My husband just got back from a TDY, and saw an old friend of ours. It turns out he is in the Washington DC guard and was the de facto mission commander over Washington skies on 911. He was circling over for seven hours.
So, all of the conspiracy "Gee! How could witnesses have heard planes when no flights were allowed in the skies after the Pentagon was hit?" is nonsense. Military planes were all over the place, to include fighters, tanker aircraft, helicopters, and even an AWACS made it out eventually (the closest ones were at Tyndall AFB in Florida).
Edited to add:
Honestly, I must wonder at this point what precisely do people think happened with the Pentagon? There was some sort of emergency procedure in the hall of Secret Agency

stating:
'Quickly! When two passenger planes hit the WTC, fire a missile directly into the wall of the Pentagon!
Or alternately....bring over some supersecret aircraft from hangar X to fly into the Pentagon! I've got it! A drone aircraft....fire some explosives! Anything! Just make sure a wall of the Pentagon comes down immediately after two passenger planes fly into the WTC!!!!! We can blame it on another passenger plane!'
QUOTE(TedN5)
Of course the witnesses saw an aircraft, but what kind? Eye witness accounts are notoriously inaccurate in the details. To settle this issue we need an authoritative answer to such evidence as this from an aerospace mechanical engineer.
I see. So, let me get this straight...highly reputable scientists with PhDs in the field analyzed this personally and are wrong, but some random guy who
says he has an engineering degree and read the press clippings on which he has based his information second-hand is right? In that case, we have engineers on this very forum, maybe you should listen to what they say.....