What happened
During service, the left main outboard landing gear wheel of a Saab 340B, registration VH-ZLX, experienced a structural failure. The event was caused by the separation of a section of the inner wheel rim, specifically adjacent to the tyre bead seat. This failure resulted in the loss of integrity of the wheel assembly.
The investigation
The investigation focused on the mechanics of the fracture and the maintenance history of the wheel. Investigators determined that the failure was caused by a progressive fatigue cracking mechanism that began at the internal bead seat transition radius. This specific area is prone to high bending stresses in conventional pneumatic wheel designs.
Analysis of the wheel's history revealed that the component was an older design (P/N 5010488) which was known to be susceptible to such fatigue. While a newer, reinforced design had been introduced to mitigate this risk, the wheel on VH-ZLX was the original version.
Investigators also examined the maintenance records following a period where the wheel had been operated with a flat tyre. They found that during a subsequent maintenance task, the personnel used a standard tyre change form that did not trigger the more rigorous inspections required for 'special cases.' Because the tyre successfully held pressure for 24 hours, a full tyre change—and the accompanying non-destructive testing—was not performed.
Findings
- The failure originated from a fatigue crack at the internal bead seat radius, a region highly susceptible to stress.
- It is probable that a crack was already present during the previous eddy current inspection but went undetected.
- The maintenance documentation used by personnel failed to direct them to the Component Maintenance Manual (CMM) for the additional inspections required after operating with a flat tyre.
- The maintenance forms did not adequately communicate the necessity of a detailed visual inspection or roundness check for this specific scenario.