What happened
During flight, a helicopter experienced an uncommanded nose-up pitch and a roll to the left. This flight instability was caused by a fracture within the right, forward pylon support link assembly. Despite the sudden movement, the pilot managed to use compensatory control inputs to stabilize the aircraft, eventually reaching a safe landing site. The aircraft sustained damage during the event, though the pilot was able to prevent a more severe outcome, likely because the fractured link remained partially caught, providing some residual support to the rotor pylon.
The investigation
Investigators examined the pylon support link assembly and identified that the failure was driven by fatigue cracking. This cracking originated from the degradation and wear of the lower elastomeric bearing. Over time, this wear created excess clearance and movement within the bearing, leading to metal-to-metal contact between the shim and the assembly. This contact created abnormal loading conditions during standard operations, which facilitated the propagation of the crack.
Evidence suggests the helicopter had been operating with a degraded bearing for an extended period. While some degradation was likely present during the periodic inspection conducted 12 flight hours prior to the incident, the most critical area of degradation was partially obscured by the flexure assembly installation. Furthermore, the maintenance process relied heavily on visual inspections for elastomer dust or debris. The presence of a protective adhesive coating on the elastomer, while intended to prevent fluid contamination, likely masked visual indicators of wear and made the degradation harder to detect.
Findings
- The fatigue cracking in the right, forward pylon support link assembly was initiated by the degradation of the lower elastomeric bearing.
- The maintenance personnel did not identify the specific requirement in the maintenance manual to remove the pylon support link assemblies for inspection during the previous periodic maintenance.
- The failure to remove the assembly for inspection significantly reduced the probability of detecting the degraded bearing.
- The use of protective adhesive coatings on the elastomer may have hindered the visual detection of bearing deterioration.