Salute!
Some thots for the Boeing MCAS folks that might be lurking here.
As many have posted here, there may be a poor fault tree analysis of the MCAS itself, and then the downstream effects of various MCAS failure modes. Some food for thought....
- Some failure modes at the back of my brain... and there are more that can be combined with other subsystems to confound the pilot ( except for the Yeager crowd that posts here)
1) MCAS doesn't work at all, as with a hard failure of the sfwe module or computer board. If no other sfwe module infected, I would not notice unless I got a high AoA and stick forces got lighter than I was used to in the other half dozen variations of he plane. No warning light, but 99.9% of we pilots would never get to the triggering criteria, or maybe not even notice, right? So no need to confuse we pilots with knowledge of the system or what it is supposed to do. And of course, no warning light that MCAS is FUBAR
.
2) MCAS does not work as designed but the sfwe module and computer is operational ( i realize I am splitting the fault tree, but who said it would be easy?)
a) It does not provide the correct noise down trim at the design AoA and mach conditions, and no warning light because it thinks it is doing O.K. "I'm O.K., Dave, don't bug me!"
b) It does not act at design AoA and mach because the flap switch indicates flaps are extended. Same answer from Hal as in 2, a above.
c) It works when it is not supposed to. This did not happen with Lion, as the AoA was clearly in the MCAS activation criteria.
i) flap switch fails in flaps up position and MCAS triggers due to momentary high AoA upon rotation. Hmmmm... does MCAS see WoW? Does that sucker give ten seconds of trimbefore reset?
ii) AoA momentarily reaches the design value due to turbulence when turning to make an approach fix and you might be about to lower gear/flaps. Does that sucker command ten seconds of trim?
And so on and so forth. Those are just a few of the things some of we old cable and pulley guys think about that have later flown very computerized planes and needed to know everything we could to ensure the safety of our SLF and own skinny butts.
And BTW, after about 200 or 300 hours in the "classic" planes, USAF let me fly 4 more for another few thousand hours that had no direct connection to any control surface with any cables, pushrods or pulleys. So that explains some of my irritation with the folks wanting to "go manual" and asserting all would be well.
Gums ....
Last edited by gums; 23rd Mar 2019 at 17:13.
Reason: usual arthritic fingers and typos