Wing Tip Clearance Hazard

Wing Tip Clearance Hazard

Description

Care is always needed during ground manoeuvring but in large aircraft with swept wings, where the wingtips are often not visible from the flight deck and are subject to ‘swept wing growth’ or ‘wing creep’, the risk of wingtip damage is exacerbated and incidents can occur all too frequently.

Taxiways are designated for use by all or only some aircraft types, depending on pavement strength and adjacent obstacles. Provided ATC do not make errors in issuing taxi clearance and aircraft flight crew comply with clearances or standard routings, the greatest risk of wing tip collision arises when aircraft are holding or manoeuvring, for example on the approach to the runway entry point. Large Aircraft may have to manoeuvre, sometimes at night, to change the queuing order. Such movement often needs to be carried out without guidance from taxiway centrelines.

The responsibility for aircraft safety when taxying remains wholly with each aircraft commander (but it is in the interests of all crewmembers not to experience a ground collision). Sometimes, the potential hazard of wingtip collision is known to the airport operators, who may mitigated their liability by ATIS or NOTAM statements such as “wingtip clearance is not assured”.

The serious incidents listed below provide examples of the way in which wing tip collision has occurred. All the aircraft involved in these occurrences were, like most modern large transport aircraft, swept wing types that are subject to a phenomenon known as ‘swept wing growth’ or ‘wing creep’. This occurs during a turn when the wing tip describes an arc greater than the normal wingspan due to the geometry of the aircraft and the arrangement of the landing gear. It is one of the reasons for the manufacturers' cautions usually found in the Flight Crew Training Manuals and can be well illustrated by a scale model of your particular aircraft. Although the effect is less noticeable at moderate curvature of turn, any turn results in some ‘swept wing growth’ that will erode the perceived wing tip clearance.

When the aircraft is pivoted on its main wheels, the wing tip and horizontal stabilizer clearance may decrease as the wing tip/tail surface arc or track moves outward[1].

Operational Considerations

General

The first defence for the operating crew is a comprehensive knowledge of their own aircraft type. Manufacturers manuals provide detail the dimensions of the aircraft and the various turn radii for points such as wing tips, nose and tails. Training pilots will be able to provide some guidance and ‘rules of thumb’ to help gauge things visually when manoeuvring. For example; when making a right angle turn, if the nose misses an obstacle, will the wing tip? It is important to understand ‘wing creep’ and its extent on your particular aircraft type.

If taxiways that you might expect to use are NOTAM’d with work in progress it can help to have an annotated photocopy of the taxi charts to illustrate where the dangers lie. An enlarged photocopy may be even more useful, as will knowing in advance which rows in the cabin permit the best view of the wingtips.

In all cases, if in doubt the safest course is to STOP.

Handling

It can be all too easy to assume that if the aircraft is on the taxiway centreline that wingtip clearance is assured. That is not always the case and information about where clearance is not assured can be hard to find, hidden in the recesses of airfield plates and related notes. Vigilance and a good look-out, as always, are the best defence but can be difficult on some aircraft types and when manoeuvring in a congested area of a busy airfield under pressure of time and ATC.

Some airfields have particularly congested and confined parking areas. In these areas it is especially important to pay very close attention to centrelines, available marshalling guidance and to keep a visual look-out. If in any doubt whatsoever, stop and seek guidance. It is considerably less embarrassing than dealing with the aftermath of contacting other aircraft or objects.

Wingtip growth is much greater than many people expect and the outer wingtip also moves fast during a turn. Do not rely on helpful ‘thumbs up’ signs from anyone except qualified marshalling teams; other airport workers will not understand swept wing geometry issues.

It can be very difficult to detect any contact with objects or aircraft from the flight deck of a large aircraft. There may be few or misleading physical clues if a collision does occur. Again, if there is any possibility in your mind that contact may have occurred seek outside assistance.

NoteICAO Doc9157 Airdrome Design Manual part 2, stipulates the clearance requirements for different aircraft codes (A/B/C/D/E/F) based on wingspan and gear dimensions. This data is used when compiling information and caution notes in AIP and airport taxi charts. It should be noted that there is no universal standard of taxiway identification for different aircraft code. They can be designated by Alpha, Numeric, Geometric Symbol or Colour code depending on the jurisdiction. Aircraft commanders must ensure they have briefed themselves on the particular identification of the taxiways that are available for their aircraft type. If there is any confusion stop and request ATC guidance or "Follow Me" escort to the parking bay.

References

  1. ^ NATA Safety 1st® eToolkit – Volume I, Issue 3 – October 15, 2004

Accidents and Serious Incidents

On 18 December 2018, a Boeing 787-9 was instructed to taxi to a specified remote de-icing platform for de-icing prior to takeoff from Oslo. The aircraft collided with a lighting mast on the de-icing platform causing significant damage to both aircraft and mast. The Investigation found that in the absence of any published information about restricted aircraft use of particular de-icing platforms and any markings, lights, signage or other technical barriers to indicate to the crew that they had been assigned an incorrect platform, they had visually assessed the clearance as adequate. Relevant Safety Recommendations were made.

On 13 April 2012 a Boeing 737-800 being taxied off its parking stand for a night departure by the aircraft commander failed to follow the clearly and correctly marked taxi centrelines on the well-lit apron and instead took a short cut towards the taxiway centreline which resulted in the left winglet striking the left horizontal stabiliser and elevator of another Ryanair aircraft correctly parked on the adjacent stand causing damage which rendered both aircraft unfit for flight. The pilot involved was familiar with the airport and had gained almost all his flying experience on the accident aircraft type.

