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Loss of Separation - ATCO-induced Situations

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Article Information
Category: Loss of Separation Loss of Separation
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL

Description

Loss of separation between aircraft sometimes occurs as a result of action taken (or not taken) by the ATCO.

Effects

  • Loss of separation from other aircraft may result in collision or the need for flight crew to take visual or ACAS avoiding action at close range to avoid one.
  • 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.

Defences

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 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.).
  • ATCO Work-Load
  • Interruption or Distraction
  • Fatigue

Solutions

Accidents and Incidents

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

  • 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.)
  • A319/A319, en-route, South west of Basle-Mulhouse France, 2010 (On 29 June 2010, an Easyjet Switzerland Airbus A319 inbound to Basle-Mulhouse and an Air France Airbus A319 outbound from Basle-Mulhouse lost separation after an error made by a trainee APP controller under OJTI supervision during procedural service. The outcome was made worse by the excessive rate of climb of the Air France aircraft approaching its cleared level and both an inappropriate response to an initial preventive TCAS RA and a change of track during the ensuing short sequence of RAs by the Training Captain in command of and flying the Easyjet aircraft attributed by him to his situational ‘anxiety’.)

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