So a civilian air traffic controller sent up a C-130 to intercept an incoming plane spotted on radar while NORAD didn't send anything from this very same Andrews Air Force Base?
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Arabesque: 911 Truth

I haven't read anything proving that the white plane was a "normal" passenger plane. However, I have read an article posted on journalof911studies.com titled The Flying Elephant: Evidence for Involvement of a Third Jet in the WTC Attacks (June 2006). The author makes a convincing case that the white plane seen circling the WTC wasn't a "normal" passenger plane. Here are a few excerpts from it....

(FYI the video links no longer work)

"No mention of a large, commercial-class aircraft loitering in the restricted airspace of lower Manhattan during the strikes on the WTC towers will be found in the 9/11 Commission Report. It does not appear in any version of the Official Story. It is largely unknown even in critical studies of 9/11. Yet substantial evidence exists to support its presence coincident with the attacks, actually orbiting in close proximity to the towers for several minutes while the North Tower burned and the South Tower was struck. Photography, video footage and eyewitness accounts, including FDNY transcripts and mainstream media audio, confirm this fact.

Why is this significant? Let us consider the commercial air traffic on a typical Tuesday morning over New York City. There are three major airports servicing the city: La Guardia and JFK International to the east, and Newark International across the Hudson to the west. Normal holding patterns for these airports do not intersect the borough of Manhattan at any point. Lower Manhattan is, and was on the morning of 9/11/01, a low-altitude flight-restricted (no fly) zone for commercial jets, as designated by the FAA, for the obvious reason that heavy, fast-moving aircraft and tall buildings pose mutual hazards. Air traffic near the WTC towers was doubly restricted, with a minimum ceiling extending two thousand feet above the towers (3,300 feet) within a radius of one nautical mile, excepting only police aviation without special permit. These were the VFR (visual flight rules) parameters in effect on the morning of 9/11. Once WTC1 was hit, the black smoke plume expanding southeast from the tower would pose an additional threat to navigation.

No avoidance warning from Air Traffic Control would be necessary, as no rational commercial pilot (no matter how curious) would risk his aircraft, crew or passengers in a "fly-by" of the burning North Tower. But in this anonymous Camera Planet segment we see a large, twin-jet aircraft (757/767-class) doing just that at approximately 8:58am (assuming the time signature is uncorrected by one hour), five minutes before WTC2 will be struck. Even disregarding the indicated time, as WTC1 is burning and WTC2 is not, the segment is clearly recorded between 8:46am and 9:03am. Note this white aircraft with dark engines and vertical stabilizer is not the aircraft that will impact WTC2.

http://terrorize.dk/911/wtc2hit13/911.wtc.yet.another.plane.wmv

This still from the video isolates the aircraft:

According to the 9/11 Commission, two F-15s were scrambled from Otis Air Force Base at 8:46am (some 33 minutes after flight controllers lost contact with AA11), and were inbound to NYC at high speed, presumably to intercept suspicious airliners. Presumably commercial flights in NY airspace would be alerted to this danger. Yet this aircraft cruises slowly near the stricken North Tower, seemingly unconcerned its behavior makes it a logical target for these fighters. Of course, the absurdly late scramble and non-arrival of the F-15s is a serious problem for the official narrative, which remains obscured by contradictory accounts from the FAA, NORAD, NEADS, the news media and the pilots themselves. (The Commission has these fighters finally arriving for Combat Air Patrol over NYC at 9:25am, after being vectored into a holding pattern off Long Island.)

[SNIP]

At 9:03am, "UA175" approaches from the south at an improbably high speed and impacts the South Tower. CNN aired this "amateur video" of the event, which captures (without notice by Aaron Brown or Paula Zahn) what is evidently the same jet seen in the Camera Planet segment, making a similar northwest pass (but farther west, approximately over Battery Park) as the South Tower hit occurs.

www.areadownload.com/video/wtc/WTC%20-%20Amateur%20Video%2004.mpg

This still from the video isolates the aircraft as "UA175" rips through the South Tower:

At 9:04am, Diane Sawyer of ABC News made remarks on-air about the "circling" jet she and her colleagues "all saw" prior to the second strike. She admits she "just assumed" it was the same one that struck the South Tower.

http://www.terrorize.dk/911/comments/911.wtc.plane.circling.around.wmv

Of interest with respect to this "mystery jet" is the phenomenon, acknowledged but unexplained by the Commission, of the "phantom Flight 11". At 9:21am, after both towers had been hit, and long after "AA11" had struck the North Tower, Boston flight control, relaying information from FAA headquarters, informed NEADS that "AA11" was still in the air and heading south, perhaps to Washington, DC. Were they tracking this "third aircraft"?

[SNIP]

Complete article here (PDF File)...
http://www.journalof911studies.com/articles/Journal_4_Jet.pdf

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"If I had just paid $20 million for the NIST report, I'd be asking for a refund!... The trouble with the NIST Report is that it isn’t even science because it's not capable of being verified or negated!" -Dr. Frank Greening