There is debate if the AoA guage is faulty or just intermittently giving erroneous readings.
The Ethiopean Airlines CVR showed that Captain and FO had different AoA readings, That was one reason why the Captain handed control over to the FO after he thought he got the plane climbing again.
I think they will find that because they did not turn off MCAS, the Captain pulling on the yoke and changed trim only temporarily reset MCAS. After 10 seconds, MCAS turns back on and wants to trim the plane to nose down. This happened as the Captain handed controls over to the FO and MCAS re-activated and put them into a nose down situation again.
My understanding is that the trim forces can be so much that manual yoke and trim cannot overcome it.
Don’t forget, they were not far from the ground, nose down, stick shaking and warning buzzer trying to get the plane to climb. They may have forgotten about the Auto Trim Off switches altogether, resulting in their deaths.
Turning Auto Trim Off is not a 737 NG procedure....and that is a problem.
The worst problem is having MCAS use only one AoA sensor input. Two is better, but then the computer has to choose which one to believe and which one to ignore. Three is always the best because the computer can look for 2 that agree and ignore the oddball.
Having 3 AoA gauges would be better, but it’s easier install them in pairs on opposite sides. I don’t really see how two AoA gauges could be enough for ETOPS. Perhaps backup could be created from processed data from other types of gauges.
Turning off runaway trim has been needed as a skill since the 727.
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Boeing Reveals 737 MAX Software Changes To Pilots (UPDATED)
Marc Cook
On Saturday, Boeing met with pilots from several airlines to review changes to the 737 MAX’s MCAS software in preparation for a more public explanation of the alterations expected to remove the beleaguered airliner from its worldwide grounding. On Wednesday, Boeing is expected to bring together “pilots and officials” from the airlines with the affected 737s in their fleets.
In addition to reviewing the software, pilots from five airlines flew simulated failures of the MCAS and were able to disengage the system and safely complete the flight. The New York Times is reporting that pilots who flew the simulation with the original software had just 40 seconds to identify MCAS as the source of the trim movement and disable it. And it's worth repeating that these pilots had been aware of the controversy swirling around MCAS since the Ethiopian Airlines crash this month.
According to reports, Boeing is set to confirm changes previously reported as considered, which include changing the MCAS configuration to accept data from both angle-of-attack sensors rather than just one, limiting the number of times the MCAS can drive the stabilizer to affect nose-down pitch, and limiting the duration of the events to 10 seconds. Preliminary reports from both the Lion Air and Ethiopian Airlines accidents suggest that the MCAS continually tried to offset the pilots’ efforts to level the aircraft. In addition, new coding will make the MCAS disengage if it sees a differential in AOA sensor readings of more than 5 degrees.
Those same pilots few the 737 MAX simulator with the revised software and were able to diagnose simulated failures and land safely with less effort, according to reports. “This is part of our ongoing effort to share more details about our plan for supporting the safe return of the 737 Max to commercial service,” Boeing says. “We had a productive session this past Saturday and plan to reach all current and many future Max operators and their home regulators.” Meanwhile, Southwest Airlines, the U.S. airline with the most 737 MAX aircraft in its fleet, is canceling approximately 130 flights a day, while American is canceling roughly 90 flights a day. Southwest has 4000 daily flights exclusively on 737s, while American has 6700 flights a day across a wide range of aircraft types.
http://flash.avweb.com/eletter/4300-full.html?ET=avweb:e4300:264646a:&st=email#232475