Nila Sagadevan is an aeronautical engineer and a qualified pilot.
"I shan't get into the aerodynamic impossibility of flying a large commercial jetliner 20 feet above the ground at over 400 MPH. A discussion on ground effect energy, vortex compression, downwash reaction, wake turbulence, and jetblast effects are beyond the scope of this article (the 100,000-lb jetblast alone would have blown whole semi-trucks off the roads. The DVD, "Loose Change - 1st Edition" contains an excellent clip of trucks being blown off the end of a runway when a jetliner powers up for take-off.)
Let it suffice to say that it is physically impossible to fly a 200,000-lb airliner 20 feet above the ground at 400 MPH.
The author, a pilot and aeronautical engineer, challenges any pilot in the world to do so in any large high-speed aircraft that has a relatively low wing-loading (such as a commercial jet). I.e., to fly the craft at 400 MPH, 20 feet above ground in a flat trajectory over a distance of one mile.
Why the stipulation of 20 feet and a mile? There were several street light poles located up to a mile away from the Pentagon that were snapped-off by the incoming aircraft; this suggests a low, flat trajectory during the final pre-impact approach phase. Further, it is known that the craft impacted the Pentagon's ground floor. For purposes of reference: If a 757 were placed on the ground on its engine nacelles (I.e., gear retracted as in flight profile), its nose would be about fifteen feet above the ground! Ergo, for the aircraft to impact the ground floor of the Pentagon, Hanjour would have needed to have flown in with the engines buried in the Pentagon lawn. Some pilot.
At any rate, why is such ultra-low-level flight aerodynamically impossible? Because the reactive force of the hugely powerful downwash sheet, coupled with the compressibility effects of the tip vortices, simply will not allow the aircraft to get any lower to the ground than approximately one half the distance of its wingspan-until speed is drastically reduced, which, of course, is what happens during normal landings.
In other words, if this were a Boeing 757 as reported, the plane could not have been flown below about 60 feet above ground at 400 MPH. (Such a maneuver is entirely within the performance envelope of aircraft with high wing-loadings, such as ground-attack fighters, the B1-B bomber, and Cruise missiles-and the Global Hawk.)"
Read entire piece here.