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Loss of Separation - ATCO-induced Situations

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Category: Loss of Separation Loss of Separation
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Loss of separation between aircraft sometimes occurs as a result of action taken (or not taken) by the ATCO. In most cases this involves one or more of the following scenarios:

  • Flight clearance does not provide adequate separation from other traffic.
  • ATCO does not detect developing potential conflict.
  • Avoiding action issued is too late or inadequate to provide safe separation.
  • Instruction not received or not understood by intended recipient due to breakdown in air-ground communications.
  • The controller issues a clearance that creates a conflict with a neighbouring aircraft due to the blind spot effect.
ATC Screen
When issuing descent clearance to ABC123, the controller spots both PQR265 and XYZ312 but overlooks DEF763

Contributory Factors

The factors listed below would usually not be sufficient (on their own) to cause loss of separation but can exacerbate the situation:

  • ATCO Work-Load. Obviously, high workload situations make people more prone to making errors. It is important to know, however, that low-workload situations may lead to the same end result:
    • A high workload situation may cause the controller to skip (intentionally or not) an important action (e.g. checking for an immediate conflict before issuing a clearance).
    • A low workload situation can result in errors due to complacency.
    • A steep peak of the workload (i.e. a "tidal wave" of complex traffic) may result in missing one out of ten important details (e.g. a sharp turn after the sector entry point or an abnormally slow/fast traffic).
    • A steep decline of workload after a (well-managed) complex situation may result in over-relaxation.
  • Volume of traffic. Even if this does not result in high complexity (e.g. a lot of conflicts, traffic avoiding weather, etc.), the sheer volume of aircraft may cause a controller to miss something or make an error.
  • Military traffic operating out of the segregated area in civil airspace normally requires special attention (e.g. more coordination effort and extended monitoring) and may drive the focus away from another situation.
  • Flight crews (military or civil) unfamiliar with the applicable rules and procedures in a particular volume of airspace could increase workload by e.g. not properly (or timely) compliance with ATC clearances and instructions, requests for repetition, etc.
  • Failure to pass an IFR aircraft timely traffic information about VFR aircraft in its vicinity.
  • Issue of a VFR clearance in airspace where the only prescribed traffic separation is IFR against IFR when the ability of the VFR aircraft to comply with its clearance and maintain an effective visual lookout may be compromised by weather conditions.
  • Poor (or missing) coordination between adjacent sectors or units.
  • Transfer on the wrong frequency may result in the inability of both controllers to issue timely instructions or a communication loss.
  • Obscured track labels (e.g. due overlapping, filters, colour representation, etc.).
  • Interruption or Distraction may draw the controller's attention away from a potential conflict or may contribute to forgetting to perform an action.
  • Fatigue in general reduces a person's working capacity and may even cause a microsleep (i.e. a person seems to be awake but is actually not).


At an organizational level, this includes:

In order to reduce the likelyhood of making errors that lead to loss of seraration, controllers should:

  • Follow the standard procedures (but be ready to deviate from them if the situation requires it).
  • Perform routine structured scan to detect potential conflicts well in advance and mitigate the "blind spot" effect.
  • Make use of the support tools available which would allow them to do more in less time, thus freeing up precious seconds.
  • Resist the urge to accomodate crew requests if unsure about their impact on the overall traffic situation. Naturally, this does not mean that such requests are to be disregarded.
  • Quickly assess a safety net warning, create a simple plan and only then execute it (without delay). Starting to speak without having decided what to do is likely to make the situation worse.

ACAS/TCAS is an onboard aircraft equipment designed to warn of potential collision with other aircraft. This barrier is meant to mitigate the consequences of a separation loss.

Accidents and Incidents

This section contains events where ATC error was considered as a contributory factor.

  • A310 / C421, en-route, northeast of Montréal Canada, 2018 (On 16 May 2018, an Airbus A310 and a Cessna 421 being positioned for ILS approaches to adjacent parallel runways at Montréal by different controllers lost separation. One controller incorrectly believed that he had transferred control of the Cessna to the other when the shift supervisor re-opened a sector which had been temporarily combined with his. The Investigation attributed the conflict to multiple deviations from standard procedures, memory lapses relating to controller information exchange of information and a loss of full situational awareness compounded by the shift supervisor also acting as an instructor whilst being distracted by his other duties.)
  • A318 / B738, en-route, Trasadingen Switzerland, 2009 (On 8 June 2009, an Airbus A318-100 being operated by Air France on a scheduled passenger flight from Belgrade, Serbia to Paris CDG in day VMC came into conflict with a Boeing 737-800 being operated by Ryanair on a scheduled passenger flight from Nottingham East Midlands UK to Bergamo Italy. The conflict was resolved mainly by TCAS RA response and there were no injuries to any occupants during the avoidance manoeuvres carried out by both aircraft.)
  • A318/B738, Nantes France, 2010 (On 25 May 2010 an Air France Airbus A318 making an automatic landing off an ILS Cat 2 approach at Nantes experienced interference with the ILS LOC signal caused by a Boeing 737-800 which was departing from the same runway but early disconnection of the AP removed any risk of un-correctable directional control problems during the landing roll. Both aircraft were operating in accordance with their ATC clearances. Investigation attributed the conflict to the decision of TWR not to instruct the A318 to go around and because of diminished situational awareness.)
  • A318/B739, vicinity Amsterdam Netherlands, 2007 (On 6 December 2007 an Airbus A318 being operated by Air France on a scheduled passenger flight from Lyon to Amsterdam carried out missed approach from runway 18C at destination and lost separation in night VMC against a Boeing 737-900 being operated by KLM on a scheduled passenger flight from Amsterdam to London Heathrow which had just departed from runway 24. The conflict was resolved by correct responses to the respective coordinated TCAS RAs after which the A318 passed close behind the 737. There were no abrupt manoeuvres and none of the 104 and 195 occupants respectively on board were injured.)
  • A319 / B738 / B738, en-route, near Lausanne Switzerland, 2013 (On 26 May 2013, an A319 in Swiss Class 'C' airspace received a TCAS 'Level Off' RA against a 737 above after being inadvertently given an incorrect climb clearance by ATC. The opposing higher-altitude 737 began a coordinated RA climb from level flight and this triggered a second conflict with another 737 also in the cruise 1000 feet above which resulted in coordinated TCAS RAs for both these aircraft. Correct response to all RAs resulted in resolution of both conflicts after prescribed minimum separations had been breached to as low as 1.5nm when 675 feet apart vertically.)
  • 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.)

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