Let's see... Gravity in fact determines the forces that the aircraft engines can generate, and the lift from aurodynamic effects. As long as the structural integrity of the aircraft is not an issu (i.e., at all times before impact) one can assume that there are no sustained accelerations larger than gravity in order of magnitude. Otherwise passenger flights would be so uncomfortable so as to be impractical. Normal lift basically balances gravity, etc.

So, we have the acceleration of gravity (9.8 m/s^2) and the speed of the plane (230 m/s from 762 ft/s). From this I can construct a characteristic length (v^2/g = 5.4 Km) and a characteristic time scale (v/g = 23 s).

Flight 77 was a 757 with a length of 50m, a wingspan of 40m and a tail height of 14m. The Pentagon is 24m tall and each of the outer walls is 280m long. A typical runway will be typically narrower than the pentagon's walls (under 100m) but at least a couple of kilometres long (the relevant dimension to compare with the height of the pentagon).

Terminal velocity is irrelevant: a plane is not spherical, to recall the famous joke about the mathematician and the cow.

This is all before impact, but there was some discussion of the target approach last night. On impact, gravity indeed seems irrelevant again, except that structures such as planes and buildings are, again, designed so that internal stresses at rest balance gravity. So gravity again can give a useful idea of orders of magnitude.

The great Richard Feynman was once giving a lecture about the forces of nature and he says "gravity is incredibly weak", at which point one of the loudspeakers in the lecture hall fell from the wall. Feynman said "weak, but not negligible".

Those whom the Gods wish to destroy They first make mad. — Euripides