Originally Posted by
Gargleblaster
If the wind abruptly changes from calm a 70 knot tailwind:
Ground speed will remain at 70 kts, however increasing for as long as it lasts.
Airspeed will abruptly drop to zero, and you shall drop from the sky as a grand piano with a closed lid.
This ^^
What you have is a an incredibly strong decreasing performance windshear and you will lose all of your airspeed (or lots of it, depending on how abrupt the change is). The following crash will be spectacular.
Note this has nothing to do with downwind turns as that scenario assumes a steady airmass while the OP's example has an abrupt change in the airmass.