Bristows had a Puma splash in on a night pilot positioning trip some years ago
Report here.
https://www.atsb.gov.au/media/24791/199100126.pdf
Talking to a Bristow pilot who was involved in test flying with the ATSB team they came to the conclusion that it was not a result of vortex ring, as the report states, but concluded that they had zeroed out the airspeed and were in fact going backwards, which precipitated the high rate of descent. Bristow pilot involved was surprised when the finding of vortex ring was published, as the test flying was unable to replicate given the prevailing conditions (AUW etc).
Any Puma driver care to comment on the possibility re vortex ring?