Wing Tip Clearance Hazard
Wing Tip Clearance Hazard
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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.
Note: ICAO 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
- ^ NATA Safety 1st® eToolkit – Volume I, Issue 3 – October 15, 2004
Accidents and Serious Incidents
On 13 February 2019, a Boeing 787 departing Amsterdam was given a non-standard long pushback by ATC in order to facilitate the use of its stand by an incoming flight and when a Boeing 747 was subsequently given a normal pushback by a single tug driver working alone who was unaware of the abnormal position of the 787 and could not see it before or during his pushback, a collision followed. The Investigation concluded that the relevant airport safety management systems were systemically deficient and noted that this had only been partially rectified in the three years since the accident.
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.
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