Originally Posted by Ka8 Flyer
What I don't understand is why the A/P kept descending at that high rate? The false G/S signal was centered - why didn't the A/P reduce the sink rate to the normal 750-800 fpm? Or was their groundspeed so high that the A/P calculated it would have to descend at that rate to keep the needle centered?
The incident report explores this in detail.
The carrier signal had a very slightly biased DDM, displaying the aircraft ever so slightly above GS, and the aircraft apparently performs zero sanity checks on vertical speed, slope, etc. So the slight bias caused the aircraft to continually push the nose down until the crew went around.
As llondel notes, there appears to be a design fault in a system that can give a "correct" indication when the system integrity has failed. It also appears silly to me that the avionics don't make better use of all the data available to check the sanity of an approach. Isn't all the data to complete a DME check already in the FMS? The humans take a while to complete those sanity checks...but thankfully they did.