What happened
During flight, the pilot of a Robinson R44 experienced severe vibration and an unusual sound during the engine start sequence, though the vibration ceased once engine speed increased. During the later stages of an approach, the pilot observed the activation of the low rotor speed warning horn and the low oil pressure caution light. These indications signaled a reduction in engine power.
In response to the mechanical issues, the pilot performed a precautionary landing in a suitable location and notified Tindal Airport ATC. Due to the loss of power and the pilot's concern regarding a potential fire, the helicopter landed heavily, which caused the landing gear skids to spread.
The investigation
An examination of the engine components revealed that the engine failure was caused by the separation of the number 4 cylinder connecting rod from the crankshaft. Investigators found evidence of fretting wear in the connecting rod bolt holes, suggesting the engine had operated after a loss of bolt tension. Specifically, one nut had completely detached, while the opposite nut was found mostly unwound.
While several potential causes for the loosening were considered—including abnormal loading, vibration, or improper parts—the exact reason the nuts loosened could not be definitively determined. However, investigators noted that during the engine overhaul, an independent inspection of the sub-assembly did not include a torque check of the connecting rod nuts. Although not a regulatory requirement, this was viewed as a missed opportunity to verify the installation.
Additionally, the investigation found that both spark plugs in the number 4 cylinder were loose. While there was no evidence this caused the engine failure, it was noted that the maintenance provider had failed to use new gaskets during the most recent periodic inspection, contrary to manufacturer requirements. The investigation also highlighted inconsistent guidance from the Civil Aviation Safety Authority regarding the use of annealed versus new spark plug gaskets.
Findings
- The engine failure was triggered by the in-service loosening of the number 4 cylinder connecting rod nuts, leading to the rod separating from the crankshaft.
- An engine overhaul inspection failed to physically verify the torque of the connecting rod nuts.
- Maintenance procedures for spark plug gasket replacement were not strictly followed during the previous periodic inspection.