Originally Posted by
A Squared
From a physics standpoint, it is completely meaningless. You are making the identical mistake that Ian was making. In your Scenario A, From the frame of reference of the ground, the airplane went from a velocity +110 knots to a velocity of -90 for a total change in velocity of 200 knots.
It's not even a mistake, let alone the same mistake. The whole point is to explain to people like Ian, that what he saw was different to what the plane actually experienced.
Now, a ball can't travel backwards, but people and planes can so + and - is a way of expressing that relative movement.