Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation.
On 22 April 2020 at about 2323 Eastern Standard Time, the crew of a Leonardo S.p.A. Helicopters AW139 was conducting a mission involving the insertion of paramedics into a remote area in support of bushwalkers who had requested assistance. A short time after the safe insertion by hoist of the paramedics and their equipment, the paramedics requested that the helicopter return to overhead their position to assist with illuminating the area.
As the helicopter moved into position, the hoist operator positioned himself to use the hoist downlight for the illumination task. This involved him holding the hoist control pendant in his left hand and reaching for the search light directional control switch on the hoist panel with his right hand. At this moment, the helicopter experienced a gust of wind that disturbed the steady hover and caused the hoist operator to partially lose his balance.
In an attempt to stabilise himself, he held the door with his left hand and his right hand remained on or near the hoist control panel. As he was looking outside, the hoist operator’s gloved hand or wrist inadvertently flicked up the cable cutter guard and depressed the cable cutter switch in one movement, severing the hoist wire and resulting in the hook assembly falling to the ground.
Figure 1: Hoist control panel
The proximity of the cable cutter guard to the searchlight directional control switch has been assessed by the operator as an issue since 2016 with various procedural controls being enacted and/or refreshed at various times. A Civil Aviation Safety Authority‑approved modification was applied to the original panel to reduce the risk of inadvertent cable cut activation, which included the cable cut shroud as well as restraint of the intercom system lead. This did not completely eliminate the risk, but did provide a measure of design protection.
Although numerous actions to reduce the risk of inadvertent cutter activation have been recorded in the operator’s safety management system since 2016, the operator considers that a design relocation of the searchlight control switch would reduce the risk of inadvertent activation to as low as reasonably practicable.
As a result of earlier occurrences, the operator implemented an engineered risk reduction solution on their fleet in 2017 that included a shroud around the cable cut switch (Figure 2 left). This shroud was present during this occurrence.
The manufacturer released a service bulletin in September 2019 that introduced the optional installation of a hoist cable cutter frame ‘to prevent inadvertent cable cut lifting actions on the hoist control panel’ (Figure 2 right). Following this occurrence, the manufacturer issued revised alert service bulletin 139-637 in June 2020 to mandate the information from operational to mandatory, and provide modification instructions for installation.
The European Union Aviation Safety Agency also issued airworthiness directive no. 2020-0131 to require installation of the frame.
Figure 2: Operator’s solution - shroud around cable cut switch (left)
Manufacturer’s solution - frame around cable cut switch (right)
Source: Operator (left) and Leonardo Helicopters Service Bulletin 139-557 (right)
The aircraft operator advised the ATSB that in response to this recent occurrence they have undertaken a preliminary assessment to have the searchlight control switch moved from the hoist control panel and have it incorporated into the hoist operator’s pendant control. This will remove the need for the hoist operator to have their hand in close proximity to the cable cut switch on the hoist control panel while operating the searchlight directional switch.
This incident serves as a reminder for all crew members that ergonomic aircraft characteristics may pose a potential hazard to the safe operation of the aircraft or its systems. Identification and communication of such hazards allows safety action to mitigate the associated risk.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.