Failure Description
The left main outboard landing gear wheel on Saab 340B aircraft VH-ZLX failed when a section of the inner wheel rim adjacent to the tyre bead seat fractured and separated. The cracking and fracture exhibited characteristics typical of progressive fatigue cracking. The fatigue crack initiated on the internal bead seat transition radius, a region known to experience high bending stresses in conventional pneumatically pressured wheel designs. Operational stresses from tyre flexure during taxi and landing can contribute to this failure mechanism.
The failed wheel was of the original design (ABSC P/N 5010488). The wheel manufacturer had introduced updated inspection methods, tyre change or cycle limits between overhauls, and a revised wheel design (P/N 5010488-1) with strengthened bead seat features to address this known issue. The original design remained susceptible to bead seat radius fatigue cracking.
Crack Development and Inspection
During non-destructive testing at each tyre change, no cracks were identified, and the wheel continued in service. The ATSB identified two possible scenarios for the fatigue crack that initiated the failure: either the crack initiated and grew rapidly after the last non-destructive inspection, or a crack was present at the time of that inspection but went undetected.
From fracture surface analysis, the ATSB could not determine the crack growth rate. However, they considered it probable that a crack had been present at the last eddy current inspection but was not detected. Supporting this view: the fatigue area was relatively large (86 mm) compared to the low number of flight cycles (97) since the last inspection; the failure occurred well before the next inspection due at 300 flight cycles; and the low number of reported failures in this wheel type over the last decade suggested the inspection interval was generally adequate.
While it was possible that the wheel operated with lower than recommended tyre pressure for a short time, which could influence crack growth rate, the ATSB deemed it unlikely that this increased the growth rate enough to cause failure at one third of the nominal inspection interval.
Maintenance Documentation
When the wheel was removed from aircraft VH-ZXK and presented to the wheel bay, personnel used the standard tyre change form to complete the inspection, noting it as a 'Repair' rather than a standard tyre change. By using this form, personnel signed for the 'general wheel check and inspection admin' task, which did not include a visual inspection (listed as a separate item). The tyre retained pressure after reinflation, so it was not changed, and a complete tyre change inspection—including non-destructive inspection—was not performed.
Because the wheel had been operated with a flat tyre (even if briefly), it should have undergone additional inspections outlined in the 'Special cases' section of the Component Maintenance Manual (CMM). However, the documentation used did not indicate this requirement nor direct personnel to the CMM for non-standard cases. The 'Special cases' section required a detailed visual inspection and roundness check. The ATSB could not determine whether these inspections would have identified the crack, but they would have provided opportunities to detect irregularities. Although not required in 'Special cases', non-destructive inspection would almost certainly have detected the crack.