Low Level Go-around - ATC Risk Management
Low Level Go-around - ATC Risk Management
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This article is based on a dedicated study made by EUROCONTROL and focuses specifically on the ATC perspective of low level go-arounds.
Description
A go-around is considered to be "low-level" if initiated below 400 ft AGL.
Since 1972, in the United Kingdom and some other countries, a limit of "not below 400 feet AGL" has been in use for planned go-arounds with the runway obstructed. This was the result of an accident in 1971 when a Trident aircraft which was intending to go around not below 100 feet, hit the tail of a Comet lined up on the runway threshold.
Generic Scenarios
There are no defned separation minima infringement measurements in the aerodrome control environment. Therefore, it is more appropriate to use the term "erosion of safety margins" rather than "loss of separation" in such cases.
A go-around event in which all parties (i.e. pilots and controllers) act within the bounds of normal operating procedures is not considered a conflict.
A dedicated study carried out by the EUROCONTROL Safety Improvement Sub-Group (SISG) defined a set of six generic scenarios of unsafe low level go-around and tested them against applicable safety barriers (discussed in next section):
- Conflict with departing aircraft from the same runway
- Conflict with other traffic on the same runway
- Conflict with departing aircraft from another parallel runway
- Conflict with departing aircraft from another crossing runway
- Conflict with other aircraft in the vicinity of the aerodrome, such as crossing or arriving traffic
- Conflict with other go-around traffic
Not all low level go-arounds lead to conflicts and turn into serious incidents. However, an analysis of reported CTR and TMA safety occurrences has shown that a disproportionally high number of events solved by providence (i.e. good luck) developed as a consequence of missed approaches initiated below 400 ft.
Safety Barriers
Prevention Barriers
These barriers, when deployed and employed correctly, are capable of alerting ATC, pilots and drivers in time to prevent normal safety margins from being eroded. Two subsets can be defined, i.e. the ones that prevent go-arounds from happening and those that prevent a conflict should a missed approach be initiated.
The barriers that are available before the initiation of a go-around are:
- Airport design and runway lighting. The limitation of this barrier is that it will not have an impact on less-than-optimum ATC or unexpected pilot action. Examples of design and lightning features:
- Clear line-of-sight from ATC to all runway touchdown zones, including remote cameras;
- Angled or wide runway entry/exit points, promoting shorter runway occupancy, e.g. rapid exit taxiways (RETs);
- Rapid exit taxiway indicator lights (RETILs).
- Airport safety nets and runway incursion prevention tools. These are various tools, both hardware and software, which can help in preventing runway incursions. This in turn prevents the presence of the traffic on the same runway, and thus prevents the need for a go-around. Airport safety nets have some limitations, though - they will not affect movement which follows an incorrect or unwise clearance and will not have an impact on long-term but forgotten clearances. They will also not affect runway vacation requests or go-around requests which are not immediately acted upon. Examples of safety nets and RI prevention tools:
- ATC clearance conformance monitoring alerts and conflict detection;
- 24-hour use of illuminated stop bars and procedures so that a lit stop bar is never crossed and ATC never clear an aircraft/vehicle to cross a lit stop bar;
- Runway entrance lights (RELs);
- Airport moving maps for aircraft and vehicles;
- Transponders in vehicles and analogous ATC functionality.
- ATC defensive controlling. This includes working techniqes which can prevent low-level go-arounds primarily caused by less-than-optimum controlling techniques. This barrier, however, will not prevent events initiated by pilot action or runway incursions. Examples of defensive controlling:
- Non-aggressive approach sequencing. Avoiding positioning aircraft too close, too fast, too high on the approach or too marginally behind preceding aircraft.
- Low-risk runway occupancy planning. Avoiding the creation of situations which are always going to be marginal, relying on everyone to expedite on and off the runway and no room for unexpected factors.
- Requesting the aircraft to enter or exit the runway at a specific position. If the ATC plan relies on an aircraft vacating the runway at a particular exit point, the pilot should be advised well before action is required.
- Constant evaluation of the plan progress and making updates as necessary as opposed to letting the situation run and hoping that things will work.
- Communicating with pilots/drivers on a level team basis when necessary in order to enhance their situational awareness and increase the likelihood of avoiding a go-around.
- Pilot defensive flying. This comprises a range of proactive cockpit ways of thinking and acting. It involves taking a share of responsibility for runway safety. Pilot defensive flying is limited for events initiated by ATC or runway incursions. Examplse of defensive flying:
- Reacting quickly and decisively to arising situations. Just like ATC, this involves not letting the situation run and hoping that things will work.
- Informing ATC of the likelihood of a go-around, thus providing an early warning and time for ATC to plan a conflict-free path.
- Informing ATC of inability to vacate or depart expeditiously, thus providing ATC with additional time to re-plan.
The barriers that are available after initiation of the go-around but before a conflict happens are:
- Conflict-free procedures. These provide sufcient separation between a go-around aircraft and other traffic such that safety margins are maintained. Unfortunately, it is not always possible to design conflict-free procedures or to use these at all times. Examples of conflict free procedures:
- Missed approach procedures and other departure/arrival routings provide procedural separation.
- ATC standard procedures in the event of a go-around, providing conflict free routings.
