Australian investigators say the engine-to-transmission main KAflex drive shaft partially failed in the fatal crash of a Garlick Helicopters Bell UH-1H Huey in northern Tasmania.
The failure is likely due to the fracture of a flex frame attaching bolt, or a flex frame element during flight, according to findings from the Australian Transport Safety Bureau (ATSB).
“This initial failure resulted in the KAflex entering fail-safe mode, which allows for a short period of continued flight, but necessitates landing as soon as possible,” said Stuart Macleod, ATSB director of transport safety.
The incident occurred during firefighting operations over the Lebrina bushfire north of Launceston, about 20 kilometers (12 miles) from Tasmania’s northern coast, on Feb. 14, 2022. The Huey helicopter collided with terrain after losing main rotor drive, investigators said.
With the KAflex in fail-safe mode, the pilot jettisoned water from an underslung bucket and diverted to clear ground, investigators said. But while the helicopter descended, the KAflex failed completely, “resulting in instantaneous loss of drive to the rotor system,” investigators said.
“Following the loss of main rotor drive, the pilot was unable to complete a survivable autorotative descent and landing, probably due to a critical reduction in main rotor speed,” Macleod said.
The crash resulted in several new safety measures, including a safety advisory notice to all UH-1H operators from the ATSB. The advisory instructed operators to look for evidence of defects in the KAflex driveshaft of their helicopters, if fitted.
Australia’s Civil Aviation Safety Authority also updated a previous airworthiness bulletin after the incident, covering pre-flight inspection requirements for the KAflex drive shaft.
U.S. regulators have also issued an airworthiness directive, with the Federal Aviation Administration requiring operators to replace KAflex driveshafts after 5,000 hours of use, or in cases when operators are unable to verify flight hours.
The 41-year-old pilot was the helicopter’s sole occupant and was working as a subcontractor to the Tasmania Fire Service, per a report from the Australian Broadcasting Corporation.
“It is a very tragic situation, especially because it has involved someone who was working hard to fight the Lebrina fires to keep our community safe,” said Jeff Harper, acting chief of the Tasmania Fire Service, at the time of the incident.
The Lebrina fire burned more than 1,600 hectares (4,000 acres) across several days prior to the helicopter incident, ABC reported. By that point, the immediate threat was reportedly over for residents, and operators grounded their firefighting aircraft.
The incident aircraft was operated by Richmond Valley Aviation, according to the ATSB investigation report. On its website, the operator describes itself as a family-owned company based at Casino Airport in the Northern Rivers region of New South Wales, Australia, about 49 nautical miles (90 kilometers) southwest of the Gold Coast.
Photos of the crash site show the wreckage strewn over a small area. Most of the tail section, including the tail rotor gearbox, separated from the fuselage, investigators said. Both main rotor blades, the battery, and the landing skids also separated from the helicopter.
The incident helicopter was originally manufactured by Bell in 1965 for the U.S. military and was converted by Garlick Helicopters for civilian use in November 2007, per the ATSB investigation report. It was powered by a Honeywell Aerospace (formerly Lycoming) T53-L-703 turboshaft engine.
