The ATSB's investigation of a Super Puma helicopter accident is seeking to determine the factors that contributed to the fracturing of the pitch change control sleeve, which manipulates the tail rotor blades to control yaw.
On 22 November 2024, the twin-turbine AS332L1 Super Puma was being flown from Broken Hill to Albury, New South Wales, the final leg of a repositioning flight from Kuala Lumpur, Malaysia, with a pilot and passenger on board.
During cruise at 3,500 ft, the ATSB's preliminary investigation report details, the pilot noted a high frequency vibration through the airframe.
In response, the pilot lowered the collective control and began a descent.
"During the descent there was a loud thud, followed by an uncommanded yaw to the left," ATSB Chief Commissioner Angus Mitchell said.
In an attempt to control the yaw, the pilot established an autorotation, then reduced the throttles to idle, inadvertently reducing no 2 engine beyond the idle gate and shutting it down.
"Reducing power halted the uncommanded yaw, and the pilot initiated a straight-in approach towards open fields below," Mr Mitchell explained.
"However, during the final flare with reduced airspeed, application of the collective control to cushion the landing resulted in the helicopter yawing again."
The helicopter landed heavily, initially impacting the ground upright but facing the opposite direction of flight, before rolling onto its side.
The pilot and passenger both sustained serious injuries in the impact, and the passenger, who was seated in the main cabin, succumbed to their injuries. The helicopter was destroyed.
Four ATSB investigators deployed to the accident site, where they examined the wreckage and collected evidence.
"On site, investigators found the tail rotor pitch change plate was detached from the tail rotor gearbox assembly, which was likely the reason for the uncommanded yaw," Mr Mitchell noted.
"Closer examination showed the splined sleeve supporting the pitch change control plate had fractured at the mounting flange, and that the pitch change control rod was also fractured."
The helicopter's manufacturer, now Airbus Helicopters, advised the ATSB that the splined sleeve in the Super Puma does not have a safe life limit (i.e. an hours-based overhaul schedule), and is instead assessed for serviceability via visual inspection at specified intervals, to ensure the absence of corrosion or surface scratches.
"Initial examinations of the fracture surface, at the ATSB's technical facilities in Canberra, found a fatigue crack had propagated around the majority of the splined sleeve's circumference, leading to fracture of the sleeve adjacent to the pitch change control plate flange," Mr Mitchell detailed.
"The ATSB will conduct further examination of the splined sleeve to determine the crack origin, and to identify the factors contributing to the cracking."
The ATSB's preliminary report notes Airbus Helicopters has already published two pieces of safety information to Super Puma owners, operators and maintainers, in response to the accident.
The first, a Safety Information Notice, highlights tail rotor assembly maintenance tasks, specific to inspection and lubrication requirements.
The second, an Alert Service Bulletin, specifies an inspection for defects of the splined sleeve radius area of the control plate mount flange.
Along with the further detailed examination of the fractured sleeve and the tail rotor gearbox, the ATSB's continuing investigation will also include an assessment of the accident's survivability aspects, a review of the helicopter's records and history, and an analysis of available recorded data (the helicopter was not fitted with cockpit or flight data recorders, but the ATSB was able to extract preliminary flight data from the helicopter usage and monitoring system).
The ATSB will release a final report at the conclusion of the investigation, detailing safety analysis and the ATSB's findings.
You can find here the final report: Loss of control and collision with terrain involving Aérospatiale (Airbus Helicopters) AS332L1, N368EV, 41 km north-north-west of Hay aerodrome, New South Wales, on 22 November 2024
