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Analysis

Summary

Tyre failure mechanism and detection during maintenance and retread

Each of the tyres examined sustained a casing break-up event as a result of fatigue damage in the sidewall of the tyre. These failures occurred at a similar retread level (R4 and R5), but occurred well before the tread was due for replacement.

Aircraft tyres of this type had several operational measures to prevent premature failure. These included non-destructive inspection (NDI) during the retread process, inflation pressure maintenance, and aircraft weight and speed limitations.

Examination of the failed tyres found that the fatigue damage was quite extensive and was considered to be possibly present at the last retread. Because the shoulder-to-shoulder NDI used by the tyre retread facility was limited to the crown (tread area) of the tyre, it could not detect damage in the sidewall unless the extent of the damage breached the inner or outer surface of the tyre resulting in a leak. The NDI technique utilised by the retread facility was physically capable of inspecting the sidewall of the tyres, but neither the Civil Aviation Safety Authority (CASA) nor the Federal Aviation Administration (FAA) required sidewall inspection for bias-ply tyres.

Diffusion of the air through the inner liner of tubeless aircraft tyres resulted in the slow loss of pressure. Thus, it is vitally important to regularly check and maintain the tyre inflation pressures. The normal practice was a daily check on cold loaded tyres. This not only ensured that the tyres were operated in the correct pressure range, but also allowed maintenance engineers to determine if a tyre was leaking at an abnormal rate.

An excessive leakage rate is a strong indication of imminent tyre failure and so it is critical that it is identified as soon as possible. The survey conducted by the operator indicated that only about one third of the low-pressure events were being recorded by the engineers. For those events that were not recorded, the only way to identify a 'leaker' (a tyre with pressure leakage in excess of the normal limits) was for a maintenance engineer to remember that the same tyre on the same aircraft was topped-up on the previous day. As the operator had 45 aircraft, each with six tyres (four main and two nose) and operated from different ports around Australia, the detection of a 'leaker' was very unlikely.

The maintenance Task Card used by the operator during the daily check listed procedures for all of the 737 variants in their fleet. This was particularly evident for the tyre inflation checks where there were three possible nominal inflation pressures. This Task Card did not directly indicate the inflation pressure range for each aircraft variant, but contained a note to refer to the Aircraft Maintenance Manual (AMM) if the pressure was below nominal by more than 5%. Although the abbreviated data on the Task Card provided the correct nominal pressure, it left the appropriate inflation pressure range open to some level of interpretation and possible confusion, as the limit for the inflation pressure in the AMM was the nominal ±5 psig, but the special maintenance procedures started at 5% (i.e. 10 psig) below the nominal pressure.

Until 2004, when the task card had the tyre nominal service pressure increased from 200 psig to 205 psig, the tyres had been operated at or below the lower limit of the normal service pressure range for extended periods. Under-inflation is a well known factor in the development of fatigue in the sidewall of tyres resulting in reduced life performance.

The examination of the incident flight data did not show any single event that would have resulted in the failure. However, a review of the taxi speed data from two aircraft that had tyre failures, indicated that they were occasionally turned at speeds approaching, or exceeding, that recommended by the aircraft manufacturer. Considering the examination of the failed tyres and the analysis presented above, the damage to the tyres was not due to a single event, but an accumulation of damage during normal operations. The effect of turning at taxi speeds above that recommended by the manufacturer would further reduce the life expectancy of the tyre casing.

Certification of tyres and qualification for retread

The basic design standard for the tyres (FAA TSO-C62d) required requalification for a tread design change. This applied to each manufacturer. If a manufacturer changed their basic tread design, such that it was the same as a retread package, they would be required to retest to show compliance with the TSO. However, the practice for retread qualification accepted by both CASA and the FAA, required only one brand of tyre to be fully tested (overpressure and dynamometer). This testing may verify the retread package, but does not capture the effect that the tread design, which may be different to the original tread design on the tyre, has on the performance of the casing of the non-tested brands.

Because the tyres were originally approved to Technical Standard Order TSO-C62d and had lasted for more than three retread lives, it was clear that there was no immediate weakness inherent in the original tyre design. As the aircraft type was fitted with several tyre brands, but only developed similar fatigue flaws in one brand, it appears that there were performance differences between the brands that the retread qualification testing of a single brand at R1 could not identify. There was no apparent compliance substantiation for the untested brands to the original design standard (i.e. TSO-C62), other than historical acceptance of similarities in the construction of bias-ply tyres.

 
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