- ATC prevention of conflict. This involves a range of actions by one or more controllers to maintain safety margins on or following a low-level go-around. These, however, may be limited by the ability to maintain visual contact with aircraft or by the use of air traffic monitors and also by early knowledge of the go-around. Examples of situation where controllers prevent a conflict:
- Advanced coordination between ATCOs in control of other runways and/or approach control.
- Tactical prevention of post-go-around conflicts by individual ATCOs.
- Pilot prevention of conflict. An example of this is the pilot's awareness of potential conflicts and taking independent action in such a manner as to prevent safety margins being eroded. Conflict prevention by pilot is generally limited to VMC flight conditions in order to allow for visual contact with the potentially conflicting traffic to be acquired and maintained.
Mitigation Barriers
These barriers, when deployed and employed correctly, are capable of alerting ATC, pilots and drivers to the initial stages of a low-level go-around conflict in sufficient time to act in order to prevent a collision.
- ATC ad-hoc conflict resolution. This is an improvised last-minute action, once safety margins have already been eroded, by one or more controllers in order to remove the risk of collision and achieve as big a safety margin as practical. Examples:
- A single ATCO issues an ad-hoc collision avoidance instruction.
- After coordination ATC implements ad-hoc collision avoidance instructions.
- ATC conflict resolution with system support. ATCO receives an alert that an aircraft is carrying out a go-around which was previously unknown, perhaps because of poor visibility, line of sight, focus elsewhere, e.g. by a Go-Around Detection System (GARDS). The ATCO is thus able to take early action to mitigate the outcome of an immediate conflict. ATC actions (both ad-hoc and system supported) may be limited by the ability to maintain visual contact or by the use of air trafc monitors and also by RTF frequency commonality.
- ATC best practice. ATC actions and consequent flight profles for each appropriate scenario on each runway are documented, understood and practised among ATCOs. This may provide faster amd more effective conflict resolution. Best practice however can only be identifed for the most common situations and are generally not available for complex situations.
- Pilot provides own conflict resolution. This barrier is generally limited to VMC flight conditions in order to allow flight crews to acquire and maintain visual contact with the conflicting traffic. Examples of pilot actions providing conflict resolution:
- Pilot flies own tactical trajectory to increase distance horizontally and/or vertically between aircraft.
- Pilot/s obtain visual acquisition of conflicting traffic and take visual avoidance manoeuvres.
- Pilot follows TCAS conflict resolution. An example of this is the correct response to TCAS resolution advisories. This mitigation is limited by functionality inhibiting RAs below 1000 ft AGL and may also be limited by the level of TCAS fitted to conflicting traffic in the aerodrome control environment.
Analysis and Conclusions
Generally, ATC defensive controlling prior to go-arounds and subsequent ATC prevention of conflicts were the barriers with the highest spread of applicability. ATC ad-hoc conflict resolution and independent pilot conflict resolution are the barriers with the highest spread of applicability to mitigate a conflict.
Nevertheless, each airport and ATC unit have different risk levels of exposure to the six scenario outcomes leading to a conflict on or after low-level go-around. Therefore, a method of weighting the potential effectiveness of each barrier could be considered which takes account of the prevalent types of runway configuration and traffic mix at individual aerodromes.
For instance, an airport with a single runway does not need to consider conflicts involving parallel or crossing runways. Airports with a broad traffic mix of IFR/VFR traffic may wish to consider the risk level of conflict with other aircraft in the vicinity of the airport to be high. Airports with a complex multi-runway operation may wish to consider the risk level involving more than one runway to be of significance.
ATC units could carry out their own study of the severity/repeatability of conflicts on/after low level go-arounds and use an appropriate method to identify what might be the most effective barriers applicable in their operational environment. The analysis may include assigning different weighing values to the different scenarios depending on the local circumstances.
Accidents and Incidents
This section contains examples of occurrences where low level go-arounds contributed to loss of separation events.
On 13 December 2011, an Airbus 320 was allowed to depart from runway 25C at Frankfurt on a left turning SID just prior to the touchdown of an A380 on runway 25L. The A380 had then initiated a low go around which put it above, ahead of and parallel to the A320 with a closest proximity of 1nm / 200 ft, in breach of the applicable wake vortex separation minima of 7nm / 1000ft. The Investigation found that there had been no actual encounter with the A380 wake vortices but that systemic ATC operational risk management was inadequate.
On 5 September 2015, a Boeing 737-800 was about to commence descent on a non-precision final approach at Porto in VMC when a green laser was directed at the aircraft. The Pilot Flying responded rapidly by shielding his eyes and was unaffected but the other pilot looked up, sustained flash blindness and crew coordination was compromised. Subsequently, the approach became unstable and a go around to an uneventful approach to the reciprocal runway direction was completed. The subsequent Investigation noted the use of increasingly powerful green lasers in this way and that such use was not contrary to Portuguese law.
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 October 2004, a Loss of Separation occurred between an A330-200, on a low go-around from Rwy 14 at Zurich Switzerland, and an Avro RJ100 which had been cleared for take-off on Rwy 10 and was on a convergent flight path.
On 28 April 2000, a British Airways Boeing 747-400 on go around at London Heathrow Airport, UK, had a loss of separation vertically from a British Midland A321 stationary on the runway waiting for take-off.
SKYclip - Low Level Go-Around
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Further Reading
- Operational Safety Study: Low Level Go Around, by SISG, 2019
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