Guarding Against a Hoist Cable Cut (Leonardo AW139, New South Wales, Australia)
At night on 22 April 2020 a Leonardo AW139 air ambulance helicopter hoisted paramedics into a remote area near Tumut. New South Wales to attend to bushwalkers who had requested assistance. The Australian Transport Safety Bureau (ATSB) report that shortly after this the paramedics requested that the helicopter assist by illuminating the area. ATSB do not identify the aircraft registration or the operator.
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.
AW139 Hoist Control Panel (Credit: via ATSB)
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.
In fact, four years earlier the proximity of the cable cutter guard to the searchlight directional control had has been noted by the operator. Various procedural controls had been “enacted and/or refreshed at various times” and recorded in the operator’s Safety Management System (SMS).
A Civil Aviation Safety Authority (CASA) approved third party modification introduced with the intent of reducing risk of inadvertent cable cut activation. This included both a cable cut shroud and restraint of the intercom lead (the latter aspect suggesting an intercom lead snag was an activation method under consideration). ATSB note that:
This did not completely eliminate the risk, but did provide a measure of design protection.
No detail is given on the design and assessment of this ineffective modification.
Safety Action
The locally modified shroud around the cable cut switch fitted at the time of the incident is illustrated below left.

Operator’s Original Modification (shroud around cable cut switch) left and Leonardo Solution (frame around cable cut switch) right (Credit: via ATSB)
Leonardo released an optional Service Bulletin 139-557 in September 2019 for a hoist cable cutter frame “to prevent inadvertent cable cut lifting actions on the hoist control panel” (above right).
After the 22 April 2020 NSW incident they issued revised Alert Service Bulletin 139-637 in June 2020 and the European Union Aviation Safety Agency (EASA) made this mandatory through Airworthiness Directive 2020-0131 (promptly effective on 12 June 2020). It stated:
This condition, if not corrected, could lead to further unintended activation of the hoist cable cutter,
possibly resulting in injury to a human load or to persons on the ground.
ATSB note that…
…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.
The aircraft operator…[has] 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.
ATSB Safety Message
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.
Safety Resources
The Royal Aeronautical Society (RAeS) has launched the Development of a Strategy to Enhance Human-Centred Design for Maintenance. Aerossurance‘s Andy Evans is pleased to have had the chance to participate in this initiative.
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. You may also find these Aerossurance articles of interest:
- SAR Hoist Cable Snag and Facture, Followed By Release of an Unserviceable Aircraft
- TCM’s Fall from SAR AW139 Doorway While Commencing Night Hoist Training
- SAR AW139 Dropped Object: Attachment of New Hook Weight
- Swedish SAR AW139 Damaged in Aborted Take-off Training Exercise
- SAR Helicopter Loss of Control at Night: ATSB Report
- Military SAR H225M Caracal Double Hoist Fatality Accident
- Marine Pilot Transfer Winching Accident: referenced in the Royal College of Art (RCA) & Lloyd’s Register Foundation Safety Grand Challenge: Safe Ship Boarding & Thames Safest River 2030
- Fatal Fall From B429 During Helicopter Hoist Training
- Hoist Assembly Errors: SAR Personnel Dropped Into Sea A UH-60M accident in Taiwan during a SAR exercise.
- USAF Helicopter Hoist Training Accident: equipment snagged on obstacle
- Fall From Stretcher During Taiwanese SAR Mission (NASC AS365N2 NA-104)
- Fatal Taiwanese Night SAR Hoist Mission (NASC AS365N3 NA-106)
- Fatal Powerline Human External Cargo Flight
- SAR Crew With High Workload Land Wheels Up on Beach
- Night Offshore Windfarm HEMS Winch Training CFIT (BK117C1 D-HDRJ)
- NH90 Caribbean Loss of Control – Inflight, Water Impact and Survivability Issues
- SAR AW101 Roll-Over: Entry Into Service Involved “Persistently Elevated and Confusing Operational Risk”
- UPDATE 14 May 2023: HH-60L Hoist Cable Damage Highlights Need for Cable Guards
Also:
- Professor James Reason’s 12 Principles of Error Management
- Back to the Future: Error Management
- Rockets Sleds, Steamships and Human Factors: Murphy’s Law or Holt’s Law?
- How To Develop Your Organisation’s Safety Culture
- Safety Performance Listening and Learning – AEROSPACE March 2017
- Learning from Adverse Events: Includes nine principles for incorporating human factors into learning investigations.
- What Lies Beneath: The Scope of Safety Investigations
The Tender Trap: SAR and Medevac Contract Design: Aerossurance’s Andy Evans discusses how to set up clear and robust contracts for effective contracted SAR and HEMS operations.
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