"It appears there was more than an altimeter problem on the Turkish jet. Boeing said the autopilot system is designed so that if it senses faulty data from the captain's altimeter on the left side, the autopilot, including the autothrottle system that controls engine thrust, is supposed to switch to the altimeter on the right side used by the co-pilot. Boeing would not comment about what may have gone wrong on the Turkish jet, saying that is part of the investigation."
Seattle Post Intelligencer writes the above cited. Sheds a completely new light on the A/T system logic. The link to the complete article:
Not quite correct from my knowledge, though I am digging deeper and deeper into the design data.
Once again for those who keep missing it, the radio altimeter did not indicate a failure to the systems or the crew!! It incorrectly indicated a altitude of -8 feet, so the A/T thought it should be in the retard mode since the aircraft was in single channel A/P.
If the A/P had been in dual channel, this would not have happened as the A/T needs to have the flare discrete from the Flight Control Computers (FCC). Yes, there is a link between the A/P and A/T, but only in dual channel operation.
FCC A uses Rad Alt #1 as primary and FCC B uses Rad Alt #2 as primary. Since the aircraft was in CMD B, the #2 Rad Alt was feeding the A/P. However, the #1 Rad Alt was still feeding the A/T since it was not sending a fail flag (NCD on the 429 bus). The A/T thought all is well with the Rad Alt and therefore would NOT switch.
