PBA and Deliv, you've hit the nail.
Helicopter in the hover with a 10 kt headwind... IAS 10kts
Turns 180 out, IAS -10 kts
voila a 20 kt decelleration.
I suppose the turn in the hover is the ultimate example of an aggressive turn, possibly a Rate 10 turn. A turn in forward flight will be less rapid to varying degrees.
All that is happening is that he is changing his IAS to maintain a fixed ground speed!
The pilot hasn't done anything. The aircraft has experienced a reduced airspeed. The aircraft has a tendancy to vary its airspeed because in varying headwind component, the Earth frame of reference is significant. Positive action from the pilot is required to maintain airspeed. In IMC, as mentioned earlier, the IAS is scanned and the pilot will conciously or subconciously increase the forward thrust slightly to nail the speed. Likewise in an E-3 doing a Rate 1 turn where the wind is 5% of the TAS, the effect will be negligable. In a slow aircraft, turning rapidly in a strong wind you WILL see in airspeed variation and you will have to adjust thrust to control it.
Goodnight