Originally Posted by
FCeng84
gums - Peace! I am honored to be able to have these exchanges with people like you who have such a wealth of flying experience over a wide range of airplanes. The last thing I want is a pissing match. Thanks for your pitching moment vs. AOA plot that you provided above. I wish I had one to share for the 737Max. I took a quick stab (no pun intended) at Google to see if I could find anything in the public domain, but no luck so far. I will keep looking. Interestingly enough one of the search hits I got was a link back to PPRUNE to another of the forum discussions we have both been on where you had previously shared the same great plot!
The sentence from my earlier post that included "plenty of pitch authority via the column to the elevator" that you reference above is one that I struggled with to add the necessary disclaimers. Here is the full sentence: The pilot has plenty of pitch authority via the column to the elevator to initiate and recover from any pitch maneuver provided MCAS either does not function or functions with the design limited authority of 2.5 degrees stabilizer at low speed and less as speed increases. Clearly repeated activations of MCAS without having returned the stabilizer to its properly trimmed position each time can, will, and did eat up the pitch authority to the point where full aft column was not sufficient to keep the nose from dropping.
I will not claim that the MCAS design is as good as it should or could be. My main reason to post about MCAS here is to convey how it operates so that all will have a better understanding of what the pilots actually faced. A clear design assumption with MCAS was that if the pilots activated manual trim then they were taking control of the stabilizer and would return it to a properly trimmed position. We all know the breakdown of the word "ASSUME" - it makes an ASS out of U and ME. Another assumption was that if the crew were flying at a steady condition and found that the automatic stabilizer control system repeatedly moved the stabilizer away from trim that would be indication of errant control action and that the crew would take the mitigating action of disabling automatic stabilizer control via the stab cutout switches. A lot of assuming going on here.
I definitely agree with your point that a full FBW system that provides stability augmentation through the elevator rather than the slow moving stabilizer would be far superior. There are many examples of that now in both the military and commercial fleets.
The bottom line for me is that we need to understand how the 737MAX system works, what the thinking was that went into that design, and what assumptions with regard to design, operation, maintenance, etc. should be challenged to reduce the risk of a repeat event to as low as possible. I strongly believe that this is a direction that all of us on PPRUNE can rally around to move us toward a better future. On that note, a Happy Valentines Day to all. (I realize that last part is belated for those east of the Atlantic.)
I think the main point you could be drawing out is that in the incident aircraft, MCAS appears to have provided an unadvertised and intermittent control input of complex algorithmic intensity.
Believe what we are left with is how unadvertised, intermittent and complex was also certified.
I vote with various other posters.
Give it at least two independent inputs, and let it disarm and notify where they disagree.
While superior solutions exist, I cannot see how an inferior solution should be supportable.