Programmed Flight Control
Hacking the Flight Computers to Take Control of the Targeted Jetliners
The most popular alternative to the official conspiracy theory that Islamic terrorists commandeered the four jetliners on 9/11/01 is that flight control computers were taken over and directed to fly the planes into the targets.
Boeing 757s and 767s apparently use hydraulic systems to drive the control surfaces, like the elevators, ailerons, and rudder. Thus they are not 'fly-by-wire' in the same sense as the more recent 777s. However, 757s and 767s can be flown entirely under the control of their flight management computer systems (FMCS), according to Boeing.
A fully integrated flight management computer system (FMCS) provides for automatic guidance and control of the 757-200 from immediately after takeoff to final approach and landing. Linking together digital processors controlling navigation, guidance and engine thrust, the flight management system ensures that the aircraft flies the most efficient route and flight profile for reduced fuel consumption, flight time and crew workload.
The precision of global positioning satellite system (GPS) navigation, automated air traffic control functions, and advanced guidance and communications features are now available as part of the new Future Air Navigation System (FANS) flight management computer. 1
Boeing also provides information on the ease of reprogramming various systems including the FMCS.
There is a question of whether the takeover of the FMCS by some means would disable the cockpit controls, preventing the flight crew from regaining control of the aircraft. If the controls have direct mechanical linkages to the hydraulic systems, then the crew could probably overcome automated control by simply applying more force to the controls. However, if something had incapacitated or killed the flight crew and passengers first, then the FMCS presumably could have flown the planes into the targets without interference. It is not difficult to imagine a scenario by which the cabin could have been filled with a potent gas at a predetermined point in its flight. For example, the fact that cabin pressure varies as a predictable function of altitude would allow a barometric-triggered device to go off at a predetermined point in a flight.
2. Onboard Loadable Software, boeing.com, [cached]