Introduction
On an unspecified date, a Robinson R44 helicopter registered VH-OOE experienced an engine failure during flight near Tindal Airport, Northern Territory. The pilot performed a precautionary landing after noting severe vibration. The aircraft sustained damage to its landing gear skids during the landing.
Engine Failure
The Australian Transport Safety Bureau (ATSB) examination of the engine components determined that the failure resulted from mechanical damage caused by the separation of the number 4 cylinder connecting rod from the crankshaft. The initiating factor was almost certainly the in-service loosening of the connecting rod nuts on the number 4 cylinder. Evidence included fretting wear in the connecting rod bolt holes, indicating engine operation after a loss of bolt tension. One nut was missing, and the opposite nut was mostly unwound. No fatigue cracking was found on the bolts or connecting rod.
Investigation Findings
The ATSB considered several possible mechanisms for the nut loosening, but could not definitively rule out most of them. These included:
- Abnormal loading or vibration: No evidence of engine overspeed, piston melting, or structural damage from severe abnormal combustion was found. Loose spark plugs were noted but had no negative impact on engine performance.
- Variation in thread condition and lubrication: Could not be fully assessed due to thread damage and the missing nut.
- Improper or defective parts: Connecting rod bolts and nut material were correct, but original condition could not be fully assessed.
- Embedding (microscopic deformation): Initial bolt tension may have reduced slightly, making the nut more susceptible to loosening.
- Inadequate installation torque: Possible that nuts were slightly under-torqued during installation and progressively loosened over the subsequent 291 hours of operation. The number 3 connecting rod nuts were found at incorrect torque during engine disassembly, supporting this scenario.
Ultimately, the reason for the nut loosening was not determined.
Maintenance Issues
The investigation identified that during the engine overhaul, the independent inspection of the engine sub-assembly did not include a torque check of the connecting rod nuts. While not a regulatory requirement and with no evidence of influence on this occurrence, the ATSB considered it a missed opportunity to verify the installation torque.
Additionally, both spark plugs in the number 4 cylinder were found loose. The reason was not determined, and no evidence indicated an influence on the engine failure. However, during the most recent periodic inspection, the helicopter maintenance provider did not refit the spark plugs with new gaskets as required by the manufacturer. Furthermore, Civil Aviation Safety Authority guidance on spark plug gasket fitment was inconsistent: Airworthiness Bulletin AWB 20-001 stated annealed gaskets could be used, while AWB 85-023 mandated new gaskets in all circumstances.
Pilot Actions
The pilot noted an unusual sound and vibration during engine start, which disappeared when engine speed increased. During flight, severe vibration prompted the pilot to assess controllability; no abnormal engine indications were observed. In accordance with the Robinson R44 Pilot's Operating Handbook, a precautionary landing was conducted at a suitable location. The pilot communicated the issue to Tindal Airport air traffic control, which increased the likelihood of a timely emergency response had one been needed.
During the final approach, the low rotor speed warning horn and low oil pressure caution light activated, indicating a reduction in power due to the mechanical failure. This reduction, combined with the pilot's concern about a possible fire and recollection of landing quicker than normal, likely resulted in a heavy landing that spread the landing gear skids.
Conclusion
The ATSB determined that the engine failure was caused by mechanical damage from separation of the number 4 cylinder connecting rod, initiated by loosening of the connecting rod nuts. The reason for the nut loosening was not established. Maintenance oversights and inconsistent guidance were noted but not directly linked to the failure.