The swift response of a pilot who landed his helicopter before a "catastrophic driveshaft failure" has been praised by the Australian Transport Safety Bureau.
The federal government body yesterday released the final report of its investigation into the hard landing of a UH-1H "Huey" helicopter north of Port Stephens on December 7 last year.
The incident occurred after the pilot, who was conducting water-bombing operations using a 1200-litre bucket on a 45-metre long line, experienced "vibrations and a buzzing noise" prior to uplifting water from the Crawford River near Bulahdelah.
The pilot aborted the lift, released the bucket and positioned the helicopter to conduct a precautionary landing in a cleared area.
When the helicopter's condition deteriorated, the pilot elected to land in a small clearing which required an approach to the hover prior to landing.
But the helicopter, at a height about three metres off the ground, started to yaw right during the approach.
The pilot attempted to stop the yaw, but could not regain directional control and the helicopter had a hard landing, which separated the main gearbox, mast, rotors and driveshaft from the main frame.
The pilot exited the helicopter without injury.
"This accident highlights the importance of pilots operating helicopters in low-level environments immediately responding to the early signs of a problem, and being prepared to commit to a precautionary landing before a situation deteriorates to the point of a forced landing," ATSB director transport safety Dr Stuart Godley said.
"In this case, the pilot responded without delay and was able to reach a safe landing site before the catastrophic driveshaft failure."
The ATSB said the driveshaft, which transmits power from the engine to the main gearbox, had failed due to a fatigue fracture of the outer flex plate attached to the main gearbox fitting.
The helicopter was fitted with a 'KAflex' driveshaft, manufactured by Kamatics Corporation in the early 1980s as part of a US Army retrofit program.
The KAflex driveshaft uses flexible plates to accommodate relative movement between the engine and gearbox, and was designed with an integral failsafe feature for continued flight in the event of a single flex frame fracture.
The KAflex is used on multiple helicopters but those fitted to UH-1Hs under the program did not have a defined service life.
The manufacturer has recommended KAflex driveshafts with the same part number as the accident helicopter should be replaced at 5,000-hours service, or if the time-in-service could not be determined, removed and replaced.
The investigation also determined that the driveshaft assembly was missing five washers, which increased the risk of a driveshaft failure.
The accident helicopter maintenance organisation reported that they could not explain how the washers came to be missing, but were confident that they were all installed prior to the accident.
