Originally Posted by bubbers44
Chris, I think the autopilot engineers didn't want the AP flying the airplane if there were two competent pilots to take over if the inputs to the computer were lost. The AP if programmed to keep flying without airspeed input would have held 3 degrees nose up until altitude hold required a different attitude, and the last N1. The AT would eventually have no way to know if it needed to change with no AS. I think that is why they wouldn't let it keep flying level. It could get too fast or too slow with no AT help. Letting the pilots fly it manually to keep it in proper speed range was their solution.
What do the pilots know that the computers don't already know? In fact everything the pilots see is filtered by computers. The AF447 pilots were just as blind and bereft of real data as the pilots. So there really is a valid question here. Why DID the pilots have to take over?
The only valid excuse that comes to mind is that the designers did not know this particular failure mode existed. Until this crash the doctrine was that such errors could not happen.
Now, the computer actually has more information than the pilots saw, it appears. At the very least it has AoA. It also has GPS, raw and processed and the inertial system that it can use to work with. It HAS altitude. It never lost it. So it can monitor its pitch and thrust and fiddle both to maintain altitude. It might be a "lumpy" process. But it would keep the plane flying. The pilots may have had the GPS/INS data displayed somewhere. It appears it was as little in their scan pattern as the trim wheels. So it might as well not have been there if it was. The computer would scan these things if the software design had it doing so.