to the provision of procedural service by the ANSP concerned.)
  • A320 / B738, vicinity Delhi India, 2016 (On 30 January 2016, an Airbus A320 crew cleared for an ILS approach to runway 11 at Delhi reported established on the runway 11 LLZ but were actually on the runway 09 LLZ in error and continued on that ILS finally crossing in front of a Boeing 737-800 on the ILS for runway 10. The Investigation found that the A320 crew had not noticed they had the wrong ILS frequency set and that conflict with the 737 occurred because Approach transferred the A320 to TWR whilst a conflict alert was active and without confirming it was complying with its clearance.)
  • A320 / B739, Yogyakarta Indonesia, 2013 (On 20 November 2013, an A320 misunderstood its taxi out clearance at Yogyakarta and began to enter the same runway on which a Boeing 737, which had a valid landing clearance but was not on TWR frequency, was about to touch down from an approach in the other direction of use. On seeing the A320, which had stopped with the nose of the aircraft protruding onto the runway, the 737 applied maximum manual braking and stopped just before reaching the A320. The Investigation faulted ATC and airport procedures as well as the A320 crew for contributing to the risk created.)
  • A320 / C56X, vicinity Geneva Switzerland, 2011 (On 17 August 2012, a Swiss A320 being positioned under radar vectors for arrival at Geneva was inadvertently vectored into conflict with a Cessna Citation already established on the ILS LOC for runway 23 at Geneva. Controller training was in progress and the Instructor had just taken control because of concerns at the actions of the Trainee. An error by the Instructor was recognised and de-confliction instructions were given but a co-ordinated TCAS RA still subsequently occurred. STCA was activated but constraints on access to both visual and aural modes of the system served to diminish its value.)
  • A320 / SW4, Calgary AB Canada, 2016 (On 2 December 2016, the crew of an Airbus A320 passing 100 knots on takeoff at Calgary saw another aircraft crossing an intersection ahead but continued because they considered that, as the other aircraft was already more than half way across, it would be clear before they reached that point. The Investigation found that the GND Controller had cleared the other aircraft to cross after forgetting that the runway was active and under TWR control. It was concluded that the response of the ANSP SMS process to a history of identical controller errors and related reports had been inadequate.)
  • A320, en-route, Denver CO USA, 2009 (On 21 October 2009, an Airbus 320-200 being operated by Northwest Airlines on a scheduled passenger flight from San Diego to Minneapolis-St Paul, with the Captain as PF, overflew its destination at cruise level in VMC at night by more than 100 nm, after the two pilots had become distracted in conversation and lost situational awareness. They failed to maintain radio communications with a series of successive ATC units for well over an hour. After a routine inquiry from the cabin crew as to the expected arrival time, the flight crew realised what had happened and re-established ATC contact after which the flight was completed without further incident.)
  • A320/E190/B712, vicinity Helsinki Finland, 2013 (On 6 February 2013, ATC mismanagement of an Airbus A320 instructed to go around resulted in loss of separation in IMC against the Embraer 190 ahead which was obliged to initiate a go around when no landing clearance had been issued due to a Boeing 737-800 still on the runway after landing. Further ATC mismanagement then resulted in a second IMC loss of separation between the Embraer 190 and a Boeing 717 which had just take off from the parallel runway. Controller response to the STCA Alerts generated was found to be inadequate and ANSP procedures in need of improvement.)
  • A321 / B734, Barcelona Spain, 2015 (On 25 November 2015, an Airbus A321 taxiing for departure at Barcelona was cleared across an active runway in front of an approaching Boeing 737 with landing clearance on the same runway by a Ground Controller unaware that the runway was active. On reaching the lit stop bar protecting the runway, the crew queried their clearance and were told to hold position. Noting that the event had occurred at the time of a routine twice-daily runway configuration change and two previous very similar events in 2012 and 2014, further safety recommendations on risk management of runway configuration change were made.)
  • A332/B738, vicinity Amsterdam Netherlands, 2012 (On 13 November 2012, a Garuda Airbus A330 and a KLM Boeing 737 lost separation against each other whilst correctly following radar vectors to parallel approaches at Amsterdam but there was no actual risk of collision as each aircraft had the other in sight and no TCAS RA occurred. The Investigation found that one of the controllers involved had used permitted discretion to override normal procedures during a short period of quiet traffic but had failed to restore normal procedures when it became necessary to do so, thus creating the conflict and the ANSP was recommended to review their procedures.)

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Further Reading

  • ICAO Doc 4444 PANS-ATM