Note TCAS does not "automatically" fly the airplane away from a potentil mid-air collision - it provides guidance to the pilot to do so.
It is an interesting topic for discussion, but at the current state of technology, my (Principal Avionics System Engineer - Commercial Aircraft) opinion is against giving automation the over-ride authority to the human pilot. The problem is not one looking for an avionics solution for several and complex reasons. An airplane is somewhat a fragile piece of complex equipment from this perspective. Even considering that acceptable level of safety (once a hijack situation is known) is less than otherwise, there are too many failure modes in the automation to worrry about, and one can only indicate it (rather than write a detailed report) in a post like this.
An automation to override the pilot meas all controls (for control surfaces and propulsion/electrical systems) have to be overridden. This means all these systems have to be routed through one (or two or three) central point(s). There has to be one or more override switches located conveniently.
How robust should the override be? To prevent the Sept 11, 2001 type scenario, it should be very robust meaning no reversion to the cockpit should be possible. What happens in case of inadvertant activation or switch failure? (This is more than conceivable and will happen more often than hijacks.) If a ground controlled reversion back to the pilot is designed in, what about the communication system failures? Even today, over the ocean, the communication is 'not very good.' Even trying to come up with an operational concept for such a system gives very unsatisfactory results.
Military technology is best suited for military where pilots/crew are trained for it and - in the extreme, can bail out of the airplane in the case of fighter aircraft.
To implement such a safe automation override (even if we choose to) will result in enormous costs to the existing fleet and take years to implement. Much better solutions for operational security are available (see pointers in the previous message(s)) including skymarshals, armed pilots (non-lethal arms, perhaps) and better doors and procedures and implementing certain no tolerance policies.
Apologies for any remaining typos.
[ 20 September 2001: Message edited by: Narada ]