Originally Posted by
PEI_3721
Takwis, #2469, IFF789, #2467,
It is difficult to understand how MCAS is wrong in principle. As a fix for a stability problem identified during flight test, the design (theory) was adequate (expedient, quick, low cost) although an ‘inelegant patch’ compared with changing the underlying aerodynamics - aircraft structure.
The weakness of MCAS was in its implementation - engineering, and thence safety assessment and certification.
For the avoidance of doubt: I don't think moving the stab to augment pitch control forces is wrong in principle, bypassing trim/pitch control safety systems to do it, is. You may consider that part of the implementation rather than the design - in which case we more or less agree. I note that STS, including the "stall-id" function, which trims down close to stall AOA,
does NOT bypass the column cutout. So, why did MCAS? That is the point it becomes, to me, wrong in principle. It appears that it was fundamental to its operation that it had to bypass pitch-control safety, in which case MCAS could not exist if it was done right - hence why I said MCAS was wrong.
It is not only wrong from a safety point of view either, it's also wrong from an effectiveness point of view. After all, this is a system which, even if it can't be overridden by the control column, does have an off-switch. If we assume the level of pilot competency (and maybe country of origin) which Boeing designers seem to have been assuming, and that, as per Tawkis #2469, the manual still documents the column cutout safety feature (without exception for MCAS, which is undocumented), then
MCAS cannot possibly work. Why? - because the pilot that sees the aircraft trimming nose down with stick full back will diagnose trim runaway and
immediately switch it off. Furthermore, they may then go on to run into what MCAS was supposed to protect them from -
because there are no documented restrictions on flight envelope after you've hit the stab cutouts.
An outstanding puzzle is why MCAS is based on AoA opposed to speed in order to cure what appears to be deficient trim-speed stability. From this arises the (false) association with stall issues, yet the existing stall alerting and protection systems remain unchanged.
Suspect this comes back to the changes made to MCAS post flight-testing. MCAS as originally designed (triggering on AOA
and g input, and with far less authority) might have been wrong in principle but not actually dangerous, even the assumption that pilots didn't "need to know" because they would never see it, might have been correct (note: not the same as "justified"). The same is true of the changes that decreased the effectiveness of the trim wheels (coupled with removing the procedure for coping with it from the manual) - they knew this wasn't right, but they thought they'd get away with it because no one would ever need it, and for the NG it turned out
they were right. Then there was the last-minute changes made to remove the g-dependency - that probably made it several orders of magnitude more likely to activate when it shouldn't, and then it was made four times more powerful if it did activate. There is no doubt in my mind that
that was the implementation change that made it
dangerous - it was
wrong before (and because it was wrong it was easy for it to become dangerous).
Since Boeing apparently didn't even fully document those changes to the FAA, let alone to the rest of us, I would suggest that we don't know whether published/public information about MCAS, from whatever source, actually refers to MCAS-as-originally-designed or MCAS-as-delivered. I now include "MCAS is not about stall protection" in that category as well - MCAS-as-designed, yeah I'll buy "not about stall", MCAS as modified and delivered but not documented to the FAA? - absolutely no idea.