Ever since that horrific day five years ago, aerospace companies have been looking for what amounts to sweet revenge -- a way to make a plane so secure that trying to hijack it is all but pointless.
Now a coalition of European aircraft makers, government agencies and universities is at work on a project that its members say could result in a hijack-proof plane. No system is perfect, the coalition members caution, but they say the first new technologies could appear on airliners as soon as 2008.
Planes of the future could be flown only by pilots who pass biometric screening -- iris or fingerprint scans that confirm identity and make the controls useless to anyone else.
They would have sniffers to detect explosives onboard before the plane leaves the gate.
If hijackers got into a cockpit and ordered a pilot to crash the plane into a building, a hazard-avoidance system would override the controls and steer it back into the sky. Control of the plane might conceivably be taken over by a computer, which would automatically land the plane.
The European plan is called Security of Aircraft in the Future European Environment, or SAFEE. It is a four-year, $45 million project, drawing on ideas from engineers, pilots and airlines.
The consortium is even working on an Onboard Threat Detection System, with cameras linked to computers that would monitor activity in the cabin and send alarms if a computer detects suspicious behavior.
"We want to detect a threat early, before somebody storms the cockpit or flight deck," said Catherine Neary, a human factors specialist in the Advanced Technology Center at BAE Systems in Bristol, England.
Software has been designed that can read people's actions in a video image. It might be able to tell if a passenger gets up when the seat belt sign is on, or makes gestures that suggest unusual activity -- or, for that matter, sits nervously.
Microphones might also be mounted around the cabin, linked to a computer with voice-recognition software to detect suspicious words.
Such signals might send low-level alerts, which engineers hope would prevent the need for more urgent action later.
They also raise obvious privacy issues, but BAE Systems said it stays mindful of that. If scanning data generates false alarms, it said, the data will be deleted before the plane even lands.
"To reassure passengers we're not listening into their conversations, we're not identifying them," Neary said. "It's all being analyzed automatically by a computer, and information is extracted.
"The plan is that the data wouldn't be analyzed by humans," she said. "It would analyzed by computer, and the data wouldn't be stored."
The SAFEE group describes its work as a last line of defense, a way to thwart a terrorist who may have eluded law enforcement and gotten through airport security and on to a plane.
So the security lines at airports are likely to continue for a while, and engineers concede fully that hijackers will try to find ways around any new security measures.
But the consortium members say that its work should make any hijacker's job harder and help "restore full confidence in the air transport industry."