Part 1 - Airworthiness Articles

AAC 1-103

Helicopter Personnel Winching

9/98

Airworthiness Advisory Article AAC 1-12 previously addressed the potential problem of inadvertent release of load from a winch hook due to so-called "D ring reversal". This article has been raised to further highlight the seriousness of the problem, the potential for a serious accident, and to emphasise the need for great care in conducting winching operations as a result.

In Australia there have been two cases reported where the load on the hoist disengaged from the hook. In both instances the load rested on a protuberance (e.g. skid, footrest) outside the helicopter door sill, thus relieving the hook of the weight of the load. With no weight on the hook it was easy to twist the "D" ring up the hook to rest on the safety latch, thus when the load was re-applied it slipped through the safety latch. However, the separation of a load from a winch hook is not limited to civil operations. In December 1995 a fatal accident occurred when a rescue strop separated from the winch hook of an Australian Navy S-70B-2 helicopter. This accident also was caused by "D-Ring Reversal" as a result of the hook being relieved of the weight of the load when it touched the ground before being winched up.

Load relief is just one way that a load may disengage from a winch hook. Our assessment has shown that hooks are susceptible to three modes of failure.

(1)	Load relief as described above. If the hook is relieved of the weight of the load, the "D" ring is able to travel up the hook, and reverse over the point of the hook, being retained only by the safety latch. When the load is reapplied the load disengages by applying a side force to the safety latch, which then fails.
(2)	Torsion - where the "D" ring-hook throat torsion caused the "D" ring to ride up the throat and subject the safety latch to a side load. The safety latch then fails under excessive side load.
(3)	Load jolt - Where, during a jolt of the load, the "D" ring jumps up around the safety latch and rests against the safety latch and the tip of the hook. As with case 1, when the load is re-applied the safety latch allows the "D" ring to slip through.

To reduce the possibility of a load disengaging, operators should:

(1)	Pay close attention to the selection of hook/D-ring combinations, because an appropriate combination of hook and D-Ring can make D-Ring reversal physically impossible. All that is required is a combination such that the size and shape of the two items together prevent the D-ring from reversing over the point of the hook.
(2)	Exercise care in attaching any D-ring to the installed hook, to ensure that new combinations are not introduced inadvertently.
(3)	Ensure that all personnel involved in winching operations, including flight crew, winch operators and anyone likely to be winched, are well briefed on the phenomenon and dangers of D-Ring reversal. Such a brief should cover how to recognise and avoid D-Ring reversal, and what to do in the event of D-ring reversal.
(4)	Provide appropriate training in winching procedures, emphasising the need to use the correct hook/D-ring combination, and to avoid relieving the hook of the weight of the load.

Navy Experience

In its investigation of the 1995 accident, the Naval Board of Inquiry found that the winch hook design permitted D-Ring reversal and subsequent separation. The hook fitted to the accident winch was based on the United States Military Standard MS 18027-2A, which is widely used on both military and civilian helicopters.

As a result of this accident the Navy tested a new design hook, developed by the US company Lifesaving Corp, known as the D-LOK hook. This hook is designed to prevent D Ring Reversal and snagging hazards. Slight modifications have been incorporated by the manufacturer as a result of these tests, and the production model has been introduced to all RAN helicopters which are involved in live winching. The D-LOK hook may be used with a wide range of D-Rings.

In parallel with installation of the improved hook design, the Navy implemented substantial training for all Navy personnel involved in helicopter winching operations, including a training video for each helicopter type. The Navy is taking the possibility of D Ring Reversal very seriously.

The purpose of this advisory material is to strongly encourage all operators involved in winching of personnel to:

be continually aware of the potential fatal consequences of this phenomenon,
to provide appropriate training to everyone involved in winching of personnel, and
to consider using the new D-LOK hook developed by Lifesaving Corp.