Ditching involving a Puma SA 330J, VH-WOF, Mermaid Sound, Western Australia, on 12 May 1991

The aircraft was tasked to carry out a marine pilot pick-up from a departing tanker. The flight was conducted by two pilots operating under night visual flight rules. Conditions were a moonless night with no defined horizon, no outside lighting other than from the ship, and a surface wind that was light and variable. The ship was steaming in a northerly direction at 12.5 kts.

The flight proceeded normally until the aircraft was established on final approach to the helideck. As the aircraft descended through 500 ft the rate of descent had increased to about 1,000 ft/min. Although the pilot in command increased main rotor pitch, the aircraft's rate of descent continued to increase until just prior to impact with the water.

Both occupants were rescued approximately 1 h after they evacuated the helicopter. The report concludes that the standard approach technique used by the pilots, coupled with the prevailing weather conditions, caused the aircraft to enter a high rate of descent shortly after the aircraft started its normal final approach to the deck.

The high rate of descent was probably the result of entry to the incipient stage of Vortex-ring state'. A lack of visual cues and inadequate management of cockpit resources prevented the crew from recognising the abnormal situation until the aircraft was well into the descent. Recovery action was commenced too late to prevent impact with the water.