What happened
On January 17, 2023, at approximately 09:39, an AXH-E230RS unmanned helicopter, registration B-AAA01397, crashed onto a beach at the Duduan Coastal Viewing Platform in Taitung County, Taiwan. The aircraft, operated by a contractor for the Coast Guard Administration's Eastern District Branch, was conducting a flight test following routine monthly maintenance.
The flight began at 09:13 in automatic mode, climbing to a cruising altitude of 60 meters. During the return leg of the mission, approximately 280 meters from the takeoff point, the aircraft began to yaw left and right. The pilot attempted to switch from automatic to manual mode to regain control, but the aircraft entered an uncontrolled leftward spin. After spinning approximately eight times, the main rotor struck the tail boom, causing the aircraft to crash at an altitude of 57 meters. There were no injuries reported.
The investigation
The Taiwan Transportation Safety Board (TTSB) examined the wreckage and flight data recorded by the Flight Control Computer (FCC) and the Ground Control Station (GCS). Physical inspection of the debris revealed that the tail boom had been struck by the main rotor, resulting in the separation of the tail rotor assembly.
Analysis of the onboard video feed showed that while the tail rotor pitch control linkage was visible and functioning normally at 09:37, it had disappeared from the camera view by 09:39, coinciding with the onset of the uncontrolled spin.
Detailed teardown of the tail rotor assembly revealed that the pitch control sleeve was unable to rotate freely. Investigators found significant wear and metal debris (consisting of iron and aluminum) on the sleeve, the bearing, and the shaft. Specifically, the internal diameter of the sleeve was measured at 8.5 mm, which matched the external diameter of the shaft, creating an extremely tight fit. This tight tolerance led to excessive friction and resistance during operation.
Findings
- The primary cause of the accident was the fracture of the tail rotor pitch control linkage, which rendered the pilot unable to control the aircraft's attitude.
- The failure was driven by excessive friction within the tail rotor assembly caused by a design flaw in the older version of the pitch control sleeve. The 8.5 mm internal diameter of the sleeve provided insufficient clearance for the 8.5 mm shaft, leading to mechanical interference and heavy wear.
- The manufacturer, Tianwu Technology, had identified this assembly issue and updated the design to a 8.7 mm internal diameter in April 2022. However, the manufacturer had not performed a risk assessment or a replacement program for existing inventory or components currently in use.
- The investigation found that the aircraft's flight control computer and the pilot's manual inputs were functioning correctly, but the mechanical failure of the tail rotor prevented the execution of control commands.