The ATSB's investigation into the loss of the main rotor drive on a Robinson R44 Raven II (AO-2011-088) helicopter found that it was associated with corrosion and subsequent fatigue failure of the main rotor gearbox gear carrier as a result of water present in the main rotor gearbox.
The helicopter, registered VH-ZWC, had departed Darwin Airport bound for Bamurru Plains in the Northern Territory on 28 July 2011. About 30 minutes into the flight the aircraft lost its main rotor drive and the pilot conducted an autorotative descent and landing. There were no reported injuries.
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, not in hangars.
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.
In May 2012 the Civil Aviation Safety Authority (CASA) released Airworthiness Bulletin 63-008, to raise awareness of the hazards associated with R44 gearbox internal corrosion due to water ingress.
The CASA bulletin made several 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.
Read the report AO-2011-088Last update 04 January 2013