212 Kts in a Lynx, (minus TOW Booms), downwind, in a Valley not far from Aviemoor one dark and very windy night in July 1994. Achieved this speed about 2/3rds of the way along the valley and continued to hold in excess of 200 Kts before being spat out of the valley at the far end. My eyes were bigger than my NVG by the time we got out and decided against a repeat performance.
The two following crews also reported ground speeds in excess of 200 Kts.