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wind speed and direction encountered by the aircraft just prior to the landing was
about 27 kts from 247 degrees true (T).
The calculated wind speed and direction over the runway 16 threshold during the
aircraft landing flare was 27 kts from 240 degrees (T). That calculated wind speed
was derived from the correlation of the aircraft’s ground track and speed from the
Melbourne ATS radar data with the SSFDR data.
Landing tyre loads
The landing gear loads remained below their design limits during the landing, and the
investigation concluded that tyre burst was not as a consequence of the actual landing
gear loads encountered during the landing.
The right MLG parameter was the first to transition from AIR to GROUND at
touchdown. That was followed by the left MLG and the centre MLG both
transitioning from AIR to GROUND. The centre MLG then transitioned back from
GROUND to AIR, followed by the right MLG, as a result of the aircraft rolling to an
attitude of 5-degrees left wing down, coincident with the touchdown. The left roll
then reduced, and the right MLG transitioned back to GROUND one half second
later. The left MLG remained in the GROUND parameter after the initial touchdown.
Because of the MLG rearward bogey tilt, the rear tyres on the MLGs would have
contacted the runway first, with the front tyres making contact following derotation
of the bogeys. The 15-degree touchdown ‘crab’ angle would initially have subjected
the rear tyres to a large groundslip angle. Under those conditions, the resultant vector
from the aircraft’s forward and lateral momentum would have imparted a rearwards,
and inwards force on the downwind-facing tyre sidewalls. Additionally, the tyre side
forces acting on the left MLG rear tyres probably reached a ‘saturation’ point at
which stage those tyres initially entered a fully-skidded state.
The short distance between the marks showing where the rear and front tyres of the
left MLG had first contacted runway 16 provided evidence of rapid derotation of the
left MLG bogey. That rapid derotation was probably the result of high touchdown
loads, and would have imparted additional vertical ‘g’ loading on the left MLG front
tyres.
Following touchdown, the MLG tyre side forces would have tended to ‘decrab’ the
aircraft into alignment with its forward momentum along runway 16. When the left
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MLG front tyres contacted the runway, the ‘decrab’ forces would have considerably
increased the tyre side forces acting on those tyres.
Once in contact with the runway surface, the heavy loads and high operating speed
during the initial stages of the landing roll would have imposed strong centrifugal
loads on all MLG tyres. Those loads would have resulted in circumferential
distortion of the tyres, causing a traction wave outward radial deflection of the tyres.
Additionally, the high landing loads would have subjected all MLG tyres to high
tensile, compression, and shear forces due to the deflection of the tyres under load.
Those forces would probably have been greater on the front tyres of the left MLG
due to the high tyre side forces acting on those tyres, as well as the additional vertical
‘g’ loading on those tyres that resulted from the rapid derotation of the left MLG
bogey at touchdown.
The investigation concluded that the failure of the number-1 wheel tyre on the left
MLG resulted from a combination of high side forces, centrifugal loads, tensile
compression, and shear forces that had acted on that tyre during the landing
touchdown in the prevailing crosswind conditions.
Crosswind risk
A combination of factors increased the risk that ‘…a safe crosswind landing…’
would not be accomplished by the flight crew in the crosswind conditions that
existed at Melbourne at the time, even though they were less than the maximum
demonstrated landing crosswind limit (gust included) of 37 kts for the A340, and the
runway was dry.
The aircraft touched down with 15 degrees right ‘crab’, on a heading of 175 degrees
M, and was rolled in a 5-degree right wing low attitude. As such, the touchdown was
technically a combination of a sideslip and a crabbed landing. However, the aircraft’s
15-degree touchdown ‘crab’ angle was greater than the ‘maximum’ 5-degrees
recommended by the aircraft manufacturer, and therefore increased the risk of
damage to the MLG at touchdown. It also increased the risk that the resultant
groundslip would be of sufficient magnitude that the MLG tyre side forces would
exceed the ‘saturation’ point, at which they would enter a fully-skidded state.
　
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