For a plane to fall 17,000 feet in 29 seconds, an average downwards velocity of 400 MPH, if it were not activelly accelerating downward with its engines (which seems questionable after a fuel tank explosion), and if it fell without air resistance, it would need an initial downward velocity of 82 MPH, accelerating to an impact speed of 700 MPH.
Presumably air resistance would be a significant factor at speeds
considerably below 700 MPH, and probably below 400 MPH, especially, as Ian
says, with the forward third of the fuselage missing. In that case you
have to assume a higher initial downward velocity. But you still need to
*average* 400 MPH, which means that unless the initial downward velocity
is that fast, then the final velocity has to be higher than 400 MPH.
I doubt that the plane was going 400 MPH in its climb through 13,000 feet. I don't know much about the climb performance of large jets but my guess is that it would be going around 200 MPH in a takeoff climb.
The conspiracy theorists say that the rising spark was a missle, and the underpressure readings were caused by the shock wave from the missile explosion, with the plane then falling from 13,800 feet. The CIA explanation has the rising spark be the plane itself, rising from 13,800 to 17,000 feet after the front of the fuselage drops off, with the underpressure readings reflecting the beginning of this climb.
I'm not sure where the timing data from explosion to ocean-impact comes from. Apparently the flight data recorder was not functional. Was this from eyewitness testimony? In that case, the only real question is whether the plan started from 17,000 or from 13,800 feet and hit the ocean in 29 seconds, right? The CIA says the former, and the conspiracy theorists say the latter.
The problem does not go away if the plane starts 3,200 feet lower than the CIA's 17,000 feet. It's still a lot of lightweight metal to get out of the sky in 29 seconds. You can just make it with a ballistic (no air resistance) fall, but it hits at 630 MPH. That's an awfully fast speed regime to be assuming no air resistance. If you can believe this then it's not that much harder to believe the 17,000 foot drop with an initial speed of 82 MPH and final impact at 700 MPH.