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

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