Technically, a feasible and not unrealistic idea, notwithstanding that Budgie's approach is probably easier to certify. I suspect that the largest difficulty in certification is that you have a system which can disable the primary cockpit controls, including fuel cocks, throttles, transponder, etc. but presumably not the radios. Not a huge problem with a modern electric jet.
I think personally that the idea of allowing the crew to disable the AP after operation is unrealistic - consider the hijacker that forces the pilot down the back and then holds a knife to his throat until he gives the code. May I suggest the following: -
(1) System can be manually operated by crew, or (with aid of ID tags or whatever) triggers automatically if no flight crew are in the cockpit for more than (say) 5 seconds.
(2) System automatically aligns for diversion field, and squawks 7500.
(3) System can only be de-activated by transmission from the ground of an encrypted code. They would no-doubt do this on a calm discussion on company frequency from the Captain, but any sounds of stress in his voice, etc. they simply refuse. At worst case severe embarrassement,some peeved SLB, and a blocked runway. Any sensible planner would only use military airfields if possible for the diversion anyway - who can cope with a blocked runway rather better than most civil airports (and have an existing supply of people with guns, which may be helpful).
I think it could only be done in an aircraft with full FBW without manual reversion (such as an Airbus), and possessing a cat-3 autoland. You also need to find some way to override the trimmers, which are usually mechanical and required in case of primary control system failure on an airliner - without taking away their availability in the case of a more mundane emergency.
I'm an airworthiness Engineer by profession and would be glad to stick my oar in if it would help, but to be honest I've never worked on these complex AFCS controlled aircraft and there are better heads than I out there.
Incidentally the existence or lack of GPS is irrelevant. A modern airliner uses a combined Nav system which takes INS, GPS, and other sources of data to give a best position. In fact reliance on GPS alone would be foolhardy, since it's extremely easy to jam. INS then ILS is probably the most secure method.
G
[ 14 September 2001: Message edited by: Genghis the Engineer ]