What happened
While operating at approximately 500 feet above a valley floor, a Robinson R44 experienced sudden vibrations followed by the failure of the clutch actuator lower bearing. This mechanical breakdown caused a total loss of drive to both the main and tail rotors. As the aircraft was flying over rising terrain, the pilot was forced to perform an autorotation into trees. The impact resulted in two serious injuries and two minor injuries among the passengers and the pilot. The helicopter sustained substantial damage during the collision.
The investigation
The investigation focused on the seizure of the lower bearing and the subsequent chain of events. Investigators determined that the lower bearing seized, which led to the failure of the fan shaft. While maintenance records indicated that the bearing had been checked for roughness 20 flight hours prior to the event, and the operator had been monitoring for unusual noises, no issues were detected on the day of the accident.
Evidence suggested that the bearing was not being maintained according to manufacturer procedures. Specifically, the lubrication intervals had been inconsistent; while the bearing was initially under-lubricated following its installation, it had recently been lubricated more frequently than required due to the hot operating environment. The presence of solid, dried grease on the lower sheave indicated that the lubricant was not effectively cooling or lubricating the component. Furthermore, the investigation noted that the maintenance procedures for the Robinson R44 did not explicitly warn against over-lubrication, nor did they identify that frequent micro-switch replacements could signal an impending bearing failure.
Findings
- The clutch actuator lower bearing was not maintained in accordance with manufacturer procedures, which likely led to its failure.
- The passengers did not receive a pre-flight safety briefing, leaving them unaware of emergency equipment or brace positions.
- The passengers exited the aircraft while the rotors were still turning without a specific briefing on how to safely clear the rotor arc.
- The aircraft was not equipped with a fire extinguisher.
- The emergency locator transmitter (ELT) failed to activate until 10 hours after the accident, partly due to a lack of clear maintenance guidance and the absence of a regulatory requirement for monthly self-tests.
- Regulatory changes regarding passenger briefings and fire extinguisher carriage were not clearly communicated to operators by the civil aviation authority.