What happened
On 1-4 February 2022, a Garlick UH-1H was engaged in aerial firebombing support for the Tasmania Fire Service near the Lebrina bushfire in northern Tasmania. While attempting to extinguish a spot fire on the western flank of the fireground, the pilot released the underslung water load prematurely. Following this release, the aircraft moved toward an open paddock but lost directional control as it slowed. The helicopter subsequently pitched forward into the terrain, resulting in one fatality.
The investigation
The ATSB examined the wreckage and identified a catastrophic failure within the KAflex drive shaft, which transmits torque from the engine to the transmission. Investigators found evidence of a torsional-induced breakout of the transmission-side interconnect tube and end fitting. Metallurgical analysis revealed high contact forces and severe friction damage on the transmission coupling and interconnect bore.
Significant evidence of fretting product, such as oxide dust, was found at a frame joint, suggesting the joint had been unstable for some time. While the attaching bolt remained intact, one washer had separated. Although the shaft initially entered a fail-safe mode, the investigation suggests the pilot likely jettisoned the water and diverted to clear ground in response to the intense noise and vibration associated with this mode.
Findings
- The engine-to-transmission KAflex drive shaft partially failed due to the fracture of a flex frame element or attaching bolt.
- The pilot likely released the water load and diverted to an open area to manage the initial mechanical distress.
- As the aircraft slowed during the approach to the paddock, the drive shaft decoupled completely, causing an instantaneous loss of drive to the rotor system.
- The complete loss of rotor drive prevented a survivable autorotative landing, likely due to a critical and rapid reduction in main rotor RPM.
- An abrupt right yaw occurred during the loss of drive, which was inconsistent with the flight manual's expected left yaw, potentially due to disproportionate tail rotor thrust reduction or flight control interference from debris.