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Loss of Separation - Pilot-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 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:

  • DH8A/DH8C, en-route, northern Canada, 2011 (On 7 February 2011 two Air Inuit DHC8s came into head-to-head conflict en route over the eastern shoreline of Hudson Bay in non radar Class ‘A airspace when one of them deviated from its cleared level towards the other which had been assigned the level 1000 feet below. The subsequent investigation found that an inappropriate FD mode had been used to maintain the assigned level of the deviating aircraft and noted deficiencies at the Operator in both TCAS pilot training and aircraft defect reporting as well as a variation in altitude alerting systems fitted to aircraft in the DHC8 fleet.)
  • C525 / B773, vicinity London City UK, 2009 (On 27 July 2009, a Cessna 525 departing from London City failed to comply with the initial 3000 feet QNH SID Stop altitude and at 4000 feet QNH in day VMC came into close proximity on an almost reciprocal heading with a Boeing 777-300ER. The 777, on which line training was being conducted, failed to follow any of the three TCAS RAs generated. Actual minimum separation was approximately 0.5nm laterally and estimated at between 100 feet and 200 feet vertically. It was noted that the Cessna had been given a stepped climb SID.)

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

  • A320 / B738, vicinity Delhi India, 2016 (On 30 January 2016, an Airbus A320 crew cleared for an ILS approach to runway 11 at Delhi reported established on the runway 11 LLZ but were actually on the runway 09 LLZ in error and continued on that ILS finally crossing in front of a Boeing 737-800 on the ILS for runway 10. The Investigation found that the A320 crew had not noticed they had the wrong ILS frequency set and that conflict with the 737 occurred because Approach transferred the A320 to TWR whilst a conflict alert was active and without confirming it was complying with its clearance.)
  • DH8D / B772, vicinity Sydney Australia, 2016 (On 9 December 2016, a Bombardier DHC8-400 departing Sydney lost prescribed separation against an inbound Boeing 777-200 after its crew failed to ensure that the aircraft levelled as cleared at 5,000 feet and this was exceeded by 600 feet. The Investigation found that the First Officer, as Pilot Flying, had disconnected the autopilot prior to routinely changing the selected airspeed because it tended to disconnect when this was done with altitude capture mode active but had then failed to re-engage it. The Captain's lack of effective monitoring was attributed to distraction as he sought to visually acquire the conflicting traffic.)

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Examples involving a level bust:

  • DH8A/DH8C, en-route, northern Canada, 2011 (On 7 February 2011 two Air Inuit DHC8s came into head-to-head conflict en route over the eastern shoreline of Hudson Bay in non radar Class ‘A airspace when one of them deviated from its cleared level towards the other which had been assigned the level 1000 feet below. The subsequent investigation found that an inappropriate FD mode had been used to maintain the assigned level of the deviating aircraft and noted deficiencies at the Operator in both TCAS pilot training and aircraft defect reporting as well as a variation in altitude alerting systems fitted to aircraft in the DHC8 fleet.)
  • A319 / A321, en-route, west north west of Geneva, Switzerland 2011 (On 6 August 2011 an Easyjet Airbus A319 on which First Officer Line Training was in progress exceeded its cleared level during the climb after a different level to that correctly read back was set on the FMS. As a result, it came into conflict with an Alitalia A321 and this was resolved by responses to coordinated TCAS RAs. STCA alerts did not enable ATC resolution of the conflict and it was concluded that a lack of ATC capability to receive Mode S EHS DAPs - since rectified - was a contributory factor to the outcome.)

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Examples related to transponder operation:

  • 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.)
  • 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.)
  • 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.)
  • B752, vicinity Atlanta GA USA, 2011 (On 11 March 2011, a Delta AL Boeing 757 departed Atlanta GA with no secondary radar indication visible to ATC and also failed to make contact with departure radar after accepting the frequency transfer instruction. During the eight minutes out of radio contact, it successively lost separation against two light aircraft and another passenger aircraft as it followed the cleared RNAV departure routing for eight minutes until the crew queried further climb on the TWR frequency and were invited to select their transponder on and contact the correct frequency.)

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