Most people seem to agree that airspeed fluctuations due to windshear are the result of the aircraft having an inertial groundspeed which causes the changes to be apparent as it experiences differing air masses and takes a finite time to regain its momentum ......................now, when flying in a stable but moving air mass where does the effect of aircraft momentum go? (why is it always ignored?)
I have observed many times descending into NRT with a tailwind of 200kts (not uncommon at certain times of year) with the autopilot in IAS hold and smooth wind conditions. The arrival called for a 90 degree heading change and whilst the IAS remained constant the rate of descent increased dramatically once the turn was complete (ie. much lower groundspeed) before stabilising back to the normal rate. This behaviour always seemed at odds with the theory.