Originally Posted by
The Bullwinkle
Can somebody please explain how the MCAS system can cause an unreliable airspeed indication after takeoff?
It doesn't, and I don't see that anyone has implied such. There is, however, a plausible common cause to both MCAS activation and IAS disagree: an erroneous AoA value by one sensor. AoA is used for calculating a correction to static pressure, and both ADIRUs independently calculate airspeed from their respective values for static and total (pitot) pressure. Since the static pressure has been adjusted for AoA, both ADIRUs will come up with different values for indicated airspeed, which will trigger an "IAS disagree" warning, which is a trigger for the unreliable airspeed procedure.
I still don’t understand why a blocked pitot isn’t even being discussed.
Because it doesn't fit the known FDR traces from the Lion Air accident. And from all we have heard, there are huge similarities between the Ethiopian and the Indonesian accident, which even Boeing has acknowledged and led them to recommend grounding the type ("out of an abundance of caution").
Could the pitot covers have been left on, or possibly a mud wasp got in there overnight?
From all we know, very very unlikely.
At the end of the day, if the engines and the hydraulics were working, why weren’t two qualified pilots able to “fly” the aeroplane in apparently fine weather back to Addis Ababa?
That is the 300-million-dollar question, isn't it? Probably because an insidious system did underhanded things, unknown to the crew, which were preoccupied with the Ureliable Airspeed procedures and distracted by a stick shaker. In end, nose-down trim overwhelmed the pilots' ability to control the aircraft.
I’m honestly confused as to why everybody seems to be jumping on the MCAS bandwagon!
Because all symptoms, and all data we know so far are consistent with an inadvertent MCAS activation, and are not consistent with most other plausible explanations.
I would think that Boeing would be the first, if there was any doubt that the Ethiopian accident was very similar to Lion Air, to vehemently deny any MCAS involvement.
Bernd