Brief, loud grinding noises precipitated the failure of a Bell 505 helicopter’s tail rotor driveshaft, an Australian Transport Safety Bureau report details.
On 15 July 2022, the Bell 505 was being used for a scenic flight from Cairns, Queensland, with a pilot and two passengers on board.
About 30 minutes into the flight, near Double Island, the pilot heard two brief, loud grinding noises, and elected to return to Cairns Airport.
Over the airfield, the grinding noise returned, and after 10-12 seconds, while approximately 10 ft off the ground, the pilot heard 2 loud bangs.
“In response, the pilot moved the throttle to idle, which stopped the yaw,” ATSB Director Transport Safety Stuart Macleod explained.
A run-on landing was performed on the grass short of the assigned helipad, and a subsequent inspection of the helicopter revealed the tail rotor driveshaft had failed.
“The ATSB determined that during landing, a combination of heat and torque due to a seized forward fan shaft bearing resulted in failure of the fan shaft just aft of that bearing,” Mr Macleod said.
Due to the amount of damage on the failed bearing, the ATSB was not able to identify the reason for the seizure, and the manufacturer, Bell Textron, advised the ATSB that it was not aware of any previous instances of bearing failure in 505 fan shaft bearings.
“Unusual sounds and responses from an aircraft can be an indication of an imminent system failure,” Mr Macleod continued.
“In this instance, the pilot’s decision to return to Cairns was probably influenced by the initial short duration of the unusual noises and overwater operation. While a safe landing on an airfield resulted, the occurrence also illustrates how quickly failures can occur.”
Mr Macleod noted pilot’s decision to adopt a shallow approach, and to reduce throttle immediately following the shaft failure, both assisted in controlling the helicopter following the uncommanded yaw, and allowed for a safe landing.
“This incident does highlight that pilots experiencing any unusual vibration or noise should land as soon as possible and have the aircraft inspected prior to further flight,” he said.
“If an immediate landing is not possible then pilot should be prepared to conduct an emergency landing or ditching if the situation deteriorates.”