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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:

  • 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".)
  • E55P, Blackbushe UK, 2015 (On 31 July 2015 a Saudi-operated Embraer Phenom on a private flight continued an unstabilised day visual approach to Blackbushe in benign weather conditions. The aircraft touched down with excess speed with almost 70% of the available landing distance behind the aircraft. It overran and was destroyed by impact damage and fire and all occupants died. The Investigation concluded that the combination of factors which created a very high workload for the pilot "may have saturated his mental capacity, impeding his ability to handle new information and adapt his mental model" leading to his continuation of a highly unstable approach.)

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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.)
  • CRJ2 / A320, vicinity Port Elizabeth South Africa, 2014 (On 10 July 2014, Bombardier CRJ-200 instructed to go around at Port Elizabeth by ATC came into close proximity with an A320 which had just taken off from the same runway and initiated avoiding action to increase separation. The Investigation concluded that the TWR controller had failed to effectively monitor the progress of the aircraft on final approach before issuing a take off clearance to the A320.)

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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.)
  • 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.)

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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.)
  • H25B / AS29, en-route / manoeuvring, near Smith NV USA, 2006 (On 28 August 2006, a Hawker 800 collided with a glider at 16,000 feet in Class 'E' airspace. The glider became uncontrollable and its pilot evacuated by parachute. The Hawker was structurally damaged and one engine stopped but it was recovered to a nearby airport. The Investigation noted that the collision had occurred in an area well known for glider activity in which transport aircraft frequently avoided glider collisions using ATC traffic information or by following TCAS RAs. The glider was being flown by a visitor to the area with its transponder intentionally switched off to conserve battery power.)
  • 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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