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Old 15th Mar 2019, 10:09
  #1450 (permalink)  
calypso
 
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In summary then:

The newer engines required to optimise the MAX have a larger diameter than that allowed by the wing/ fuselage geometry. In order to fit those engines Boeing had to move them forward which changed the aerodynamic characteristics of the aircraft. That change consisted on insufficient pitch down moment when close to the stall in clean configuration. In order to meet certification criteria Boeing introduced a system that automatically moved the stabiliser to artificially induce the pitch down moment . In order to introduce this system with minimal costs existing equipment was used including a data input from a single source - the AOA probe. In order to reduce qualification costs this system was not mentioned in the MAX operation manuals.

In the Lion Air case the AOA probe was not calibrated properly, meaning the direction and magnitude where correct but the value was 20 degrees out. When the flaps where retracted the MCAS started to work as intended -based on the erroneous aoa data- by introducing bursts of down trim, those bursts got interrupted every time the pilot trimmed manually via the electric trim. At a certain point the crew started to pull hard at the yoke and the automatic stab trim cutout switch operated, with the effect that electric manual trim no longer worked, MCA trim in the other hand did continue to operate but it no longer got interrupted by manual pilot trim eventually trimming all the way nose down, a position that exceeds the authority of the elevator. At that point they only had two options left
1. Offload control column pressure to release the automatic stab trim cut-out then trim back to neutral.
2. Manually disconnect the electric stab trim via the override switches at the base of the throttle quadrant then use manual trim for the remainder of the flight.

Three crews managed the above but the fourth got overwhelmed in the Lion Air case. The Ethiopian on the face of it looks quite similar but there isn't enough data yet to even have a guess. A bit of a mess but where I tend to agree with Boeing is in which way is this any different from a classic runaway stabiliser? Ok there is more distractions going on but bottom line is if the stabiliser starts to run and you dont get on those disconnect switches soon enough you will end up in the same place.
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