An airport is a complex interface between the air and the ground environments, where access must be controlled and separation between aircraft or between aircraft and vehicular traffic must be maintained and optimised. While most occurrences on airport aprons and taxiways do not have consequences in terms of loss of life, they are often associated with aircraft damage, delays to passengers and avoidable financial costs.
This article examines collisions whilst aircraft are on, approaching or departing their assigned gate. The article Taxiway Collisions provides insight into collisions occurring on the airport taxiway environment.
Approaching Gate Occurrences
The aircraft can approach the gate either under its own power or under tow. Although collisions during the parking process most often occur as a result of the aircraft striking items in its path, it is also possible that the aircraft itself could be struck by support vehicles, improperly secured equipment or by other aircraft, either manoeuvring or under tow. Some of the more commonly encountered causes of collisions include:
- Failure to follow lead-in guidance - "cutting the corner" whilst approaching a gate can cause a wingtip to strike an aircraft on an adjacent gate
- Inappropriate gate assignment - not all gates can accommodate all aircraft types. If the gate assigned is too small for the aircraft type, the aircraft could strike airport infrastructure, equipment outside of the safety zone or aircraft on adjacent gates
- Inappropriate stop guidance or failure to stop when directed - a late stop can result in the aircraft striking the Passenger Boarding Bridge or other support equipment
- Vehicle traffic failing to yield right-of-way to aircraft - this action can lead to a collision but is even more likely to result in harsh braking action by the aircraft and potential personnel injuries
- Compromised safety zone - vehicles or equipment positioned within the gate's aircraft safety zone may be struck as the aircraft approaches the stop point
- Compromised taxiway clearance - there are numerous reasons that a parking aircraft might stop short of the gate (lack of marshaller, inappropriate automated guidance, safety zone violations, etc). However, in doing so, the taxiway behind the aircraft may be compromised and there is risk that another taxiing aircraft could strike the tail of the one in the process of parking
- Premature positioning of support equipment - no personnel or equipment should approach the aircraft until the aircraft beacon has been turned off, indicating that it is safe to do so. Failure to adhere to this rule can lead to collision due to further movement of the aircraft or due to the effects of jet blast
On-gate collisions most commonly involve the parked aircraft being struck by support equipment but could also be caused by other aircraft manoeuvring or under tow. Whilst being struck by another aircraft is generally obvious to all parties concerned, accidents involving support vehicles may go unnoticed or unreported. The aircraft damage resulting from collisions with ground vehicles or objects can be a significant safety risk if not identified and remedied prior to flight. It is therefore critical that all accidents are reported and that the Flight Crew Pre Flight External Check includes a thorough visual inspection. Some of the more common occurrences include:
- Aircraft struck by Passenger Boarding Bridge - a faulty bridge or an inadequately trained operator can result in the bridge striking the aircraft causing damage to the aircraft door(s) and/or fuselage
- Aircraft struck by ground support equipment - below the wing services make use of a great deal of mechanised equipment such as belt loaders, forklifts, split-loaders, catering trucks and lav and water trucks. Faulty equipment, poorly trained operators, inattention or lack of appropriate marshalling can all lead to collision with the aircraft
- Fuel truck damage - Damage from fuel trucks can occur in many ways such as improper or inappropriate positioning of the bowser, damage due to the hose lift platform striking the underside of the wing, improper hose coupling/decoupling, or from failure to detach hoses from the aircraft before moving the fuel truck
- Non secured equipment - damage can be caused by improperly secured vehicles or support equipment, such as Unit Load Devices (ULD), being blown against the parked aircraft by the wind or by jet blast
- Aircraft struck by other aircraft - other aircraft under tow or under their own power have the potential to strike a parked aircraft due to improper entry to adjacent gates, failure to follow taxi lanes, inattention or malfunction
Pushback occurrences are those which whilst the aircraft is being pushed off the gate and positioned in the taxi lane to either taxi under its own power or be towed to another location. The most common occurrences involve the pushback aircraft striking another aircraft, object or vehicle but that aircraft can also be struck by other aircraft or vehicles manoeuvring on the apron. Some of the more common occurrences include:
- Use of inappropriate equipment - use of incorrect towbar or inappropriate tug can result in the aircraft being struck and damaged by the tug as it manoeuvres
- Commencing pushback prior to retracting the Passenger Boarding Bridge - this can result in door and fuselage damage
