9/11: A Boeing 757 Struck the Pentagon, page 73

what apartment building? It hit light poles, which would have damaged the wings, but not taken them off, and then impacted the Pentagon. The white smoke trail could have been a problem developing with the engine or engines. They aren't meant to be run constantly at high speed as these were for long periods of time before they fail. There was a good talk about why it didn't hit the ground in another thread. It's called ground effect. When a plane gets to a height equal to its wingspan above the ground it "bounces" in the air for lack of a better term. The amound of air being displaced under the wings balances so that if you don't have flaps/slats/gear/spoilers deployed the plane will stay at that height until you do something to either climb or force it to descend. When the aircraft is close to the surface -- in ground effect -- coming into contact with the surface modifies the almost cylindrical vortex-induced circulation around the wing. This flattens the cylindrical circulation pattern and reduces the downwash angle of the air behind the wing. This flattening of the cylindrical circulation spreads the pattern outwards below the wing and increases the effective span of the wing. The aerodynamic aspect ratio of the wing is also increased. (The aerodynamic aspect ratio of the wing is measured between the cores of the vortices, which occur at about 80% of the geometric wingspan outside of ground effect. This aerodynamic aspect ratio has a strong inverse effect on lift-induced drag.) When the aircraft flies close enough to the ground that the sag of the vortices trailing the wingtips is restricted by coming in contact with the ground, the backward-tilting angle of the total lift vector is reduced, thereby reducing its horizontal component and reducing induced drag. The combination of the reduction in the downwash angle of the air behind the wing and the increases of both effective wingspan and aerodynamic aspect ratio of the wing occur when the wing is close to the surface. These increases in aerodynamic efficiency of the wing are what we call ground effect. When an aircraft enters ground effect during the landing flare, the aircraft may tend to float because the lift-induced drag is reduced quite dramatically as the aircraft descends below one wingspan distance from the ground. Any excess speed at all -- you know, the 10 knots for Ma and the kids -- will cause this float to become excessive. This can cause an inexperienced pilot to grope for the ground and possibly induce pitch oscillations. At any rate, the bent nose gear and the excess wear on the tires and the brakes are of no use to anyone except the folks who sell spare parts.