SAR AW139 Dropped Object: Attachment of New Hook Weight (Sapporo City Fire Department JA17AR)
The Japan Transport Safety Board (JTSB) explain in their safety investigation report (issued 18 February 2021) that the helicopter was undertaking the final of 6 hosting exercises planned for that flight.
The right-hand door was opened, a rear crew member attached two connected 5lb ARS Hoist Hook Weights (a total mass of 10 lb, 4.54 kg) to the hook to increase stability and let go of the hoist hook.
The weights however “came off from hoist hook hole and dropped from altitude 180 ft (55 m)”. They were subsequently found in the national forest near the airfield. They had not hit anyone but the Dropped Objects Prevention Scheme (DROPS) calculator confirms this as a potentially fatal incident.
JTSB Safety Investigation
The JTSB report that upon examination no defects were found with either the hoist hook or the attachment hook on the weights, a component also widely used on mountain-climbing gear. The investigators comment that:
The weight hook can be opened by squeezing levers on the both side and remains open. When you insert an object to hook, the object pushes the guide and close the hook. The weight hook consists of two hook-shaped parts that are overlapped symmetrically and attached with a shaft, and the spring is pushing the two parts to close from both sides.
When the hook is closed, the spring force is applied in a direction to close, and when it is open, the force is applied in a direction to open. When you connect the weight to the hoist hole it is difficult to visually confirm situation of the weight hook whether it is open or closed for the weight hook works inside the hole.
The hook weights had only recently been purchased by the Fire Department, differed from the traditional Karabiners and had only been used “a few times”.
Upon introducing the weights, rescue members conducted familiarity training to confirm their usability and installation certainty, but did not anticipate the possibility that the weight hook may be disengaged.
Fortuitously video footage was available of the incident to aid the investigation.
The enlargement of the hoist hook image…
…shows that the lever on one side can be seen, but the other side cannot. In addition, the black upper part of the hook seems to be open on its right side. From this, it was confirmed that one side of the hook was open when the weights were released.
The image above shows show that the crewmember placing their hands on the lever when attaching the weights to the hoist hook hole.
The instruction manual of the hook manufacturer, describes a warning that it should never hold the lever during connection. However, when they purchased the weight[s], no manual was attached.
The crew member confirmed they were aware of the need to apply tension to confirm secure attachment but unconsciously omitted that check.
During testing, it was found that weights were able to hang as shown below.
If the weight was put outside in this state, weights would sway to the left, and therefore, the weight hook would turn clockwise as the arrow indicates. It was confirmed due to the shape of the hook tip, the hoist hook opened the left side of the weight hook, so both sides of the hook were opened, and weights were unhooked.
JTSB Probable Cause
The JTSB concludes that the probable cause of this serious incident was the weight hook was almost certainly not properly closed when attached on the hoist hook, and when the weights with the hoist hook were released [i.e. deployed] outside of the rotorcraft, the [weight’s] hook opened and weights dropped.
Safety Lessons & Actions
JTSB comment that:
When introducing new equipment, it is necessary to confirm in advance, the usage including appropriate selection, and sufficiently identify the risk factors.
The Sapporo City Fire Department Air Corps took following safety actions.
- They suspended to use this weight.
- Revised the confirmation procedures when positioning the weight on the hoist hook as follows. 1) The weight installation work shall be done inside the aircraft 2) After the weight installation, it shall be confirmed by applying tension 3) The connection state of weights shall be cross-checked by two crew members and only then the weights would be untethered from the aircraft
The European Safety Promotion Network Rotorcraft (ESPN-R) has a general helicopter safety discussion group on LinkedIn and a Hoist Operation Safety Promotion LinkedIn group.
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