Well, for what it's worth, I wondered the same thing while reading the official explanation of the airplane "streaking" upward after the CWT explosion.
Without going thru the numbers again, what I recall was that if you assumed that the airplanes velocity was suddenly directed upward, without significant loss of energy the altitude gain suggested was at leas semi-plausible.
The problems with that of course was that the reason the airplane suddenly turned up was the CWT explosion blew the nose off.
This introduces two problems:
1) The huge amount of drag from having a gaping hole in the fuselage instead of the forward half would not be a low drag feature.
2) The post explosion airplane was assumed to achieve a stable zoom climb following the loss of of the forward fuselage. This is inconsistent with the part of the theory which says that the loss of the forward fuselage caused it to depart stable flight upward. The loss of the forward fuselage either caused it to become unstable, or it didn't. It can't really do both, which is what the theory requires.
You can only figure that it climbed thousands of feet if you completely neglect drad and aerodynamics and focus only on how high the kinetic energy would take it in a vacuum.
The "zooms upward" scenario fails on an entirely different level. The raison d'etre for the "zoom upward" theory was to explain why so may people reported seeing something "streak" upward. It serves no other purpose, other than to provide an explanation for the witness accounts of a fire streak going up. The theory being that the post-explosion airplane was what they saw "streak" upward. The problem with that was the distance, altitude and speeds involved. From memory, the airplane was at 14,000 ft (or was it 15K?) when the explosion occurred, and (again from memory) it was something like 40 miles away from the nearest witness who reported something 'streaking upward"
We have all watched airplanes fly. an airliner at 14,000 ft altitude, and even just 10 miles away does not "streak" across the sky. It crawls. So if from the observers perspective it appears to be crawling across the sky, if that same airspeed is suddenly directed perfectly upward without loss of speed, it's not going to appear to "streak" upward any more than it was "streaking" across the sky horizontally.
Now, I'm not trying to start a conspiracy theory discussion either. I have no idea what happened to TWA 400, There are all kinds of reasons why a shoot-down is really implausible, and there are all kinds of reason why a CWT explosion seems the most plausible explanation.
I'm just saying that the post-explosion climb as an explanation for witness reports of fiery streaks going up to the airplane is a complete farce. And that is the only reason it exists.