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Loss of Separation - Pilot-induced Situations

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Category: Loss of Separation Loss of Separation
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Content control: EUROCONTROL EUROCONTROL

Description

Loss of separation between aircraft sometimes occurs as a result of an aircraft deviating from the cleared track or level without clearance. This may happen for a variety of reasons, captured in the following scenarios:

  • Due to pilot inattention, equipment malfunction or the mis-setting of aircraft equipment. An example of this is flying with the transponder switched off or in standby mode.
  • Action to avoid a visually-perceived loss of separation from another aircraft.
  • Action to avoid severe weather if IFR or to remain in VMC if VFR.
  • Pilot failure to follow ATC clearance or delaying their actioning of an accepted clearance.
  • Instruction not received or not understood by pilot due to ineffective air-ground communications.
  • Pilot taking a clearance intended for another aircraft due to callsign confusion.
  • Pilot receiving a TCAS RA but fails to follow it correctly.
  • Pilot entering notified airspace without clearance.

Contributory Factors

The following factors, on their own, are unlikely to cause a loss of separation. They can, however, contribute to the reduction of a pilot's situational awareness which in turn may lead to action (or inaction) that would cause separation breach.

Defences

A number of activities are performed so that the risk of loss of separation due to pilot actions is reduced or the consequences of such loss are mitigated so that collision is avoided. The most notable of them are:

  • Standard Operating Procedures, on the flight-deck, which detail procedures to be followed to reduce the chance of loss of separation.
  • Onboard aircraft equipment designed to warn of potential collision with other aircraft (TCAS).
  • Pilot training, especially in:
  • Development and improvement of safety nets, e.g. STCA.
  • Air traffic controller training emphasizing the importance of:
    • Air-ground communication. Appropriate communication reduces the risk of misunderstanding and, consequently, unexpected traffic behaviour.
    • Monitoring pilot compliance with the issued clearances. This allows early detection of aircraft deviation which may help prevent a loss of separation.
    • Appropriate planning, especially in emergency/abnormal situations and weather avoidance scenarios.

Accidents and Incidents

This section contains examples of occurrences where pilot actions or inactions lead to loss of separation. Note that some events may fall into more than one category. Also, there are situations where both pilot and controller actions contributed to the outcome.

Examples where a TCAS RA was not properly complied with:

  • F100 / EC45, vicinity Bern Switzerland, 2012 (On 24 May 2012, a Fokker 100 descending visual downwind to land at Berne and an EC145 helicopter transiting the Bern CTR (Class 'D' airspace) VFR came within 0.7 nm horizontally and 75 ft vertically despite early traffic advice having been given to both aircraft. The Investigation attributed the conflict to the failure of the F100 crew to follow either their initial TCAS RA or a subsequent revised one and noted that although STCA was installed at Berne it had been disabled "many years before".)
  • A320/B738, vicinity Delhi India, 2013 (On 2 September 2013, a B737 crew were not instructed to go around from their approach by ATC as it became increasingly obvious that an A320 departing the same runway would not be airborne in time for a landing clearance to be issued. They initiated a go around over the threshold and then twice came into conflict with the A320 as both climbed on similar tracks without ATC de-confliction, initially below the height where TCAS RAs are functional. Investigation attributed the conflict to ATC but the failure to effectively deal with the consequences jointly to ATC and both aircraft crews.)

... further results

Examples where the pilot failed to comply with an ATC clearance:

  • B738/B738, vicinity Oslo Norway, 2012 (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.)
  • B763, en-route North Bay Canada, 2009 (On 19 June 2009 a Boeing 767-300 was level at FL330 in night IMC when the Captain’s altimeter and air speed indicator readings suddenly increased, the latter by 44 knots. The altimeter increase triggered an overspeed warning and the Captain reduced thrust and commenced a climb. The resultant stall warning was followed by a recovery. The Investigation found that a circuitry fault had caused erroneous indications on only the Captain’s instruments and that contrary to the applicable QRH procedure, no comparison with the First Officer’s or Standby instruments had been made. A related Operator FCOM error was also identified.)

... further results

Examples involving a level bust:

  • B773, vicinity Houston TX USA, 2014 (On 3 July 2014, a Boeing 777-300 departing Houston came within 200 feet vertically and 0.61nm laterally of another aircraft after climbing significantly above the Standard Instrument Departure Procedure (SID) stop altitude of 4,000 feet believing clearance was to FL310. The crew responded to ATC avoiding action to descend and then disregarded TCAS 'CLIMB' and subsequently LEVEL OFF RAs which followed. The Investigation found that an inadequate departure brief, inadequate monitoring by the augmented crew and poor communication with ATC had preceded the SID non-compliance and that the crew should have followed the TCAS RAs issued.)
  • A321 / B738, en-route, south eastern Bulgaria, 2016 (On 8 September 2016, an Airbus A321 en route in Bulgarian airspace at FL 350 was given and acknowledged a descent but then climbed and came within 1.2nm of a descending Boeing 737. The Investigation found that the inexperienced A321 First Officer had been temporarily alone when the instruction was given and had insufficient understanding of how to control the aircraft. It was also found that despite an STCA activation of the collision risk, the controller, influenced by a Mode ‘S’ downlink of the correctly-set A321 cleared altitude, had then added to the risk by instructing the 737 to descend.)

... further results

Examples related to transponder operation:

  • B738 / E135, en-route, Mato Grosso Brazil, 2006 (On 29 September 2006, a Boeing 737-800 level at FL370 collided with an opposite direction Embraer Legacy at the same level. Control of the 737 was lost and it crashed, killing all 154 occupants. The Legacy's crew kept control and successfully diverted to the nearest suitable airport. The Investigation found that ATC had not instructed the Legacy to descend to FL360 when the flight plan indicated this and soon afterwards, its crew had inadvertently switched off their transponder. After the consequent disappearance of altitude from all radar displays, ATC assumed but did not confirm the aircraft had descended.)
  • F15 / E145, en-route, Bedford UK, 2005 (On 27 January 2005, two USAF-operated McDonnell Douglas F15E fighter aircraft, both continued to climb and both passed through the level of an Embraer 145 being operated by British Airways Regional on a scheduled passenger flight from Birmingham to Hannover, one seen at an estimated range of 100 feet.)
  • E170 / F900, en-route, east of Varna Bulgaria, 2015 (On 30 June 2015 the crew of an en route Embraer 170 failed to notice that their transponder had reverted to Standby and the ATC response, which involved cross border coordination, was so slow that the aircraft was not informed of the loss of its transponder signal for over 30 minutes by which time it had already passed within 0.9nm of an unseen Dassault Falcon 900 at the same level. The Investigation found that the Embraer crew had failed to follow appropriate procedures and that the subsequent collision risk had been significantly worsened by a muddled and inappropriate ATC response.)
  • F2TH / GLID, vicinity St Gallen-Altenrhein Switzerland, 2017 (On 15 October 2017, a Falcon 2000EX on base leg for an easterly ILS approach at St Gallen-Altenrhein came into close proximity with a reciprocal track glider at 5000 feet QNH in Class ‘E’ airspace in day VMC with neither aircraft seeing the other until just before their minimum separation - 0.35 nm horizontally and 131 feet vertically - occurred. The Investigation attributed the conflict to the lack of relevant traffic separation requirements in Class E airspace and to the glider not having its transponder switched on and not listening out with the relevant ATC Unit.)

... further results


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