A standard industry practice of re-using self-locking nuts on Robinson helicopters is the subject of a safety advisory notice issued by the Australian Transport Safety Bureau this week.
The bureau, which has been reviewing the August 2017 accident in which Cloncurry Mustering Company pilot Brent Acton died, has found that re-using the self-locking nuts may inadvertently result in the omission to replace them with corrosion-resistant ones on critical fasteners.
While the re-use is an industry practice, the ATSB concluded that in consideration of the evidence they'd been examining, it could lead to the omission.
It advised that the finding of increased risk applied to all approved maintenance organisations for Robinson helicopters and said that at present, it had not been identified as a contributing factor to Mr Acton's accident.
On August 2, 2017, Mr Acton was seven kilometres north west of the Cloncurry Airport when the fatal crash occurred on Lanark Station.
The initial report provided to the ATSB indicated the helicopter had contacted powerlines adjacent to the accident site.
However an analysis of the available evidence, including photography of powerlines, did not indicate any contact between them and the helicopter.
The ATSB safety notice said there was "an anomaly with the helicopter's bellcrank in the cyclic control system".
The notice said the fastener, which attached the horizontal push-pull tube to the bellcrank, was missing.
"The bellcrank's remaining fasteners were all attached. The bellcrank and missing fastener bolt were recovered, but the reason for the separation of the nut from the bolt has not yet been determined," the notice said.
It highlighted a 2014 service letter that stated that whenever maintenance that involves the disassembly and reassembly of a critical fastener is performed, the MS21042L or NAS1291 nut should be replaced with a D210-series nut.
This requirement was necessary after cracking from hydrogen embrittlement of the type of nuts fitted to Robinson helicopters was previously identified on other aircraft.
"In the course of interviewing personnel employed by the maintenance organisation, the ATSB noted a low level of awareness of the need to replace MS21042L/NAS1291-series nuts with the D210-series nuts when critical fasteners were reassembled.
"It is a standard practice within sectors of the aviation industry to re-use self-locking nuts provided the nut cannot be turned onto the bolt thread by hand and the published torque value for the fastener is achieved."
Brent Acton's wife Shona said she was relieved with the ATSB findings.
"From our tragedy, my ultimate outcome of Brent's ongoing investigation conducted by our ATSB commission was to ensure that if there was any safety issues or risks brought to light from his accident, that they were identified and rectification made to ensure the safety of all pilots so they can all confidently come home at the end of the day to their loved ones," Mrs Acton said.
"This safety advisory notice I think has done just that and is positively promoting safety and awareness amongst the industry."