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The failure of a Robinson R44 helicopter’s clutch shaft due to an undetected fatigue crack led to a loss of control before the helicopter collided with a vehicle, an ATSB investigation report details.
The R44 had been conducting aerial spraying operations at a property near Clare, South Australia on 22 December 2020. As the pilot was descending the helicopter to the landing site to reload the chemical hopper, a loud bang emanated from the rear of the helicopter, and the flight controls appeared to be jammed.
The helicopter collided heavily with the loading vehicle, coming to rest on its side. The pilot and ground crewman were uninjured, however the helicopter was substantially damaged.
“Our investigation determined that the forward yoke of the clutch shaft had failed due to an undetected fatigue crack that originated from an indent in one of the arms of the yoke,” said ATSB Director Transport Safety Dr Stuart Godley.
“This resulted in loss of drive to the main and tail rotor systems, and the loss of hydraulic fluid, which removed hydraulic power assistance to the flight control servos.
“The pilot experienced difficulties in controlling the helicopter and executed an emergency descent from a low height without hydraulic power assistance and no tail rotor control.”
The investigation found that the helicopter manufacturer’s maintenance instructions requiring verification that no cracks, corrosion or fretting were present on the yoke, lacked specific instructions on the method to be employed. The visual inspection that was employed increased the risk that a crack in the yoke arm may not be detected
The manufacturer has since developed new maintenance inspection requirements for the clutch shaft forward yoke, which were included in the R44 maintenance manual in August 2022.
Earlier, in June 2021, the ATSB released a Safety Advisory Notice to R44 operators that based on the investigation’s preliminary finding of fatigue cracking, to look for the presence of corrosion, fretting or cracking, which may not be visually obvious during all inspections of the clutch shaft yoke.
The Civil Aviation Safety Authority also issued an Airworthiness Bulletin, (AWB 63-010) advising industry of the failure of the yoke based on the ATSB investigation preliminary finding.
“This occurrence highlights how non-life limited components such as a drive train yoke may still develop defects and fail in-flight,” said Dr Godley.
“When conducting inspections, pilots and maintenance engineers should be prepared for the unexpected, and remain vigilant for defects in parts with an established history of reliability.”
Read the report: Loss of control and collision with terrain involving Robinson R44 II, VH-HOB, near Clare, South Australia, on 22 December 2020