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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.


  • Injury, especially to cabin crew or passengers, may result from any unusual manoeuvres to avoid a collision.
  • Injury to the occupants of an aircraft, especially Cabin Crew, may also result from wake vortex turbulence encounter.
  • High levels of stress for the pilots and controllers involved, which may lead to reduced performance.


  • Standard Operating Procedures, in the ATSU, which detail procedures to be followed to reduce the chance of loss of separation.
  • Routine structured scan helps controllers detect potential conflicts well in advance and mitigate the "blind spot" effect.
  • Onboard aircraft equipment designed to warn of potential collision with other aircraft (ACAS/TCAS).
  • Use of controller support tools, e.g. Tactical Controller Tool (TCT)
  • Ground-based equipment designed to warn of potential collision with other aircraft: STCA (STCA).
ATC Screen
For ABC123 - the controller spots both PQR265 and XYZ312 but overlooks DEF763

Typical 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.
  • The controller issues a clearance that creates a conflict with a neighbouring aircraft due to the blind spot effect.

Contributory Factors

  • Volume of traffic;
  • Military traffic operating out of the segregated area in civil airspace
  • Flight crews (military or civil) unfamiliar with the applicable rules and procedures in a particular volume of airspace;
  • Failure to pass an Instrument Flight Rules (IFR) aircraft timely traffic information about Visual Flight Rules (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.).
  • ATCO Work-Load
  • Interruption or Distraction
  • Fatigue


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