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Old 16th Nov 2018, 04:56
  #1296 (permalink)  
Join Date: Nov 2016
Location: Lakeside
Posts: 428
Originally Posted by LEOCh View Post
The Leeham News article is quite informative of why the MCAS is required on the MAX, and fills in some blanks. I originally thought heavier engines further forward should if anything bring COG forward (unless balanced by other modifications), creating a more longitudinally stable MAX. However the idea that at high AoA the nacelles become aerodynamic surfaces sufficient to longitudinally destabilise the aircraft is interesting. It's a big aside, but a similar problem was noted on the WWII Supermarine Spitfire. As development to more powerfull engines dictated more and more propeller blade area, the non obvious effect of this as an aerodynamic surface way forward of the aerodynamic center caused degraded longitudinal stability margin.

So when you are at high AoA, your MAX may become pitch divergent (without MCAS). At the same time you are likely to be slow but with high thrust beneath the wings, pitching you upward into the stall.

One thing that keeps coming up considering how much importance the AoA signal now has in the MAX, is whether there is redundency in the two vanes. I think the answer is not really, as many conditions (rolling, yawing!) will lead to expected AoA disagree between the two sides of the nose. A warning (i.e. stick shaker) based on a single high alpha vane is reasonable, but any kind of consensus signal from two vanes only with severe consequence for faulty data is quite problematic.

So the current two vanes are really only one sensor for the purposes of redundancy. I wonder whether to have such a high degree of reliance on a AoA signal you should incorporate three vanes on each side.
That’s changes to Pitch done and dusted. If the manual did not include something as tricky as sole source AoA data FAIL, what else is missing? Are there new issues as regards OEI? Roll problems? Does Max need bigger ailerons? Is the Rudder effective in OEI? ETOPS? Will I fly to Hawaii on MAX? Yaw damping? What manner of control issues are different? MCAS is not so much about erroneous data recovery as it is about the whole picture. How do we trust that MAX is just the same as NG? Because with the fit of MCAS, Boeing admits it int the same aircraft....then disguises that finding by casually excluding the mitigation to show “it isn’t that important...” Who says?

Slow rolling the aircraft’s Test program with pax? Testing potential failures that demonstrably don’t meet statistical proofs with manufacturers prior “approval”?

What else is new and exciting? What other issues are too complex in failure to overwhelm the “average pilot” such that they need not to know and train for them?

Iceberg. Tip of....?

Last edited by Concours77; 16th Nov 2018 at 05:16.
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