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Analysis

Summary

The investigation could not determine the origin of the right upper wing or when it was installed. Therefore, its history and airworthiness could not be determined. Despite the extent of damage incurred by the right upper wing during the 1993 accident, the fabric was not removed from the wing to conduct a thorough inspection of the wing structure. During the course of the repair, the wing structure was noted as appearing old. However, had a comparison with the logbook description of the wing been made at the time, it may have been evident that a deeper examination of the wing, to preclude the possibility of more extensive damage to the aged and unknown wing structure, would have been prudent.

It is possible that the compression failures and shakes found in the wooden wing structures subsequent to the 1998 accident, were caused by break-up forces. However, due to the degree of reported damage to the wings during the 1991 and 1993 accidents, it is likely that the damage was present prior to the accident flight.

The presence of either microscopic compression failures or visible shakes would have seriously reduced the load-bearing capacity of the wood. The 1993 inspection was conducted, as recommended by the maintenance manual, visually through holes cut in the fabric. The appropriateness of this type of inspection could be questionable considering the difficulty associated with visually detecting compression failures and shakes in wood components. If the compression failures and shakes existed prior to the accident, then once the wood was subjected to a load in excess of its reduced load-bearing capacity, it would have catastrophically failed without any warning.

The effect of any mis-rigging of the aircraft's upper right wing could not be determined. However, if the centre section was out-of-square, then the right upper wing may have carried extra loading for an extensive period. Although the right wing rear spar had evidence of significant weakening around the fuselage attachment fitting, it was considered unlikely that this was the area that initiated the wing break-up because the spar was predominantly subjected to compression loads at that point.

Witness evidence and wreckage disposition indicated that the right upper wing failed while the pilot was pulling out from a loop. The wing failed in the area of the right upper wing spar where the inter-plane strut was attached. Evidence indicated that the upper right wing spar was significantly weakened around the inter-plane strut attachment point by the effects of fungal decay and a partially de-bonded doubler.

Because the loop profile was described as being egg-shaped and the aircraft was possibly being flown at a lower height than normal, the pilot might have used more nose-up elevator control than usual during the pullout. The pullout from the loop may have induced a higher than usual g-loading on the wing structure, however, the loading could not be determined from the available evidence. In any case, the g-limits for the DH-82A were not published and the aircraft was not fitted with a g-meter. Therefore, the pilot was probably unaware of the aircraft's g-limit and of the g-loading he was putting on the aircraft structure just prior to the accident.

Despite the aircraft's flight manual requiring the wing slats to be locked before conducting aerobatics, it is possible that the pilot might not have locked them before commencing the looping manoeuvres. However, any opening of the wing slats should not have caused a serviceable wing to fail, although the upper right wing forward spar was already structurally weakened by fungal decay, delaminated doubler at the inter-plane strut area and possibly by pre-existing microscopic compression failures and shakes. A violent opening of the right slat may have applied some additional loading to the most critical area of the spar. If the slat deployed during the pull out from the loop, the additional loading may have contributed to the failure of the already weakened right upper wing spar.

CONCLUSIONS

Findings

The pilot may have conducted a more positive pullout from the loop than usual and in doing so probably applied a higher than normal g-loading to the aircraft. This, associated with the possible deployment of the wing slats, contributed to the in-flight failure of the already weakened upper right wing.

 
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