'Parcel of air'
Hmmm…. I am a little shocked that people battle with this basic concept. At no stage did I mention the turn being executed with reference to a ground feature. The result WILL be the same if the said manoeuvre was carried out on instruments. If it has nothing to do with inertia and ground speed, then explain wind shear!?!
The block of air theory is all good and well, till we come to overcoming inertia. If we were flying into a 60kt head wind with 60kt TAS, we would all agree that the groundspeed is zero. If this headwind component drops to zero, the IAS would drop to zero. This is due to inertia.
In wind shear conditions, headwind component turn into tailwind component; the aircraft is not able to accelerate fast enough, with spectacular results. It requires that aircraft accelerate from 0kts groundspeed to 120kts groundspeed(60kts w/v and then 60kts TAS) in a matter of seconds (if the head/tail wind is 60kts). This phenomenon is not in question. If you have any doubts, the FAA has some good reading on the topic.
Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.
QED