Loss of Separation - Pilot-induced Situations

Loss of Separation - Pilot-induced Situations

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:

On 23 February 2018, an Embraer 195LR and an Airbus A320 on SIDs departing Brussels lost separation after the 195 was given a radar heading to resolve a perceived third aircraft conflict which led to loss of separation between the two departing aircraft. STCA and coordinated TCAS RA activations followed but only one TCAS RA was followed and the estimated minimum separation was 400 feet vertically when 1.36 nm apart. The Investigation found that conflict followed an error by an OJTI-supervised trainee controller receiving extended revalidation training despite gaining his licence and having almost 10 years similar experience in Latvia.

On 4 June 2016, a Boeing 737-800 instructed to climb from FL340 to FL380 by the controller of one sector in Bulgarian upper airspace came into sufficiently close proximity to an Airbus A320 under the control of a different sector controller to trigger co-ordinated TCAS RAs. Separation was eventually restored after the 737 followed its RA despite the A320, which had already deviated from its clearance on the basis of a prior TCAS TA without informing ATC, ignoring their RA. The Investigation found that the root cause of the conflict had been inadequate coordination between two vertically separated ATC sectors.

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.

On 20 November 2011, a problem in reading the altitude labels on the ATC radar control display led to a Finnair Boeing 757 being cleared to make a descent which brought it into proximity with a Thomas Cook Boeing 757 in day VMC. Co-ordinated TCAS RAs were generated onboard both aircraft but when the Finnair aircraft failed to respond to its Climb RA and continued descent, the other aircraft, which had responded correctly to its initial RA, received a further RA to reverse their descent to a climb. The Finnair aircraft reported retaining visual contact with the other aircraft throughout.

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.

On 14 October 2016, two Bombardier DHC8-400s received coordinated TCAS RAs as they came into opposite direction conflict near Sudbury, an uncontrolled airport, as one was descending inbound and emerging from an overcast layer and the other was level just below that layer after departing. Both aircraft crews ignored their RAs and their respective visual manoeuvring brought them to within 0.4nm at the same altitude. The Investigation noted that the conflict had occurred in Class E airspace after the departing aircraft had cancelled IFR to avoid a departure delay attributable to the inbound IFR aircraft.

On 17 October 2013, a Falcon 900 climbing as cleared to FL 340 and being operated as a State Aircraft equipped with TCAS II v7.0 initially responded to a TCAS RA against crossing traffic at FL 350 in day VMC in the opposite direction to the one directed and prescribed separation was lost as a result. The Investigation concluded that the F900 crew had commenced a climb on receipt of a TCAS RA 'ADJUST VERTICAL SPEED' when a reduction in the 800 fpm rate of climb was required. Safety Recommendations were made in respect of TCAS RA requirements for State Aircraft.

On 30 October 2014, a descending Airbus A320 came close to a Boeing 737-800 at around FL 220 after the A320 crew significantly exceeded a previously-instructed 2,000 fpm maximum rate of descent assuming it no longer applied when not reiterated during re-clearance to a lower altitude. Their response to a TCAS RA requiring descent at not above 1,000 fpm was to further increase it from 3,200 fpm. Lack of notification delayed the start of an independent Investigation but it eventually found that although the A320 TCAS equipment had been serviceable, its crew denied failing to correctly follow their initial RA.

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.

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.

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

On 13 October 2019, an Airbus A320neo inbound to Zurich had been cleared to the lowest available Class ‘C’ airspace level when a light aircraft crossing its intended track below in uncontrolled airspace began to climb into the same Class C airspace without clearance. An ATC Conflict Alert was activated and the controller put the A320 on an avoidance radar heading and safe separation was thereby achieved. The reason for the incursion was not determined but the event was considered yet another example of yet-to-be-addressed airspace infringement risk and a corresponding safety recommendation was made to the State Safety Regulator.

