As Takata has pointed out that is only partly true as below a certain speed the drag will rise again due to ending up in a less good cl/cd area (higher Alpha) of the wing polar. So at the end you can probably leave excess thrust largely out of the equation.
Indeed, to confirm, the drag curve consists of profile drag + induced drag + Mach drag. The absolute value of the sum of these can indeed increase below a minimum drag speed, particularly approaching a stall, but between the normal cruise and maneouvring speed unlikely to increase much if any at all.
== DutchM ==
I really dont think you need
ground speed for calculating those energy exchanges, TAS is fine surely... Unless you state you are allowing for windshear of thermal gusts , which we aren't are we, because we can't can we, because we don't know the figures do we. So we do the basic steady state air-mass TAS calculations, and bear in mind some variations may occur in practice... but if we're within the right cricketpitch, then fine.. that's what engineers do, get in the right region, check they've got the order of magnitude correct and then use some commonsense to make further deductions and corrections!
===========
Possibly worth consideration (as airline pilots seem increasingly unfamiliar these days with hand-flying through heavy thermal activity, wicked windshear or terrible turbulence - ?)
Flying in a +ve going thermal, with increasing x and z wind-vector components, glider pilots can find that stick back for extended periods is quite OK, with an almost unstallable
feel to the aircraft (for reasons possibly explained some while ago)
Conversely, when this 'free energy' phase is exited, you are effectively in heavy adverse windshear, known to some as '
going over the falls' for obvious reasons! It's a horrible feeling, as if your very lifeblood was being drained... and if you don't respond, it may well be. (I've heard hang-gliders say they've had to hand-stand on the bar to prevent -ve 'g' tuck during some exits from the back-side of thermals.
Now, I'm definitely not comparing a 300 kt IAS airliner with an 80 lb hang-glider, even though the latter does have a sensible AR (8 to 10), span as big as a Spitfire and several hundred square feet of wing.
Only by strong pitch stability or quick and considerable ND inputs, is a very quiet & lonely sensation avoided, with a subsequent stall or wing drop...
So, it is interesting that a strong right wing drop seems to have had to be countered early on in this series of events, around when the climb started (strong and persistent wing drop, is frequently one wing entering quite different air than another). Then after quite safely holding NU (and a high angle of attack) for some time, we have the new THS* position... and a quite telling one I'm sure the report will atest to.
It is this * that our (hypothetical) glider pilots wouldn't have had to deal with... just a firm but controlled ND to accelerate back down through dirty colder slow air (!)