What Happened
On 11 January 2022, a Robinson Helicopter Company R22 Beta helicopter, registered VH-VHE, was conducting a mustering operation approximately 25 kilometres south-west of Mitchell, Queensland. At about 1015, 25 minutes into the second mustering flight of the day, the pilot heard a 'pop' and a 'crack' and experienced a significant uncommanded yaw and abnormal vibration. The pilot executed a forced landing from an altitude of approximately 3 to 5 metres, shut down the engine, and inspected the helicopter. The inspection revealed that the tail rotor drive shaft had fractured.
ATSB Findings
The Australian Transport Safety Bureau (ATSB) determined that the intake valve in the engine's number-four cylinder had sustained thermal damage. This damage led to reduced engine performance, which necessitated the forced landing. During the landing, the tail rotor drive shaft fractured due to torsional overstress. The reason for the overstress could not be determined. Additionally, an unapproved modification was present on the leading edge of the tail rotor blades, although it probably had no influence on the occurrence.
Engine Power Loss
Approximately 25 minutes into the flight, VH-VHE experienced a reduction in engine power, induction backfires, and a yawing event that resulted in a forced landing. The unusual engine behaviour was likely due to thermal damage to the intake valve in the number-four cylinder, which reduced the effectiveness of the seal between the valve face and the seat. Neither the pilot nor the maintenance organisation had observed any indications of the progressing damage, including during a successful compression test conducted 17.6 flight hours prior. While not required, a borescope inspection may have increased the likelihood of detecting the valve damage.
Tail Rotor Drive Shaft
Examination identified that the tail rotor drive shaft fractured in overstress from torsional loading. The reported pedal control, up until the skids touched the ground, suggested that the drive shaft fracture was concurrent with ground contact. Noting the absence of identified mechanical or material issues with the tail rotor drive train, the ATSB considered two hypotheses: the tail rotor blades encountered a foreign object, or the altered engine performance coupled with pilot control inputs and ground contact led to the fracture. However, there was insufficient evidence to support either hypothesis, leaving the cause of the drive shaft fracture unknown.
Unapproved Blade Modification
The tail rotor blade modifications found on VH-VHE were unapproved and had the potential for significant consequences. The ATSB emphasised that using approved and documented engineering processes for changes to aircraft systems is important to ensure compliance with applicable airworthiness standards or an equivalent level of safety.