Loss of Separation - Pilot-induced Situations
Loss of Separation - Pilot-induced Situations
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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.
- Weather (e.g. thunderstorm activity). A pilot may start a weather avoiding action without informing the controller. The latter may not anticipate this and consequently their plan may become inadequate. It should be noted, however, that controller training generally emphasises on this possibility.
- Aircraft performance (e.g. a high or low rate of climb or descent may lead to a level bust that, in turn, may result in a loss of separation).
- Interruption or Distraction.
- Pilot workload.
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:
- Air-ground communications safety;
- Loss of situation awareness;
- Use of TCAS.
- 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.
Examples where the pilot failed to comply with an ATC clearance:
On 14 September 2022, a Boeing 787-9 inbound to Mumbai lost separation against a Boeing 787-8 also inbound to Mumbai on the same airway when it began a descent from FL410 to FL340 at night without clearance close to an air traffic control sector boundary. ATC soon observed the error but a delay in achieving contact with the 787-9 resulted in it descending to FL389 before being instructed to return to its cleared level. As it passed through FL390 descending and then again when climbing, it came within 3nm of the same-direction 787-8 which was level as cleared at FL390.
On 29 April 2021, the crew of an Airbus A319 that had just taken off from runway 18 at St Petersburg saw a Cessna M337 about to cross their intended track close to the same height, and levelled off to ensure they passed beneath it, achieving a 100-foot separation. The conflict was attributed to the controller’s general loss of situational awareness and his failure to properly scan airspace in the vicinity of the airport. The Cessna 337 pilot’s failure to fly at the standard circuit height or track downwind at the standard distance from the runway was deemed contributory.
On 12 January 2022, an Embraer 170 and a Cessna 525 crossed tracks without the prescribed minimum separation, with neither ATC nor the Embraer crew being aware. Although ATC had issued acknowledged clearances to keep the Embraer 1,000 feet above the Cessna, it actually passed beneath it, violating minimum lateral separation. The underlying cause of the event was found to be an unrectified recurrent intermittent fault in one of the Cessna’s air data systems. Poor Cessna crew/controller communication during the event, systemically poor safety culture at its operator, and shortcomings in the Textron Aircraft Maintenance Manual were considered contributory.
On 13 July 2022, an Airbus A330-300 inbound to Madrid and descending on the 32L ILS and a Cessna Citation 550 which had just departed from Torrejón Air Base lost separation. The aircraft came within 400 feet vertically and 0.6nm horizontally after the Citation failed to follow its assigned and acknowledged departure clearance. A TCAS RA "DON’T CLIMB" message was annunciated on the A330 as the opposite-direction Citation, which the A330 crew subsequently reported having had briefly in sight, passed just above it. Detection of the conflict risk to ILS traffic inbound to Madrid was delayed by sub-optimal civil/military ATC coordination.
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.
Examples involving a level bust:
On 21 March 2022, an Airbus A320 level at FL 360 lost separation with another Airbus A320, which continued its descent beyond its cleared level. A predictive conflict alert prompted the controller to issue multiple calls confirming the clearance limit but with no response. When both aircraft tracks crossed at FL 360, lateral separation was reduced to 3.8 nm. It was concluded that the immediate cause of the conflict was the failure of the descending aircraft to respond to ATC alerting calls, but that its origin was an undetected incorrect readback of the descent clearance.
On 29 April 2021, the crew of an Airbus A319 that had just taken off from runway 18 at St Petersburg saw a Cessna M337 about to cross their intended track close to the same height, and levelled off to ensure they passed beneath it, achieving a 100-foot separation. The conflict was attributed to the controller’s general loss of situational awareness and his failure to properly scan airspace in the vicinity of the airport. The Cessna 337 pilot’s failure to fly at the standard circuit height or track downwind at the standard distance from the runway was deemed contributory.
On 12 January 2022, an Embraer 170 and a Cessna 525 crossed tracks without the prescribed minimum separation, with neither ATC nor the Embraer crew being aware. Although ATC had issued acknowledged clearances to keep the Embraer 1,000 feet above the Cessna, it actually passed beneath it, violating minimum lateral separation. The underlying cause of the event was found to be an unrectified recurrent intermittent fault in one of the Cessna’s air data systems. Poor Cessna crew/controller communication during the event, systemically poor safety culture at its operator, and shortcomings in the Textron Aircraft Maintenance Manual were considered contributory.
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.
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.
Related Articles
- Loss of Separation
- Loss of Separation - ATCO-induced Situations
- Accident and Serious Incident Reports: LOS contains examples of pilot-induced loss of separation.
- Visual Scanning Technique
- Loss of Separation During Weather Avoidance
Further Reading
- Safety Management System Manual v10.1, by the British Gliding Association (BGA), 26 February 2016
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