Incident Description
On 15 July 2022, a Bell Textron 505 helicopter registered VH-VTB was conducting a scenic flight near Double Island, Queensland, with a pilot and two passengers on board. Approximately 30 minutes into the flight, the pilot heard two brief, loud grinding noises and decided to proceed to Cairns Airport. While over the airfield, the grinding resumed and continued without stopping. After 10–12 seconds, at an altitude of about 10 ft, the pilot heard two loud bangs, and the helicopter yawed to the right. The pilot responded by moving the throttle to idle, which stopped the yaw. A run-on landing was performed on the grass short of the assigned helipad. Post-incident inspection revealed that the tail rotor driveshaft had failed.
Findings
The Australian Transport Safety Bureau (ATSB) found that a section of the tail rotor driveshaft failed due to a bearing that seized and overheated in flight. The reason for the bearing seizure could not be determined. An examination of the bearings two days prior had revealed no problems, and the adjacent bearing was well lubricated, with some very small points of damage on the bearing race indicating potential contaminants.
During the failure, a combination of heat and torque due to the seized forward fan shaft bearing resulted in failure of the fan shaft just aft of the bearing. The bearing seizure was evidenced by a disintegrated inner race, metal deposits on the bearing seat, and signs of excessive heat around the bearing components and fan shaft. Scoring on the outer race and hanger indicated that at some stage, the bearing and fan shaft had also spun together. It could not be determined when the bearing seized, but the noises heard by the pilot were likely either the fan shaft spinning on the bearing or the bearing spinning on the hanger. The noises heard two days before the incident could not be definitively linked to the bearing failure.
Due to the extent of damage, the reasons for the bearing seizure could not be identified. The disintegrated inner race, corrosion within the outer race, and damage to the hanger could all have been results of the seizure rather than contributing factors. No grease was observed after the failure, but it could have been expelled or dissociated by the excessive heat. If the bearing had not been properly greased, it would likely have produced abnormal noise or excessive heat during the visual inspection two days earlier, but neither was detected.
The aft fan shaft bearing showed no signs of corrosion or damage associated with improper lubrication, only small regions of spalling. Such localized damage was most likely due to a foreign contaminant within the bearing, as other possibilities such as poor tolerances, misalignment, or manufacturing defects would have resulted in more widespread damage. The forward bearing failure could have resulted from exposure to similar contaminants, but the faces of the inner and outer race were too damaged for assessment.
Landing and Aftermath
The pilot reacted quickly to the fan shaft failure, reducing the throttle in accordance with Federal Aviation Administration guidelines. Despite having sufficient power for a normal approach, the pilot's decision to adopt a shallow approach provided the advantages of ground effect and translational lift, reducing the required power. The combination of higher airspeed and lower power from the shallow approach and running landing mitigated the yaw effect and enabled a safe landing.