atomic_fungus (atomic_fungus) wrote,

#6588: The best explanation, well worth reading

A pilot discusses why the 737MAX MCAS system was a bad idea. It's a Faceboob post but includes the link to a PDF. It goes into detail right down to "Sunday supplement" level, so if you don't know anything about aviation you'll still understand what went wrong and why.
If I have not been clear, so far, let me say it succinctly. Boeing produced a dynamically unstable airframe, the 737 MAX. That is big strike #1. Boeing then tried to mask the 737's dynamic instability with a software system, similar to the systems used in dynamically unstable fighter jets (though those jets are fitted with ejection seats). Big strike #2. Finally, the software system relied on systems known for their propensity to fail (angle of attack indicators) and did not appear to include even rudimentary provisions to cross check the outputs of the angle of attack sensor against other sensors, including the other angle of attack sensor. Big strike #3.
"Dynamic instability" is another way of saying that if the airplane goes nose-up, it will tend to go more nose-up. "Positive feedback" is another way of saying it.

The big engines, placed where they are, actually provide lift at high angles of attack. Because that lift is ahead of the center of lift of the wings, it forces the nose up. So, yeah--get into a nose-high attitude, and the nose tries to go higher because the engines are pushing it up, and the stall warning starts going off and the pilots try to stop it and your flight profile starts looking suspiciously like the time-versus-altitude plot of the plane involved in the Ethiopia crash. /\/\/\_

The more I learn about this, the worse it looks. Yeesh.
Tags: #1, #2, #3

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