Speed
Could he slow? Yes. 30 - 40 knots is possible but is it advisable and therefore what a pilot would chose to do if he thought at that time he had options?
Helicopter discs are less efficient in the hover as airflow moves from above the disk to below. With forward motion the airflow changes, entering the disc from in front and being deflected down / rearward. This increases disc efficiency and therefore we have the same lift at lower power setting, and we are safer.
Increasing speed also increases parasitic drag of the airframe, hence there is a trade-off leading to an optimal speed. This is usually between 60 to 80 knots but it is individual to particular helicopter design. However, 30 or 40 knots is definitely on the back side of the power curve.
Slowing below this optimal speed means increase power usage, flying the back-side of the curve. So personally I would not chose to slow down below that without very good cause.
We don't know what speed PB was flying but with half the height normally used then even 70 knots would look fast from the viewpoint of a member of the public. The AAIB report will have this data.
Last edited by John R81; 19th Jan 2013 at 13:14.
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