On 20 April 2010, the left wing of an Antonov Design Bureau An124-100 which was taxiing in to park after a night landing at Zaragoza under marshalling guidance was in collision with two successive lighting towers on the apron. Both towers and the left wingtip of the aircraft were damaged. The subsequent investigation attributed the collision to allocation of an unsuitable stand and lack of appropriate guidance markings.

On 14 April 2011, a Ryanair Boeing 737-800 failed to leave sufficient clearance when taxiing behind a stationary Boeing 767-300 at Barcelona and the 737 wingtip was in collision with the horizontal stabiliser of the 767, damaging both. The 767 crew were completely unaware of any impact but the 737 crew realised the close proximity but dismissed a cabin crew report that a passenger had observed a collision. Both aircraft completed their intended flights without incident after which the damage was discovered, that to the 767 requiring that the aircraft be repaired before further flight.

On 18 June 2010 a Sun Express Boeing 737-800 taxiing for a full length daylight departure from runway 06 at Prague was in collision with an Airbus 321 which was waiting on a link taxiway leading to an intermediate take off position on the same runway. The aircraft sustained damage to their right winglet and left horizontal stabiliser respectively and both needed subsequent repair before being released to service.

On 19 June 2016, an Airbus A320 failed to follow the clearly-specified and ground-marked self-positioning exit from a regularly used gate at Ibiza and its right wing tip collided with the airbridge, damaging both it and the aircraft. The Investigation found that the crew had attempted the necessary left turn using the Operator’s ‘One Engine Taxi Departure’ procedure using the left engine but then failed to follow the marked taxi guideline by a significant margin. It was noted that there had been no other such difficulties with the same departure in the previous four years it had been in use.

On 19 May 2015, an Airbus A319 crew attempted to taxi into a nose-in parking position at Lisbon despite the fact that the APIS, although switched on, was clearly malfunctioning whilst not displaying an unequivocal ‘STOP’. The aircraft continued 6 metres past the applicable apron ground marking by which time it had hit the airbridge. The marshaller in attendance to oversee the arrival did not signal the aircraft or manually select the APIS ‘STOP’ instruction. The APIS had failed to detect the dark-liveried aircraft and the non-display of a steady ‘STOP’ indication was independently attributed to a pre-existing system fault.

On 30 March 2017, a Boeing 787 taxiing for departure at night at Singapore was involved in a minor collision with a stationary Airbus A380 which had just been pushed back from its gate and was also due to depart. The Investigation found that the conflict occurred because of poor GND controlling by a supervised trainee and had occurred because the 787 crew had exercised insufficient prudence when faced with a potential conflict with the A380. Safety Recommendations made were predominantly related to ATC procedures where it was considered that there was room for improvement in risk management.

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 24 November 2016, a Dassault Falcon 7X being marshalled into an unmarked parking position after arriving at London City Airport was inadvertently directed into a collision with another crewed but stationary aircraft which sustained significant damage. The Investigation found that the apron involved had been congested and that the aircraft was being marshalled in accordance with airport procedures with wing walker assistance but a sharp corrective turn which created a 'wing growth' effect created a collision risk that was signalled at the last minute and incorrectly so by the wing walker involved and was also not seen by the marshaller.

On 26 December 2016, the wing of an Airbus A340-300 being repositioned by towing at Copenhagen as cleared hit an Avro RJ100 which had stopped short of its stand when taxiing due to the absence of the expected ground crew. The RJ100 had been there for twelve minutes at the time of the collision. The Investigation attributed the collision to differing expectations of the tug driver, the Apron controller and the RJ100 flight crew within an overall context of complacency on the part of the tug driver whilst carrying out what would have been regarded as a routine, non-stressful task.

On 13 January 2016 ice was found on the upper and lower wing surfaces of a Boeing 777-300ER about to depart in the late morning from Lisbon in CAVOK conditions and 10°C. As Lisbon had no de-ice facilities, it was towed to a location where the sun would melt the ice more quickly but during poorly-planned manoeuvring, one of the wingtips was damaged by contact with an obstruction. The Investigation attributed the ice which led to the problematic re-positioning to the operator’s policy of tankering most of the return fuel on the overnight inbound flight where it had become cold-soaked.

On 7 October 2014, a locally-based Boeing 737-800 taxiing for departure from runway 34 at Dublin as cleared in normal night visibility collided with another 737-800 stationary in a queue awaiting departure from runway 28. Whilst accepting that pilots have sole responsible for collision avoidance, the Investigation found that relevant restrictions on taxi clearances were being routinely ignored by ATC. It also noted that visual judgement of wingtip clearance beyond 10 metres was problematic and that a subsequent very similar event at Dublin involving two 737-800s of the same Operator was the subject of a separate investigation.

On 26 February 2014, an ATR 72-202 which had been substituted for the ATR42 which usually operated a series of night cargo flights was being marshalled out of its parking position with a new flight crew on board when the left wing was in collision with the structure of an adjacent hangar. The Investigation found that the aircraft type had not been changed on the applicable flight plan and ATC were consequently unaware that the aircraft had previously been parked in a position only approved for the use by the usual smaller aircraft type.

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.

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