Loss of Separation - ATCO-induced Situations
Loss of Separation - ATCO-induced Situations
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Description
Loss of separation between aircraft sometimes occurs as a result of action taken (or not taken) by the ATCO. In most cases this involves one or more of the following scenarios:
- Flight clearance does not provide adequate separation from other traffic.
- ATCO does not detect developing potential conflict.
- Avoiding action issued is too late or inadequate to provide safe separation.
- Instruction not received or not understood by intended recipient due to breakdown in air-ground communications.
- The controller issues a clearance that creates a conflict with a neighbouring aircraft due to the blind spot effect.
When issuing descent clearance to ABC123, the controller spots both PQR265 and XYZ312 but overlooks DEF763
Contributory Factors
The factors listed below would usually not be sufficient (on their own) to cause loss of separation but can exacerbate the situation:
- ATCO Workload. Obviously, high workload situations make people more prone to making errors. It is important to know, however, that low-workload situations may lead to the same end result:
- A high workload situation may cause the controller to skip (intentionally or not) an important action (e.g. checking for an immediate conflict before issuing a clearance).
- A low workload situation can result in errors due to complacency.
- A steep peak of the workload (i.e. a "tidal wave" of complex traffic) may result in missing one out of ten important details (e.g. a sharp turn after the sector entry point or an abnormally slow/fast traffic).
- A steep decline of workload after a (well-managed) complex situation may result in over-relaxation.
- Volume of traffic. Even if this does not result in high complexity (e.g. a lot of conflicts, traffic avoiding weather, etc.), the sheer volume of aircraft may cause a controller to miss something or make an error.
- Military traffic operating out of the segregated area in civil airspace normally requires special attention (e.g. more coordination effort and extended monitoring) and may drive the focus away from another situation.
- Flight crews (military or civil) unfamiliar with the applicable rules and procedures in a particular volume of airspace could increase workload by e.g. not properly (or timely) compliance with ATC clearances and instructions, requests for repetition, etc.
- Failure to pass an IFR aircraft timely traffic information about VFR aircraft in its vicinity.
- Issue of a VFR clearance in airspace where the only prescribed traffic separation is IFR against IFR when the ability of the VFR aircraft to comply with its clearance and maintain an effective visual lookout may be compromised by weather conditions.
- Poor (or missing) coordination between adjacent sectors or units.
- Transfer on the wrong frequency may result in the inability of both controllers to issue timely instructions or a communication loss.
- Obscured track labels (e.g. due overlapping, filters, colour representation, etc.).
- Interruption or Distraction may draw the controller's attention away from a potential conflict or may contribute to forgetting to perform an action.
- Fatigue in general reduces a person's working capacity and may even cause a microsleep (i.e. a person seems to be awake but is actually not).
Defences
At an organizational level, this includes:
- Standard Operating Procedures in the ATSU, which detail procedures to be followed to reduce the chance of loss of separation.
- Implementation of controller support tools, e.g. Tactical Controller Tool (TCT), Probe (What-if), MTCD, etc.
- Implementation of ground-based equipment designed to warn of potential collision with other aircraft, e.g. Short Term Conflict Alert (STCA).
- Use of two-person teams on one sector (normally designated a planner and an executive controller). Having two controllers handling the same traffic increases the likelihood of error detection.
- Improved standard of ATCO training (including refresher training), especially in:
- Air-ground communications safety;
- Loss of situation awareness;
- Use of structured scan;
- Response to safety net warnings.
In order to reduce the likelihood of making errors that lead to loss of separation, controllers should:
- Follow the standard procedures (but be ready to deviate from them if the situation requires it).
- Perform routine structured scan to detect potential conflicts well in advance and mitigate the "blind spot" effect.
- Make use of the support tools available which would allow them to do more in less time, thus freeing up precious seconds.
- Resist the urge to accommodate crew requests if unsure about their impact on the overall traffic situation. Naturally, this does not mean that such requests are to be disregarded.
- Quickly assess a safety net warning, create a simple plan and only then execute it (without delay). Starting to speak without having decided what to do is likely to make the situation worse.
ACAS/TCAS is an onboard aircraft equipment designed to warn of potential collision with other aircraft. This barrier is meant to mitigate the consequences of a separation loss.
Accidents and Incidents
This section contains events where ATC error was considered as a contributory factor.
On 10 November 2023, an Airbus A320 inbound to Delhi going around from an approach to runway 29L came into lost required separation with a Boeing 787-8 that had just taken off from the closely spaced parallel runway 29R. The aircraft were on two different radio frequencies. Two successive TCAS RAs occurred and were followed, which prevented a collision. Prior to that point, both aircraft had been flown in accordance with their clearances. The conflict occurred because the aircraft flight paths as cleared created a collision risk. Inadequacies in related air traffic control procedures and practices and in controller performance and support were assessed as causal.
On 8 February 2024, a Boeing 737-900ER taking off from runway 14 at Nassau in accordance with its clearance was instructed to reject its takeoff at high speed. This happened when the controller recognised that a Bombardier CRJ200, which had been on final approach to land on intersecting runway 10 had begun a go-around from low level after not receiving a landing clearance. The report says it appears the tower controller may have become overwhelmed during complex operations when there was no tower supervisor to ensure coordinated tower and approach control functions, due to inadequate manning.
On 16 February 2023, an Airbus A321 crew did not promptly begin a night takeoff from Sarasota runway 14 on receiving clearance after confirming that the flight was ready for takeoff. The crew had been advised of a Boeing 737-800 at 3 miles to land on the same runway. When the A321 was still rolling as the 737 crossed the runway threshold, the 737 crew announced they were commencing a go-around. This alerted the controller, who was working alone, to a potential airborne conflict. He instructed the 737 to immediately begin a right turn which created 0.6 nm of same-altitude horizontal separation.
On 3 December 2020 an Embraer 195-200 on final approach to Sao Paulo Congonhas was given clearance to land on a runway where the same controller had already cleared a Boeing 737-800 to line up and wait. Despite good day visibility and a late call from the E195 crew querying about an aircraft on the runway, only a call from the 737 prompted the controller to instruct the E195 to go around. The E195 crew then commenced the go-around after a repeat instruction. The resulting vertical clearance between the E195 and the 737 was calculated as 22 metres.
On 29 April 2022, an Airbus A320 and a Boeing 737-800 came into close proximity at Barcelona at night - 247 metres laterally and 44 feet vertically - after the A320 received landing clearance for runway 02 and the 737 was then given takeoff clearance on runway 06R. Both aircraft were following ATC instructions issued shortly after runway use had changed from daytime (parallel runways in use) to nightime configuration (near-intersecting runways in use). The landing clearance was issued contrary to procedures by a controller who had not reconfigured his position when runway use changed.
Further Reading
HindSight Articles:
- HindSight1 Analysis of an AIRPROX between a B737 and an A340;
- HindSight2 Loss of Separation - an Incident at Paris/Orly;
- HindSight3 Loss of separation: The Blind Spot;
- HindSight4 Loss of Separation - A Lesson for the Instructor;
- HindSight5 Loss of Separation - TCAS and STCA - Not Just Anagrams
- HindSight6 Loss of Separation - Mid-Air Collisions, Elephants, and Systems Approaches
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