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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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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.


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:

  • GLF5 / A319, south-eastern France, 2004 (On 16 September 2004, a loss of separation occurred over Geneva between Air France A319 and a Gulfstream 5 which commenced descent without clearance by ATC and with coordinated TCAS RAs not followed by either aircraft.)
  • 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, en-route, Sydney Australia, 2007 (On 11 January 2007, an Air New Zealand Airbus A320 which had just departed Sydney Australia for Auckland, New Zealand was observed to have turned onto a heading contrary to the ATC-issued radar heading. When so advised by ATC, the crew checked the aircraft compasses and found that they were reading approximately 40 degrees off the correct heading.)
  • B734 / MD81, en-route, Romford UK, 1996 (On 12 November 1996, a B737-400 descended below its assigned level in one of the holding patterns at London Heathrow in day IMC to within 100 feet vertically and between 680 and 820 metres horizontally of a MD-81 at its correct level, 1000 feet below. STCA prompted ATC to intervene and the 737 climbed back to its cleared level. Neither aircraft was fitted with TCAS 2 or saw the other visually.)

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Examples involving a level bust:

  • B744 / B773 / B773, en-route, Delhi India, 2018 (On 22 December 2018, a Boeing 747-400 crew began to climb from FL310 without clearance and prescribed separation was lost against both an opposite direction Boeing 777-300 at FL 320 and another same direction Boeing 777-300 cleared to fly at FL330. The Investigation found that the 747 crew had requested FL 390 and then misunderstood the controller’s response of “level available 350” as a clearance to climb and gave a non-standard response and began to climb when the controller responded instructing the flight to standby for higher. Controller attempts to resolve the resultant ‘current conflict warnings’ were only partially successful.)
  • H25B / B738, en-route, south eastern Senegal, 2015 (On 5 September 2015, a Boeing 737-800 cruising as cleared at FL350 on an ATS route in daylight collided with an opposite direction HS 125-700 which had been assigned and acknowledged altitude of FL340. The 737 continued to destination with winglet damage apparently causing no control impediment but radio contact with the HS 125 was lost and it was subsequently radar-tracked maintaining FL350 and continuing westwards past its destination Dakar for almost an hour before making an uncontrolled descent into the sea. The Investigation found that the HS125 had a recent history of un-rectified altimetry problems which prevented TCAS activation.)

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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.)
  • B738 / C172, en route, near Falsterbo Sweden, 2014 (On 20 July 2014, the pilot of a VFR Cessna 172 became distracted and entered the Class 'C' controlled airspace of two successive TMAs without clearance. In the second one he was overtaken by a Boeing 738 inbound to Copenhagen with less than 90 metres separation. The 738 crew reported a late sighting of the 172 and "seemingly" assessed that avoiding action was unnecessary. Although the 172 had a Mode C-capable transponder, it was not transmitting altitude prior to the incident and the Investigation noted that this had invalidated preventive ATC and TCAS safety barriers and compromised flight safety.)
  • 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.)

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