Reply #10: Correction: Ground effect is reduced with reduced speed

[Ezlivin]

Ground effect is also influential in landing. As the airplane flies down from free air into ground effect, the reduction of induced drag as it nears the runway comes into effect to make the airplane float past the point of intended touchdown. In the common case of an airplane coming in with excessive speed, the usable portion of the runway may slip by with the airplane refusing to settle down to land. A go around will probably be necessary. On short fields, approach as slowly as is consistent with safety. (http://www.pilotfriend.com/training/flight_training/aft_perf.htm)

The hijackers wanted to "land" the 757 at the Pentagon. The touchdown point would be on the ground just before the building, in this case, since the aircraft impacted the bottom floor. (Graphic depictions as well as the Pentagon videos show an aircraft very close to the ground.) To get the aircraft down to the ground while traveling at high speed (400+ knots) and in ground effect would be highly improbable.