How good a pilot would you have to be to hit the World Trade Center? For the record: We’re short of facts, what follows is inference and speculation, the conclusions offered here could all turn out to be wrong.

Still, on the basis of the evidence today, two conclusions seem very likely. One is that the original pilots on the United and American flights were removed from the controls, probably by being killed. The other is that the people who replaced them in the cockpit were skilled pilots, probably with jetliner experience and training, and at least with special preparation in the systems and controls of these planes.

The first conclusion may seem obvious, but it’s worth spelling out the reasoning. A pilot with a gun to his head might well be forced to fly the plane on a suicidal mission into the ground. (What’s his alternative? If he refuses on principle and lets himself be killed rather than give up his passengers’ lives, the pilotless plane will eventually crash anyway.) But it is very difficult to imagine that an unwilling pilot could be forced to fly an airplane into a building–in effect, forcing him to murder countless additional victims. It would be just too easy for the pilot to thwart the plan. In the last two or three seconds before impact, the pilot would need only to have shoved the control yoke to one side–or up or down, in any way missing the skyscraper and instead crashing the plane into New York Harbor. Forcing him to hit the target would be like trying to force a sniper to pick off one of his own troops.

So what about the people who actually steered the planes into the towers? As anyone who has used a flight-simulator program knows, simply guiding a plane in flight is not necessarily that hard. Moreover, the controls of a big Boeing or Airbus are fundamentally similar to those in a little Cessna or Bonanza, and in a pinch any trained pilot could sort of handle any plane. (The amateur pilot’s problems would come when he tried to land the airliner, since everything would happen much faster and with much more momentum than he was accustomed to.)

But changing what an airplane does can be difficult, especially doing it in a smooth, controlled way. All of these planes changed course and altitude substantially–and apparently without the out-of-control veering that an inexperienced pilot would have encountered. According to flight-tracking software, one of the planes that hit the World Trade Center, American Flight 11, made a dramatic change from its original course–turning south, toward New York, rather than continuing west toward Los Angeles. At a minimum, accomplishing that turn would require a pilot who knew how to switch off the “Flight Management Systems” and autopilot that were programmed to guide the plane to Los Angeles. Amateur pilots, like me, would know how to handle the autopilots on small planes; they wouldn’t necessarily know how to do it in a Boeing.

More impressive, Flight 11 was at 29,000 feet before it made that turn. It had to lose 28,000-plus feet of altitude by the time it hit the tower. Making a plane descend is easy if you know what you’re doing. But if you were experienced only in small airplanes, a descent in a big airplane could be terrifying. You wouldn’t know how much to reduce the power, to prevent the plane from gaining too much speed as it went down. (A descent at full power can push a plane past its “never-exceed” speed and lead to structural failure.) You wouldn’t know how or when to arrest the descent, to be sure you could level off and be under control when you neared the target. Everything would be faster, bigger, and heavier than you were used to. You’d do what almost every pilot does when encountering a bigger, more powerful plane: “over-control” it, making the plane lurch from side to side and up-and-down, and “getting behind the airplane,” reacting with ever-growing lag time to what the airplane was doing. The result would less likely be a suicide attack, like the ones in New York and Washington, than simple suicide.

In short, odds are that at least three of the four hijacked airplanes were flown by experienced pilots, who one way or another had gotten big-jet training. And conceivably, a difference in piloting experience may explain why the fourth hijacked plane simply crashed in Pennsylvania, rather than crashing into a target. When the facts are out, of course, the real explanation for that fourth crash could be something else entirely.

Next question?

James Fallows is the national correspondent of the Atlantic Monthly and author of Free Flight.