On 6 January 2018, a Boeing 737-900 and an Airbus A320 both inbound to Surabaya with similar estimated arrival times were cleared to hold at the same waypoint at FL100 and FL110 respectively but separation was lost when the A320 continued below FL110. Proximity was limited to 1.9nm laterally and 600 feet vertically following correct responses to coordinated TCAS RAs. The Investigation found that all clearances / readbacks had been correct but that the A320 crew had set FL100 instead of their FL110 clearance and attributed this to diminished performance due to the passive distraction of one of the pilots.

On 29 November 2017, a Boeing 737-900 on an ILS approach at Atlanta became unstable after the autothrottle and autopilot were both disconnected and was erroneously aligned with an occupied taxiway parallel to the intended landing runway. A go-around was not commenced until the aircraft was 50 feet above the ground after which it passed low over another aircraft on the taxiway. The Investigation found that the Captain had not called for a go around until well below the Decision Altitude and had then failed to promptly take control when the First Officer was slow to begin climbing the aircraft.

On 12 March 2019, a Bombardier DHC8-300 which had requested and been granted a visual approach to Wellington was instructed to follow another of the operator’s DHC8 aircraft already in the circuit but instead turned in front of it after the its crew identified the Airbus A320 as the other DHC8. The conflict was detected by ATC and advised and coordinated TCAS RAs then followed. The Investigation noted that whilst the inability of the second DHC8 crew to correctly identify the aircraft they should follow had been causal, procedures had delayed the ATC response to the automatically detected conflict.

On 29 February 2020, an Airbus A320 inbound to Delhi lost separation against an outbound A320 from Delhi on a reciprocal track and the conflict was resolved by TCAS RA activation. The Investigation found that the inbound aircraft had correctly read back its descent clearance but then set a different selected altitude. Air Traffic Control had not reacted to the annunciated conflict alert and was unable to resolve it when the corresponding warning followed and it was noted that convective weather meant most aircraft were requesting deviations from their standard routes which was leading to abnormally complex workload.

On 4 February 2020, an Airbus A350-900 initiated a go around from its destination approach at 1,400 feet aal following a predictive windshear alert unsupported by the prevailing environmental conditions but the First Officer mishandled it and the stop altitude was first exceeded and then flown though again in a descent before control as instructed was finally regained four minutes later. Conflict with another aircraft occurred during this period. The Investigation concluded the underlying cause of the upset was a lack of awareness of autopilot status by the First Officer followed by a significant delay before the Captain took over control.

On 12 November 1996, an Ilyushin IL76TD and an opposite direction Boeing 747-100 collided head on at the same level in controlled airspace resulting in the destruction of both aircraft and the loss of 349 lives. The Investigation concluded that the IL76 had descended one thousand feet below its cleared level after its crew had interpreted ATC advice of opposite direction traffic one thousand feet below as the reason to remain at FL150 as re-clearance to descend to this lower level. Fifteen Safety Recommendations relating to English language proficiency, crew resource management, collision avoidance systems and ATC procedures were made.

On 26 June 2017, an Airbus A319 which had just taken off from Stuttgart came into conflict in Class ‘D’ airspace with a VFR light aircraft crossing its track and when, at 1,200 feet agl, the TCAS RA to descend which resulted was followed, an EGPWS Mode 3 Alert was generated. Clear of Conflict was annunciated after 10 seconds and climb resumed. The Investigation concluded that the light aircraft pilot had failed to follow the clearance which had been accepted and had caused the flight path conflict which was resolved by the response of the A319 to the TCAS RA.

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.

On 11 October 2012, the crew of a Ryanair Boeing 737-800 did not change frequency to TWR when instructed to do so by GND whilst already backtracking the departure runway and then made a 180° turn and took off without clearance still on GND frequency. Whilst no actual loss of ground or airborne safety resulted, the Investigation found that when the Captain had queried the receipt of a take off clearance with the First Officer, he had received and accepted a hesitant confirmation. Crew non-compliance with related AIP ground manoeuvring restrictions replicated in their airport briefing was also noted.

