Improper stowage of helicopter winch hook assemblies can lead to excessive movement and accelerated wear of the cable, which if undetected, can lead to catastrophic failure of the winch cable, an Australian Transport Safety Bureau (ATSB) Safety Advisory Notice advises.
The Safety Advisory Notice comes after an incident involving an Airbus Helicopters AS350 B3 helicopter on 5 February 2020 near Bulga, New South Wales. While conducting winching operations of personnel and equipment during the recent bushfires, the helicopter crewman detected the outer strands of the hoist cable toward its termination into the hook assembly had loosened. Such loosening is known to occur during repeated short length winch deployment and retrieval cycles.
The crewman attempted to reset the cable strands using an accepted maintenance practise known as cable conditioning, however the following day the cable strands had not reset and remained loose, which prompted a second cable conditioning operation. During that operation, and while under load, the cable fractured at the hook assembly, releasing the dummy weight to the ground. There was no damage to the helicopter or injuries to personnel.
As part of an ATSB investigation into the incident, an examination of the helicopter’s winch system found that the cable failed near to the swaged ball-end that terminated into the hook assembly. The ATSB identified that the wear associated with the cable failure probably occurred due to improper stowage of the hook assembly following hoisting operations.
“A hook that is not firmly seated or with the bump stop spring not sufficiently compressed can move during exposure to airframe vibrations and rotor downwash buffeting during normal helicopter operations, which can lead to accelerated wear of the hoist cable close to the ball-end fitting where it enters the hook assembly” ATSB Director, Transport Safety, Stuart Macleod said.
“Should the cable fail while under load during operation, personnel being winched may sustain serious or fatal injuries.”
The ATSB is advising all helicopter operators and flight crew involved in rescue hoist operations to review their current operational practices to ensure hoist operation and hook stowage are in accordance with the hoist manufacturers’ published procedures.
“In addition, the ATSB advises those operators, flight crew, and maintainers, to closely review the pre- and post-flight inspection requirements of the hook and cable assembly, along with any recurring scheduled maintenance of the hoist system to ensure that they are completed in accordance with the manufacturers’ instructions,” Mr Macleod said.
In response to the failure, on 6 April 2020 winch system manufacture Breeze-Eastern published a Service Information Letter to operators, hoist maintainers and personnel working on the BL-29700 series rescue hoist. The letter reinforced the requirement for proper homing of the hook assembly following operation of the hoist, the importance of pre- and post-flight inspections of the hook assembly, and the recommended maintenance practises for the hoist system.
The Service Information Letter can be accessed from Breeze-Eastern’s website.
The Civil Aviation Safety Authority has also issued an Airworthiness Bulletin (AWB-25-034) warning of accelerated winch cable wear.
The ATSB’s investigation into the rescue hoist cable failure (AO-2020-013) is continuing.
Read the Safety Advisory Notice: Rescue hoist cable failure