Transponders were switched off to prevent aircraft being tracked by air traffic control
American Airlines flight 11 and United Airlines flight 175, both Boeing 767s bound for Los Angeles, took off from Boston Logan airport within a minute of each other at 07:59 and 07:58 local time. They both headed north-west initially and came close to colliding at one point, according to an FAA area controller, who told the Washington Post that AA011's erratic behaviour indicates that the hijackers took only minutes to intervene after take-off. AA011's transponder was switched off as the aircraft changed course for New York, but the aircraft remained visible on primary radar. Controllers learned of the hijack when from time to time the pilot pressed the transmit switch so that they could hear the hijackers talking. The controllers alerted the security services. At one point a hijacker said that if no-one "did anything stupid" no-one would get hurt. The aircraft, heading south, hit the north tower of New York's World Trade Center (WTC) at 08:45.
UA175 flew normally for longer, and the hijackers appear to have left at least one United pilot in his seat, possibly for all or almost all the flight. Controllers who knew of the first aircraft hitting the WTC say they did not expect UA175 to be used in a similar way. It passed New York heading south-west and then turned north-east, eventually to hit the WCT south tower at 09:05. Just before impact the 767 looked as if it was being flown to pass the WTC with the building very close to its left wingtip, but in the last seconds it banked left to hit the tower.
At 08:01 flight UA093 took off. It was a Boeing 757 from New York Newark bound for San Francisco. The aircraft headed west for about an hour before turning south and then east. This change of course towards Washington DC sparked the evacuation of the Capitol and the White House. Cellphone messages well into the 2h flight indicated that both pilots were under guard at the rear of the aircraft with cabin crew and passengers.
They also indicated a realisation among the passengers that their best chance of survival might be to over-power the hijackers. A US Air Force F-16 fighter shadowed the aircraft in the latter part of the flight. The aircraft crashed steeply in countryside south-east of Pittsburgh at 10:10, with no clue yet as to what the hijackers real intention may have been.
At 08:10 flight AA077, another 757, took off from Washington Dulles airport, only about 50km west of the hijackers' ultimate target, the Pentagon. The aircraft headed west for it's scheduled destination, Los Angeles, then turned east and was lost to radar. But it is not clear whether this was the result of switching off the transponder or low flying. It crashed into the Pentagon, just across from the Potomac river from Washington DC, at 09:40.
Source: Flight International
