I go back to the UK AAIB G-TIGH report. I am not sure how may have read this report, but please consider this from the report:
From the Analysis part of the report:
2.4.1 Wind Limits.
The wind at no time precluded flight, but it did provide particular problems. The prime one, which the commander appeared not to have considered was the effect of turning downwind whilst flying solely by external references, a potentially hazardous manoeuvre that is emphasised in basic flying training.
Although the helicopter can hover at zero airspeed, it requires much more power to do so than it does to maintain forward level flight with forward velocity. A power setting required to maintain forward level flight will, therefore, be inadequate to maintain hovering flight and any loss of airspeed will cause the helicopter to descend if not checked. Such a reduction in airspeed is the likely outcome of turning away from a substantial headwind whilst visually maintaining a constant groundspeed.
This appears to be exactly what occurred as 'GH' lifted from Cormorant 'A' and immediately entered a 180° climbing turn: whilst the perceived ground speed remained fairly constant, the airspeed decreased to zero and an inevitable descent ensued.
From causes:
b. Causes
The following causes were identified:
(i)The handling pilot's failure to recognize the rapidly changing relationship between airspeed and groundspeed which is a fundamental problem associated with turning downwind in significant wind speeds
(ii)The Commander, who was the handling pilot at the time, shortly after takeoff inadvertantly allowed the airspeed and then the height to decrease whilst turning away from a strong gusting wind
(iii)Despite the application of maximum power, the helicopter was incapable of arresting its established descent within the height available. Incipient Vortex Ring state and down draughts may have contributed to this problem, as may the height of the wave crests.