Helicopter operators and flight crew involved in rescue hoist operations should review their operational practices to ensure hoist operation and hook stowage are in accordance with the manufacturers' published procedures, the Australian Transport Safety Bureau (ATSB) urges.
The advice comes as the ATSB publishes its final report into a rescue hoist cable failure which occurred when a New South Wales National Parks and Wildlife Service-operated AS350 B3 Squirrel helicopter was conducting personnel and equipment winching near Bulga, NSW on 5 February 2020. A crewman operating the helicopter's Breeze Eastern-manufactured winch detected the outer strands of the load cable toward its termination into the hook assembly had loosened in respect of the inner core.
During a subsequent 'cable conditioning' hoist operation intended to tension the cable and realign the wires to restore the cable integrity, the cable fractured at the hook assembly while under load, releasing a 160 kg weighted bag and the hook assembly to the ground. There was no damage to the helicopter or injuries to personnel.
The ATSB's investigation found that variations in the operator's hook stowage practices over an extended period of winching operations led to inadequate compression of the hook assembly and subsequent wear of the load cable. The wear damage was due to vibration and movement of the hook assembly during periods of helicopter operation. This led to a significant reduction in the cross-sectional area of the cable, fatigue and fracture of the strands and an associated reduction in cable strength.
It is likely that specific post-flight inspection requirements for the Breeze Eastern rescue hoist required in a Civil Aviation Safety Authority's Airworthiness Directive were not being adequately completed by the operator, the investigation found. The inspections were targeted at minimising wear damage to the load cable by ensuring correct stowage of the hook assembly at the end of each flight.
The ATSB also found that the operator's method of cycle counting during operation of the rescue hoist led to an accumulation of cycles that significantly exceeded the helicopter manufacturer's recommended life-limit. That exceedance probably compounded the level of wear damage sustained by the load cable.
"The ATSB recommends that pre- and post-flight inspection requirements of the hook and cable assembly, along with any recurring scheduled maintenance of the hoist system, are closely reviewed to ensure that they are completed in accordance with the manufacturers' instructions," Director Transport Safety Stuart Macleod said. "Improper stowage of the hoist hook assembly can lead to excessive movement and accelerated wear of the cable, which, if undetected, could have a fatal outcome."
Mr Macleod reiterated that should a load cable exhibit an increased frequency of outer strand loosening requiring a condition operation, operators should be particularly mindful to check for narrowing or 'necking' of the cable at the ball end within the swivel hook assembly.
"Narrowing or necking of the cable can signify that the cable has become damaged due to extreme wear and may no longer be safe to use," he said.
The ATSB first highlighted this critical safety messaging in April 2020, approximately two months after the incident, when it published a Safety Advisory Notice (AO-2020-013-SAN-001) addressed to helicopter operators and flight crew involved in rescue hoist operations.
You can find here the final report - Rescue hoist cable failure involving AS 350 B3, VH-UAH, 1 km south west of Bulga New South Wales, 5 February 2020 (AO-2020-013)
