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A UH-1H Huey helicopter was substantially damaged during a hard landing while conducting fire control operations in December 2019 following a driveshaft failure caused by a fatigue crack, a new Australian Transport Safety Bureau (ATSB) investigation details.
The pilot of the helicopter, who was using a 1,200 litre fire-fighting bucket on a 150 foot ‘long-line’ to conduct firebombing operations, experienced vibrations and a buzzing noise just prior to uplifting water from the Crawford River, north of Port Stephens, New South Wales. The pilot aborted the uplift, released the bucket and began positioning the helicopter to conduct a precautionary landing in a cleared area. However, after assessing that the helicopter’s condition was deteriorating, the pilot elected to land in a small clearing, which required an approach to the hover prior to landing.
On approach to the hover, at a height of about 10 feet above the ground, the helicopter started to yaw right. Despite the pilot’s attempt to stop the yaw, directional control could not be regained, which resulted in a hard landing. The main rotor blades then struck the ground, which resulted in the main gearbox, mast, rotors and driveshaft separating from the airframe.
The pilot was able to exit the helicopter, uninjured.
“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,” said ATSB director transport safety, Dr. Stuart Godley. “In this case, the pilot responded without delay and was able to reach a safe landing site before the catastrophic driveshaft failure.”
The ATSB’s investigation determined that the driveshaft, which transmits power from the engine to the main gearbox, failed due to a fatigue fracture of the outer flex plate attached to the main gearbox fitting.
The accident helicopter had been fitted with a ‘KAflex’ driveshaft, manufactured by Kamatics Corporation in the early 1980s as part of a U.S. Army UH-1H driveshaft 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 widely on a number of different helicopter types, and those fitted to UH-1Hs under the U.S. Army retrofit program did not have a defined service life (and were considered ‘on condition’).
“The U.S. Federal Aviation Administration is currently reviewing a position paper submitted by the driveshaft manufacturer, which recommended that 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,” said Godley. “Any legacy driveshafts can be sent to the manufacturer for modification to a new ‘safety of flight’ part number.”
The investigation also determined that the driveshaft assembly was missing five washers, which increased the risk of a driveshaft failure. The accident helicopter maintenance organization 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.