- Commencing pushback without clearance or in the wrong sequence - commencing push without clearance or not in accordance with the cleared sequence could result in collision
- Pushing back without wing and tail "walkers" - the tug operator cannot adequately judge the wing and tail clearances of a large aircraft when conducting a pushback. Use of the appropriate number of guides is essential to prevent incident
- Inappropriate engine power settings during pushback - whilst normal engine start during pushback is routine, actions, such as a cross-bleed start, which involve high power settings should not be undertaken until the push is complete. Doing otherwise could result in the tug operator losing directional control of the aircraft, shearing of the towbar safety pin and/or collision of the aircraft with the tug or other object
- Simultaneous pushback from close or adjacent gates - gate proximity and configuration can result in loss of separation and possible collision of aircraft during concurrent pushback
- Commencement of taxi before equipment is clear - In many cases, it is not possible to see the connected pushback equipment and crew from the flight deck. Crews must ensure that all equipment has been disconnected and moved to a safe location and that all personnel are clear prior to commencing taxi
The great majority of collisions occurring whilst approaching, on, or leaving the gate are preventable. Robust training, appropriate and well promulgated procedures, consistent adherence to those procedures and a mature Safety Management System (SMS) all play a role in collision prevention. In particular flight and ground crew should:
- ensure gate assignment is appropriate to the aircraft type
- maintain the gate centerline whilst approaching and pushing off the gate
- ensure that the gate safety areas are clear whilst the aircraft is approaching or departing the gate
- be aware of wing and tail clearance at all times and using wing walkers when appropriate
- make appropriate use of marshalling services both when the aircraft is approaching the gate and when vehicles are approaching the aircraft
Accidents and Incidents
On 28 September 2022, a Boeing 777-300 taxiing for departure at London Heathrow was in collision with an arriving Boeing 757 which had turned onto its assigned gate prior to the stand entry guidance system being available without informing ATC. The 757 was taxiing as cleared following the illuminated taxiway centreline lighting. The airport AIP entry was found to state that in the absence of stand entry guidance, aircraft must remain on the taxiway centreline. The Investigation noted that lack of stand entry guidance is a common occurrence at this airport and needs to be addressed by all those involved.
On 8 August 2017, a Boeing 767-300 departing Delhi was pushed back into a stationary and out of service Airbus A320 on the adjacent gate rendering both aircraft unfit for flight. The Investigation found that the A320 had been instructed to park on a stand that was supposed to be blocked, a procedural requirement if the adjacent stand is to be used by a wide body aircraft and although this error had been detected by the stand allocation system, the alert was not noticed, in part due to inappropriate configuration. It was also found that the pushback was commenced without wing walkers.
On 5 January 2018, an out of service Boeing 737-800 was pushed back at night into collision with an in-service Boeing 737-800 waiting on the taxiway for a marshaller to arrive and direct it onto the adjacent terminal gate. The first aircraft s tail collided with the second aircraft s right wing and a fire started. The evacuation of the second aircraft was delayed by non-availability of cabin emergency lighting. The Investigation attributed the collision to failure of the apron controller and pushback crew to follow documented procedures or take reasonable care to ensure that it was safe to begin the pushback.
On 19 December 2013, the left engine of a Boeing 777-200 taxiing onto its assigned parking gate after arrival at Singapore ingested an empty cargo container resulting in damage to the engine which was serious enough to require its subsequent removal and replacement. The Investigation found that the aircraft docking guidance system had been in use despite the presence of the ingested container and other obstructions within the clearly marked 'equipment restraint area' of the gate involved. The corresponding ground handling procedures were found to be deficient as were those for ensuring general ramp awareness of a 'live' gate.
On 30 September 2010, an A330-200 was about to take off from Khartoum at night in accordance with its clearance when signalling from a hand-held flashlight and a radio call from another aircraft led to this not taking place. The other (on-stand) aircraft crew had found that they had been hit by the A330 as it had taxied past en route to the runway. The Investigation found that although there was local awareness that taxiway use and the provision of surface markings at Khartoum did not ensure safe clearance between aircraft, this was not being communicated by NOTAM or ATIS.
- ICAO Doc 9157 Aerodrome Design Manual Part 4 : Visual Aids (4th edition 2004)
- Visual Aids Handbook, UK CAA, CAP 637 (2007).