The Australian Transport Safety Bureau (ATSB) report that a helicopter, registered VH-ZWC, which departed Darwin Airport bound for Bamurru Plains in the Northern Territory on 28 July 2011 lost its main rotor drive and the pilot conducted an autorotative descent and landing. No injuries were reported as a result of the incident.
The ATSB investigation found that the helicopter manufacturer advised that there had been a similar instance of corrosion-related gearbox failure in an R44 helicopter. In each case the aircraft had been operated in similar climatic conditions in Australia's tropical north and had been stored outside.
The ATSBU report that the helicopter manufacturer has now modified the design of the gear carrier to incorporate a metallic cadmium surface plating to improve the corrosion resistance of the assembly.
An Airworthiness Bulletin number 63-008 has been released by the Civil Aviation Safety Authority (CASA), to raise awareness of the hazards associated with R44 gearbox internal corrosion due to water ingress.
The CASA bulletin makes recommendations to reduce the risks to those carrying the original assemblies, including:
- that operators and maintainers requesting their maintenance facility conduct a MRGB oil inspection for any contaminants such as water, rust or paint
- where appropriate store the rotorcraft under cover, or cover the main rotor mast and head assembly during inclement weather conditions
- during lengthy periods of storage or inactivity in tropical conditions take additional preservation action (seek advice from Robinson Helicopter Company)
- in the event of a MRGB warning chip light indication, land the helicopter immediately and have the issue investigated by a LAME in accordance with the manufacturer's instructions
- report all water, rust and paint contamination of the MRGB oil system and any corrosion to CASA.
More information on this incident, including a copy of the report can be found on the ATSB website.