willo said:..excerpt...
Air defense stand down theory
A common claim among conspiracy theorists is that the North American Aerospace Defense Command (NORAD) issued a stand down order or deliberately scrambled fighters late to allow the hijacked airplanes to reach their targets without interference. According to this theory, NORAD had the capability of locating and intercepting planes on 9/11, and its failure to do so indicates a government conspiracy to allow the attacks to occur.[72] The Web site emperors-clothes.com argues that the U.S. military failed to do their job. StandDown.net's Mark R. Elsis says, "There is only one explanation for this .... Our Air Force was ordered to Stand Down on 9/11."[75][76]
In September 2001, NORAD generals said they learned of the hijackings in time to scramble fighter jets. Later, the U.S. government released tapes claiming to show the Federal Aviation Agency (FAA) did not tell the military about the hijackings until three of the four planes had crashed, a fact that would indicate that the FAA repeatedly lied to other U.S. government agencies.[77]
Phil Molé of Skeptic magazine has explained that it is neither quick nor easy to locate and intercept a plane behaving erratically, and that the hijackers turned off or disabled the onboard radar transponders. Without these transponder signals to identify the airplanes, the hijacked airplanes would have been only blips among 4,500 other blips on NORAD’s radar screens, making them very difficult to track.[72][75]
According to Popular Mechanics, only 14 fighter jets were on alert in the contiguous 48 states on 9/11. There was no automated method for the civilian air traffic controllers to alert NORAD.[75] A passenger airline had not been hijacked in the U.S. since 1979.[78] "They had to pick up the phone and literally dial us," says Maj. Douglas Martin, public affairs officer for NORAD. According to Popular Mechanics, only one civilian plane was intercepted in the decade prior to 9/11, which took one hour and 22 minutes.[7
Link http://en.wikipedia.org/wiki/9/11_conspiracy_theories
pretty simple and reasonable explanation. My intuition said the same thing, the planes would be almost crashed before anybody realised anything was unusual.