On 28 July 2011, at around 1615 Central Standard Time, a Robinson R44 Raven II helicopter, registered VH-ZWC, departed Darwin Airport on a charter flight to Bamurru Plains, Northern Territory. Approximately 30 minutes into the flight, the aircraft lost main rotor drive and the pilot conducted an autorotative descent and landing. There were no reported injuries.
What the ATSB found
The ATSB’s investigation found that the loss of main rotor drive 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.
What has been done as a result
The helicopter manufacturer has 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 among operators and maintenance providers of Robinson R44 helicopters of the hazards associated with gearbox internal corrosion due to water ingress. The bulletin made several recommendations aimed at reducing the associated risks.
Operators and maintainers of Robinson R44 helicopters are alerted to the potential for the ingress of water into the main rotor gearbox, and for the subsequent corrosion and possible fatigue cracking of componentry, which could lead to a loss of main rotor drive while in flight. Responsible persons are referred to the recommendations contained within CASA AWB 63-008, which are aimed at limiting the likelihood of water ingress and provide guidance on remedial action should water ingress be suspected.