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affecting the inner plies. Shear forces then develop between the various layers in the
tyre because of the force gradient that exists between the inner and outer plies of the
tyre.
Crosswind landing forces
As stated previously, large groundslip angles result if an aircraft lands with a large
drift or ‘crab’ angle in strong crosswind conditions. Additional forces are imparted
on an aircraft’s tyres on their initial contact with the ground under those conditions.
If an aircraft is drifting across the runway at the point of touchdown, the resultant
vector from the aircraft’s forward and lateral momentum will impart a rearwards, and
inwards force on the downwind-facing tyre sidewalls. Additional upwards forces will
be applied on the tread portion of the tyre in contact with the runway because of the
landing vertical ‘g’ loading25. On a dual bogey landing gear assembly, such as that on
the A340 MLG, the bogey derotation will impart additional vertical ‘g’ loading on
the front MLG tyres. Ground contact vertical ‘g’ loading on the front MLG tyres will
therefore be greater than the SSFDR-recorded vertical accelerometer ‘g’ loading at
the touchdown point.
During the investigation, the ATSB provided the French BEA with photographic
evidence of the partially disintegrated tyre and of the separated tyre tread. The BEA’s
assistance was sought in order to obtain an analysis from the French tyre
manufacturer of the tyre forces encountered during the touchdown, and which
appeared to have resulted in the separation of the tyre tread from the number-1 wheel
tyre on the left MLG. Based on its assessment of the photographic evidence, the tyre
manufacturer reported that yaw forces had been significant.26 However, the tyre
manufacturer reported that, without physical access to the tyre debris, it was unable
to establish the ‘…most probable origin of the tyre separation and damage…’.
Crosswind hazard and risk
A hazard is a source of potential harm, or a situation with a potential to cause loss or
damage. If not correctly managed, a crosswind may represent a hazard to a landing
aircraft as a result of its potential to cause damage to the aircraft, and possibly to its
occupants.
Risk is related to the potential for an event to occur that will have an impact upon
objectives.27 It is measured in terms of the consequences of the event and of the
25 Stress applied to a structure as a multiple of that in 1 ‘g’ flight.
26 Those yaw forces would have included as a result of the 15-degree touchdown drift or ‘crab’ angle,
and the ‘decrab’ forces after touchdown that acted to bring the aircraft into alignment with the
vector of its forward momentum along the runway.
27 Australian/New Zealand Standard RISK MANAGEMENT AS/NZS 4360:2004 1.3.13.
– 25 –
likelihood that the event might occur. The likelihood or probability of an event can
be expressed in either qualitative or quantitative terms
If crosswind affects a landing area, the residual risk posed by that crosswind will
depend on the intensity of the crosswind, and the existing risk controls. Those
controls may include factors such as: an operator’s standard operating procedures;
the crew’s awareness of the prevailing wind speed and direction, training and
experience and aircraft handling technique; the condition of the runway surface; and
the prevailing ambient light and visibility.
In terms of the consequences of a crosswind landing event, relevant risk controls
might include the availability of an aerodrome RFFS and of appropriate aerodrome
emergency plans.
Following the publication of the ATSB Preliminary Report into this occurrence, the
International Federation of Air Line Pilots' Associations (IFALPA) published
information about the occurrence in the June 2006 issue of IFALPA News.
The IFALPA article noted that the crosswind landing risks in this event were
‘mitigated by the fact that, despite the wind, [the meteorological] conditions were
otherwise good…’. The IFALPA also reported that it was developing a Safety
Bulletin to address the following safety issues relative to crosswind operations:
• lessons to be drawn from the Melbourne occurrence
• crosswind landing techniques
• the impact of a crosswind landing on a wet or contaminated runway on a crew’s
ability to maintain directional control of an aircraft, and whether that was a factor
to be taken into account when planning or continuing an approach
• whether a review of the utility of published ‘maximum demonstrated crosswind’
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