Examples involving a level bust:

On 6 January 2018, a Boeing 737-900 and an Airbus A320 both inbound to Surabaya with similar estimated arrival times were cleared to hold at the same waypoint at FL100 and FL110 respectively but separation was lost when the A320 continued below FL110. Proximity was limited to 1.9nm laterally and 600 feet vertically following correct responses to coordinated TCAS RAs. The Investigation found that all clearances / readbacks had been correct but that the A320 crew had set FL100 instead of their FL110 clearance and attributed this to diminished performance due to the passive distraction of one of the pilots.

On 29 February 2020, an Airbus A320 inbound to Delhi lost separation against an outbound A320 from Delhi on a reciprocal track and the conflict was resolved by TCAS RA activation. The Investigation found that the inbound aircraft had correctly read back its descent clearance but then set a different selected altitude. Air Traffic Control had not reacted to the annunciated conflict alert and was unable to resolve it when the corresponding warning followed and it was noted that convective weather meant most aircraft were requesting deviations from their standard routes which was leading to abnormally complex workload.

On 4 February 2020, an Airbus A350-900 initiated a go around from its destination approach at 1,400 feet aal following a predictive windshear alert unsupported by the prevailing environmental conditions but the First Officer mishandled it and the stop altitude was first exceeded and then flown though again in a descent before control as instructed was finally regained four minutes later. Conflict with another aircraft occurred during this period. The Investigation concluded the underlying cause of the upset was a lack of awareness of autopilot status by the First Officer followed by a significant delay before the Captain took over control.

On 12 November 1996, an Ilyushin IL76TD and an opposite direction Boeing 747-100 collided head on at the same level in controlled airspace resulting in the destruction of both aircraft and the loss of 349 lives. The Investigation concluded that the IL76 had descended one thousand feet below its cleared level after its crew had interpreted ATC advice of opposite direction traffic one thousand feet below as the reason to remain at FL150 as re-clearance to descend to this lower level. Fifteen Safety Recommendations relating to English language proficiency, crew resource management, collision avoidance systems and ATC procedures were made.

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.

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.

On 8 July 2010 an Easyjet Airbus A319 on which line training was being conducted mis-set a descent level despite correctly reading it back and, after subsequently failing to notice an ATC re-iteration of the same cleared level, continued descent to 1000 feet below it in day VMC and into conflict with crossing traffic at that level, a Boeing 737. The 737 received and actioned a TCAS RA ‘CLIMB’ and the A319, which received only a TCAS TA, was given an emergency turn by ATC. The recorded CPA was 2.2 nm and 125 feet.

On 31 December 2011 a USAF C12 Beech King Air descended 700 feet below the cleared outbound altitude on a procedural non precision approach to Stornoway in uncontrolled airspace in IMC and also failed to fly the procedure correctly. As a result it came into conflict with a Saab 340 inbound on the same procedure. The Investigation found that the C12 crew had interpreted the QNH given by ATC as 990 hPa as 29.90 inches, the subscale setting units used in the USA. The Saab 340 pilot saw the opposite direction traffic on TCAS and descended early to increase separation.

On 15 October 2011, a Loganair Saab 340 in uncontrolled airspace and inbound and level at 2000 feet QNH on a procedural non precision approach in day IMC to runway 18 at Stornoway received a TCAS RA ‘DESCEND’ when a second Loganair Saab 340 outbound on the same procedure descended prematurely to the same altitude contrary to ATC clearance. The subsequent investigation concluded that the failure of the controller to re-iterate the requirement to remain at 3000 feet outbound until advised had contributed the crew error. Minimum separation after the TCAS RA was less than 0.1nm horizontally when 500 feet vertically.

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.

Examples related to transponder operation:

On 28 November 2020, in uncontrolled Class ‘G’ airspace, an Airbus A320 inbound to and in contact with Ballina and an en-route light aircraft tracking abeam Ballina both listening out on a shared Common Traffic Advisory Frequency (CTAF) did not recollect hearing potentially useful CTAF calls and converged on intersecting tracks with the light aircraft TCAS only selected to Mode ‘A’. The A320 received a TCAS TA but neither aircraft visually acquired the other until the minimum separation of 600 feet with no lateral separation occurred. Changes to the air traffic advisory radio service in the area were subsequently made.

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.

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.

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.

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.

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.

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