Originally Posted by
OldnGrounded
I can't possibly say it better than this. I'm pretty sure everyone agrees that the Turkish crew was "guilty" of multiple serious errors. However, that doesn't change the fact that the manufacturer had failed to let crews know that the left RA was feeding the autothrottle even when the right FCC was active -- a very significant hole in the cheese, all by itself. Remember, they noticed that the left RA reading was bogus, but they didn't think it mattered.
Talking about cheese: did you know that the NG's autothrottle also uses the left
airspeed / Mach reference, even when the right FCC is active? I discovered that by accident during an actual unreliable airspeed event (I'm actually not sure if this information was mentioned somewhere in the FCOM at that time - if it had, the "discovered" in this sentence should be replaced by "re-discovered", but the end result was the same regardless).
We were confronted with a blocked static port at cruising altitude (probably had something to do with excessive rain ingestion on the ground during storm conditions earlier that flight): we noticed a steadily increasing difference in altitude indications, which then exceeded the threshold for the Altitude Disagree flag to appear. Thus, we had to descend to a flight level below RVSM airspace. Starting the descent it became immediately apparent that the left-hand indicated altitude remained constant, even though the aircraft was very obviously descending. This, in turn, caused an erroneous overspeed annunciation on the left-hand PFD, plus the associated aural distraction of the clacker. But no immediate concerns with regard to flight-path control, as we already had autopilot B engaged and LVL CHG appeared to work flawlessly, with the throttle closed during descent - as one would expect. Then the surprise: no automatic throttle-up when levelling off at the new lower cruising altitude. The autothrottle speed control left the throttle closed as it was, relying on the erroneous left-hand airspeed value that was already indicating deep into the barberpole.
Boeing will probably argue that using a single source airspeed here is perfectly acceptible and completely safe. After all, my "superiour western piloting skills" obviously saved the day - we didn't stall the aircraft.
Still, it would have been nice if the system had just disengaged the autothrottle automatically with the associated warnings...