On 12 October 2011, the pilot of a Robinson R22 helicopter, registered VH-JNP, was performing aerial work near Saxby Downs in Queensland, when he heard a rattling noise behind the cabin and noted that the clutch light had illuminated. The pilot opened the clutch actuator circuit breaker and, at the same time, noted a burning rubber smell, prompting him to make an immediate precautionary landing and shut down the helicopter.
What the ATSB found
The problems with the helicopter’s drive system were traced to the clutch assembly where a group of MS21042L-4 locking nuts on the drive belt upper sheave had cracked and fractured. This premature nut failure had stemmed from the likely embrittling effect of residual hydrogen generated during the cadmium electroplating process applied during manufacture. The nut failures consequently led to a series of mating part failures and a breakdown of the clutch assembly, producing the symptoms experienced by the pilot and prompting the precautionary landing.
Importantly, after recognising the aural and visual warnings of problems developing with the helicopter’s drive system, the pilot followed the required emergency procedures and made an immediate and safe precautionary landing. Taking this prompt, prescribed action limited the damage sustained and very likely prevented a more serious outcome.
What was done as a result
At the time of this occurrence, the brittle failure of MS21042L-series nuts was an emerging airworthiness issue and several associated safety actions had already been implemented. In August 2011, 2 months before this occurrence, the helicopter manufacturer issued Service Letters alerting owners, operators and maintenance personnel to the potential for cracking of MS21042L-series self-locking nuts and requiring the immediate replacement of any cracked nuts found during inspections. The service letters had been issued in response to reports of cracked nuts being discovered on Robinson and other helicopter types.
On 12 October 2011 (the date of this occurrence), the Australian Civil Aviation Safety Authority (CASA) issued an Airworthiness Bulletin (AWB 14-002), alerting pilots and maintenance personnel of the need to closely monitor the condition of high-strength steel hardware (such as these nuts) with a view to identifying any failures that may have resulted from hydrogen-induced cracking.
On 4 April 2012, the manufacturer of the specific MS21042L-series nuts in question issued a Technical Quality Notice Bulletin, addressing in detail many procedural improvements that were being introduced to reduce the potential for hydrogen-related failures of this nut type.
A potentially serious accident was avoided by the prompt actions of the pilot, who recognised the symptoms of a drive system malfunction and promptly followed the emergency procedure requirements by landing immediately.
This occurrence highlights the importance of maintained vigilance during pre-flight and maintenance inspections, where close attention must be paid to the condition of all components within the helicopter’s critical flight systems. It also highlights the importance of pilots and maintenance personnel remaining attentive to the release of any information regarding new or emerging airworthiness issues that may affect the safety of their flight operations.