MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

[GordonR_Cape]

Originally Posted by Gilles Hudicourt

737: The MAX Mess | bit-player

Thanks for the link, which I had not seen before. It included a description of an issue that I raised recently (which elicited some skepticism):

That experience taught me just enough to recognize something odd about MCAS: It is way too slow to be suppressing aerodynamic instability in a jet aircraft. Whereas the X-29 controller had a response time of 25 milliseconds, MCAS takes 10 seconds to move the 737 stabilizer through a 2.5-degree adjustment. At that pace, it cannot possibly keep up with forces that tend to flip the nose upward in a positive feedback loop.
There’s a simple explanation. MCAS is not meant to control an unstable aircraft. It is meant to restrain the aircraft from entering the regime where it becomes unstable. This is the same strategy used by other mechanisms of stall prevention—intervening before the angle of attack reaches the critical point. However, if Brady is correct about the instability of the 737 MAX, the task is more urgent for MCAS. Instability implies a steep and slippery slope. MCAS is a guard rail that bounces you back onto the road when you’re about to drive over the cliff.
Which brings up the question of Boeing’s announced plan to fix the MCAS problem. Reportedly, the revised system will not keep reactivating itself so persistently, and it will automatically disengage if it detects a large difference between the two AoA sensors. These changes should prevent a recurrence of the recent crashes. But do they provide adequate protection against the kind of mishap that MCAS was designed to prevent in the first place? With MCAS shut down, either manually or automatically, there’s nothing to stop an unwary or misguided pilot from wandering into the corner of the flight envelope where the MAX becomes unstable.

My bold added.

Edit: I understand that it is an old article (Apr 2019), but the fundamental control theory issue remains the same, and still not clarified by Boeing or the FAA.

Edit: IMO the author Brian Hayes is the B737 MAX equivalent of Richard Feynman and the Space Shuttle Challenger. From Wikipedia:

He is a senior writer and regular columnist for the magazine American Scientist, and was editor in chief for the magazine from 1990 to 1992. He has also edited and written columns for Scientific American, as well as writing for Computer Language and The Sciences.