"Ground effect" is a phenomenon known to pilots whereby a plane, flying at extremely low altitudes, encounters a sudden increase in lift. It can be difficult to descend further until lift is reduced. According to the official record, Flight 77 approached the Pentagon at a very shallow angle, very low to the ground, and very fast -- too low, and too fast, for too long. The ground effect buffer would have forced the plane to stay higher, and the pilot could not have overcome this.

Reality: The effects of ground effect can be mitigated by changing the plane's angle of attack -- in this case, that meant adjusting its nose-down pitch. This low-level finesse would have been a challenge for Hani Hanjour, but by no means impossible, especially if he'd coordinated his final descent with help from the autopilot, which can make the needed adjustments easily.

Watching on their screens, some air traffic controllers believed Flight 77's radar track was that of a military plane.

Reality: Why wouldn't they have thought so? How many civilian jetliners zoom around a city, spiraling down to treetop level at 400 knots?

Much has been made of the 757's comparatively tiny impact crater in Shanksville, Pa. The hole was barely a few yards across, filled with nothing more than shredded debris. Reportedly, no bits of aircraft bigger than about two feet were recovered. Compare and contrast with the massive crater caused by the downing of Pam Am 103 in Lockerbie, Scotland. The bombed 747 destroyed 20 houses and plowed a crater 155 feet long that displaced more than 1,500 tons of earth. Large segments of fuselage and intact bodies were scattered in and around Lockerbie. This, some argue, proves that Flight 93 either never existed or had safely diverted elsewhere (the fate of its occupants is never made clear), and that the crash scene was put together as badly improvised theater.

Reality: Another apples and oranges comparison. To begin with, a 747 clocks in at nearly quadruple the weight of a 757. Pan Am 103's fuel load alone (240,000 pounds) was heavier than United 93 in total. Flight 103 broke apart well prior to impact, but the wings and main wing-box structure -- a chunk of wreckage not much smaller than an entire 757 -- fell square in the middle of a residential neighborhood.

The damage caused by Flight 93 in Shanksville was more extensive than many Web sites portray, and the lack of sizable pieces is fully consistent with an aircraft striking the ground at a tremendous rate of speed (over 500 miles per hour). According to the black boxes, the skyjackers put the 757 into a near vertical dive at maximum power. Similar to the case of American 77, this sets up a crash dynamic entirely different not only from that of Pan Am 103 but from that of the vast majority of airplane accidents. For the sake of comparison, have a look at the debris field from the crash of American Eagle Flight 4184 near Roselawn, Ind., in 1994. This was a commuter plane that dived into soft earth at half the speed of American 77.

This is a separate, and somewhat less egregious, myth from those alleging the plane was never seized, or never existed, to begin with. Many remnants of the 757, including an engine, were discovered at considerable distances from the main impact zone, fueling speculation that the jet was fired on by one or more U.S. military fighters, causing it to burst apart in midair. Sections of the 9/11 Commission Report pertaining to the cockpit voice recorder are therefore misunderstood or bogus.

Reality: High-energy impacts can eject fragments over startlingly long distances. It's also quite probable that the violent, high-speed maneuvers induced by the skyjackers caused one or both of the plane's engines to detach and/or partial breakup of the main structure. Comparatively benign plummets of aircraft in the past have resulted in the separation of engines, control surfaces and even entire wings. Debris can be carried aloft for many miles by the wind. And had Flight 93 been blown up with a missile, destruction of the airframe would not have been as complete, with portions falling to earth at a lower, less disintegrative velocity.

Elsewhere in this rat's nest, I cannot speak for aspects that extend beyond the aviation side -- such as the purported demolition of the twin towers, etc. Simple extrapolation tells us to be wary.

It's distressing that so many people become married to a preposterous idea based on little more than erroneous interpretations of some pictures and selective, manipulative use of evidence. But in debating this stuff now and again, you learn that it can be a bit like arguing religion. Evidence, or lack of it, has little to do with what motivates many believers. At the heart of their convictions is something utterly unprovable. It's faith.

Having said all that, I don't wish to belittle the idea that perhaps some important truths have been concealed, and it is hardly my intention to give our fearless leaders undue credit. Considering the extent to which we've been chicaned, shystered and condescended to these past six years, why should we trust them? We shouldn't, frankly, but remember Carl Sagan's famous quip about extraordinary claims requiring extraordinary proof.

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Deeper reading: the debunkers. I recommend the analysis by Aerospaceweb.org and, even better, Jim Hoffman's careful vetting of the Pentagon "booby trap." Hoffman does not dismiss the possibility of a cabal, but considers the Flight 77 quarrel a red herring.

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