The technical information shared in this thread showed that the 737 design predate the deployment of aircraft ADIRU redundancy, as found in more recent design, and that the adaptation made in the various 737 iterations did keep the same *split sides" design. Implementing the MCAS function on the 737 FCC design require specific safety precautions because there is only a single AoA sensor / ADIRU available on the FCC data link. That's the technical facts.
Okay, I checked out some of the posts dated 12/10/2018, and FlyXLsa posted a functional AOA/SMYD/FCC block diagram.
From that diagram, it implies (using plural AOA "sensors") multiple AOA sensors input to SMYD subsystem. This implies data validation/source selection (DV/SL) happening within the SMYD.
It also shows ADIRU input to SMYD. But not for AOA I'm guessing, but for the AOA-corrected IAS and ALT data.
It also shows outputs going to FCC, but doesn't detail which, although one would suspect ADIRU pass through (after SMYD DV/SL? probably not since it isn't plural) and AOA sensor/sensors data (likely after DV/SL, else why have multiple inputs?).
Then someone (you?) shared that each AOA sensor on the 737 Max has dual outputs, one going to the SMYD, and one going to it's own side's ADIRU. (Can we have the source reference for that information?)
Assuming it's accurate, there are total of four AOA channels:
Left AOA - to - Left ADIRU
Left AOA - to - SMYD : source selection for FCC?
Right AOA - to - SMYD : source selection for FCC?
Right AOA - to - Right ADIRU
Since stall detection depends on accurate AOA and IAS, there's probably DV/SL within the SMYD for that as well.
So, unless there's something I missed, this all supports the case of source selection on multiple AOA sensors on the 737 Max.
I discovered (only now) this page that explain all in deep details:https://www.satcom.guru/2018/11/737-fcc ... mmand.html
I encourage anyone to read it with attention. I think this is the kind of analysis that should be in the final report.
Some key points about SYMD that support your findings:
* There are two Stall Management Yaw Dampers (SMYD) on a 737ng. The 737MAX does not use a separate SMYD "box", rather the functions have migrated to other "boxes".
* SMYD 1 is used for primary yaw damping and is connected to the both ADIRUs and left AOA Sensor for inputs.
* SMYD 2 is used to match SMYD 1 primary yaw damper commands, and is available as a backup under certain conditions when SMYD 1 is not available. SMYD 2 uses both ADIRUs and the right AOA sensor for inputs.
* The Yaw Damper architecture is an example of a dual-channel philosophy.
Some key points about the FCC:
* Each FCC has two 16-bit CPUs. The CPUs calculate different commands.
* For Mach trim, Speed trim, and possibly MCAS; the single active FCC CPU#1 command is made regardless of CPU#2, and regardless of the non-active FCC.
* Active FCC CPU#1 commands are made based on a single sensor set
. Active FCC CPU#2 raises an alert if the output command disagrees with the CPU#1 calculation, but does not stop CPU#1 command.
* It appears CPU#2 is using the same sensor data as CPU#1, making it susceptible to a common failure, using the same valid but false data.
* There is a cross-talk bus between the FCC's to allow sharing of sensor data. This may be already occurring in some cases.
So it appear that the data link architecture of the 737 NG and MAX have the capability to cross check the both sides ADIRU. But for some reason this was not [always] the case at least for the MCAS. The same page have a explanation in his conclusion:
"The FCC engaged-mode calculations, even single channel, depend on two sensor systems to prevent inappropriate response to a single sensor failure. Yet no such feature exists for FCC commands while not engaged
." [my emphasis]
This could be the very detail that forced Boeing to write that the MCAS rely on a single AoA sensor in manual flight only
. If all this are true, this greatly improve the understanding of the MCAS design context. It's maybe possible that there simply added the MCAS function where the mach trim and speed trim already exists, and those was already defective in manual flight but not as dangerous as the MCAS in case of a AoA fault.