Great when it works. I was at Duxford, the IWM real world site this afternoon at about 1715 when control tower announces a fly past of a Chinook helicopter coming from west over Royston. Joined circuit from north over officers' mess and Junction 9 of M11 and proceeded east west north of runway line doing a slow rocking left right left motion. Rear door open as it passed. Proceeded south and climbed away, probably eventually going round to east again as it faded into the distance. Only spectator, apart from control tower, appeared to be yours truly, who gave a jolly wave. Sorry, no camera. Lovely deep sound. Good of the crew to think of Duxford visitors.