flybymike wrote:By what means are you establishing the wind speed and direction?
It doesn't matter. Imagine two anemometers, if you like, measuring the wind at the two levels in question - one where the wind is coming from in front at 20 kt, and the other where it's coming from behind at 10 kt.
The point is, at the instant
the wind changes, the aircraft has a speed over the ground of 60 kt forwards. If there was suddenly no wind at all, its airspeed would be 60 kt, right? (@OP, if you don't get this, stop reading here and say so, because what follows will not help).
But actually, the wind is 10 kt from behind. So the aircraft at that instant is going only 50 kt faster than the air it's moving through. That is what the airspeed is, by definition.
Now, as the aircraft was flying at an airspeed of 80 kt just before, it must have been developing enough thrust to overcome the drag at that airspeed. At risk of a very slight oversimplification that you can ignore for these purposes, now that the airspeed is only 50 kt, the drag is less, so that thrust will be enough to accelerate the aircraft until it's doing 80 kt through the air again.
The point is, at the instant after the wind changed, the airspeed is 50. This might be below the stall speed - the question is intended to illustrate the potential dangers of abrupt wind fluctuations especially close to the ground.
@OP - if you still think it should be 70, or if you don't get that it would be 60 if the wind changed to being no wind at all, post an explanation of how you reached your answer.