Originally Posted by
Brercrow
The difference between these two examples is that in one the windspeed is less than the airspeed and in the other the windspeed is greater than the airspeed.
This is true. But why does it matter?
In both cases airspeed is positive.
This is true too.
In the first case the groundspeed reverses direction and in the second case the groundspeed is in the same direction.
Also true...
That means that in the first case you are entitled to take the difference between the groundspeeds but in the second case you have to take the sum.
Hold the phone! Why does that justify taking the sum? That makes no sense. The whole premise is that we are comparing the
difference between the two groundspeeds (i.., before and after) which, by definition of the mathematical operation, is performed by subtraction. In no universe is addition meaningful to any of this. There is a difference in one of the initial conditions (the airmass is so fast compared to the airspeed that the groundspeed starts out negative) but this does not justify changing the manner in which we compare the initial and final condition. Groundspeed is airspeed minus headwind (equivalent to airspeed plus tailwind) always.