Loss of Separation - ATCO-induced Situations

Loss of Separation - ATCO-induced Situations


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 likelihood of making errors that lead to loss of separation, 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 accommodate 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.

On 17 January 2018, two Airbus A320s both inbound to Surabaya at night were vectored to the same waypoint to hold, one at FL210 and the other at FL200 but the one initially given FL210 was then re-cleared to also descend to FL200. The two aircraft subsequently received and followed coordinated TCAS RAs which restored prescribed separation. The Investigation found that before the conflict, the experienced controller involved had made several transmissions to aircraft other than the intended ones and noted that the usually-available ATC conflict alerting system had been temporarily out of service without any consequent mitigations in place.

On 3 January 2019, prescribed vertical separation was lost between a Bombardier DHC8-400 and a Piaggio P180 on converging cruise tracks at FL 220 and only restored after a resulting TCAS RA was followed. The Investigation found both aircraft were being flown in accordance with their clearances and that the controller involved had not been aware of corresponding traffic and conflict alerting system activations. A specific traffic display fault which arose from failure to follow a routine software upgrade procedure correctly and the shift supervisor failing to recognise the need to act as controller when workload increased were assessed contributory.

On 16 July 2019, a Boeing 737-800 inbound to Malaga and another Boeing 738-800 inbound to Seville and under area radar control lost separation after the Malaga-bound aircraft was unexpectedly given radar headings to extend its destination track miles after early handover to a control  sector which it had not yet entered. With no time to achieve resolution, the two aircraft, both descending, came within 1.3 nm of each other at the same level. The Investigation attributed the conflict to an overly-permissive Letter of Agreement between Seville Centre and Malaga Approach and recommended that it be revised to improve risk management.

On 23 February 2018, an Embraer 195LR and an Airbus A320 on SIDs departing Brussels lost separation after the 195 was given a radar heading to resolve a perceived third aircraft conflict which led to loss of separation between the two departing aircraft. STCA and coordinated TCAS RA activations followed but only one TCAS RA was followed and the estimated minimum separation was 400 feet vertically when 1.36 nm apart. The Investigation found that conflict followed an error by an OJTI-supervised trainee controller receiving extended revalidation training despite gaining his licence and having almost 10 years similar experience in Latvia.

On 12 April 2019, a Boeing 717-200 commenced a go around at Strasbourg because the runway ahead was occupied by a departing Bombardier CRJ700 which subsequently, despite co-ordinated TCAS RAs, then came to within 50 feet vertically when only 740 metres apart laterally as the CRJ, whose crew did not see the 717, passed right to left in front of it. The Investigation attributed the conflict primarily to a series of flawed judgements by the TWR controller involved whilst also noting one absent and one inappropriate ATC procedure which respectively may have provided a context for the resultant risk.

On 27 September 2019, an Airbus A320 and an Embraer 145 both inbound to Barcelona and being positioned for the same Transition for runway 25R lost separation and received and followed coordinated TCAS RAs after which the closest point of approach was 0.8nm laterally when 200 feet vertically apart. The Investigation found that the experienced controller involved had initially created the conflict whilst seeking to resolve another potential conflict between one of the aircraft and a third aircraft inbound for the same Transition and having identified it had then implemented a faulty recovery plan and executed it improperly.

On 12 May 2019, a Boeing 737-800 making its second procedural ILS approach to runway 25 at Reus came into conflict with an opposite direction light aircraft as the latter approached one of the designated VFR entry points having been instructed to remain well above the altitude which normally ensures separation of IFR and VFR traffic. The collision risk was resolved by TCAS RA promptly followed by the 737. The Investigation concluded that limiting the TWR radar display to the ATZ for controller training purposes had resulted in neither the trainee controller nor their supervisor being aware of the risk.

On 4 June 2016, a Boeing 737-800 instructed to climb from FL340 to FL380 by the controller of one sector in Bulgarian upper airspace came into sufficiently close proximity to an Airbus A320 under the control of a different sector controller to trigger co-ordinated TCAS RAs. Separation was eventually restored after the 737 followed its RA despite the A320, which had already deviated from its clearance on the basis of a prior TCAS TA without informing ATC, ignoring their RA. The Investigation found that the root cause of the conflict had been inadequate coordination between two vertically separated ATC sectors.

On 31 October 2012, a Boeing 737-800 on go around after delaying the breaking off of a fast and high unstable ILS approach at Oslo lost separation in IMC against another aircraft of the same type and Operator which had just taken off from the same runway as the landing was intended to be made on. The situation was aggravated by both aircraft responding to a de-confliction turn given to the aircraft on go around. Minimum separation was 0.2nm horizontally when 500 feet apart vertically, both climbing. Standard missed approach and departure tracks were the same.

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.

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