Originally Posted by
Smythe
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What concerns me is that the initial CFD modelling showed at 0.6 degrees would be enough to mitigate the issues. On flight test, that was found not to be sufficient, and somewhere, the 0.6 turned in 2.5 degrees.
That is an exponential difference, and I would really like to see the modelling and assumptions that supported either numbers...
There is also the NYT report where the test pilot stated MCAS orginally used G force, and that was dropped somehwere along the line...now what was that used for? It seems that the high G manuevers the pilot was talking about such as wake turbulence or avoidance (that caused stall?)
Again, the CFD assumptions that required MCAS or where mitigated by MCAS would be very, very interesting to look at.
I’ve seen no mention of CFD modeling, so I don’t know if they used CFD or actual flight tests to determine the design parameters.
According to the article you reference, MCAS was originally designed to mitigate undesirable handling characteristics during a high-speed accelerated stall. The g-meter input was used along with the AOA as a sanity check on the accelerated stall condition. At the higher speeds, the 0.6 unit input was probably enough.
Later on, it was decided to expand MCAS’s role to correct control feel issues during low-speed, 1-g stalls. This required removing the g-meter from the loop and increasing the input to 2.5 degrees because at the lower speed a greater stab movement was needed. At this point someone should have noticed that 1) there was no redundancy to prevent the single-point failure, and 2) there was no test for high-speed (0.6 input) versus low-speed (2.5) stab input.
Too many chefs, to little communications, and no one providing a thorough top to bottom review of the final design.