Originally Posted by Dutch M
Yeah, suppose the windspeed goes from 100 knots back to zero, does this change the Ekin of the airplane ? If not, then you will need to take the groundspeed as speed reference.
The energy exchange Ekin vs Epot, is in general only valid for speeds in the direction of the change of Epot (ie Height) and not when trading speed between perpendicular axis.
This is why I use the surface of the Earth as a reference. Then you find it easier to include the effects of thrust, drag, and wind. If the wind stops there is no change in the energy of the aircraft except through changed drag and lift.