ATSB identifies cause of fatal Super Puma crash

The twin-turbine AS332L1 Super Puma was being flown from Broken Hill to Albury on 22 November when the incident occurred, leading to four ATSB investigators being deployed to the crash site.

The organisation’s chief commissioner, Angus Mitchell, said his team discovered the tail rotor pitch change plate was detached from the tail rotor gearbox assembly, which was likely the reason for the uncommanded yaw.

“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,” Mitchell said.

The full report reveals that during the cruise at 3,500ft, 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,” 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.

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“Reducing power halted the uncommanded yaw, and the pilot initiated a straight-in approach towards open fields below,” 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.

The helicopter’s manufacturer, now Airbus Helicopters, advised the ATSB that the Super Puma’s splined sleeve does not have a safe life limit (i.e., an hours-based overhaul schedule). Instead, it is 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,” Mitchell said.

“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 noted Airbus Helicopters 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.