Originally Posted by
Mach2point7
The "Boeing Statement on AOA Disagree Alert" tells us that the MAX has had an AoA disagree alert function from the outset, but in error, it was not displaying to the aircrew. There is a long and convoluted explanation/justification of why that deficiency may not have been critical.
The Boeing spokesperson is essentially correct. Again, I am not defending the overall sloppiness that this particular piece displays, and it is just another example in a growing list of how the MAX development program wasn't managed within the high standards that are normally expected of an aircraft manufacturer.
Strictly from a technical perspective, however, it needs to be pointed out that if a 737 crew were to receive an "AOA Disagree" message and go to the appropriate NNC, that checklist would simply direct them to the "IAS Disagree" checklist and/or the "ALT Disagree" checklist as appropriate. The "IAS Disagree" checklist would then direct the crew to the "Airspeed Unreliable" checklist, which has already been discussed extensively. In short, if the AOA disagreement becomes significant enough to impact the flight instruments or other systems, there will be plenty of other indicators for that problem (stick shaker, "IAS Disagree" alert) and so the "AOA Disagree" message is somewhat redundant in terms of directing the crew to the proper procedure.
The real issue is that if the AoA disagree signal system was available " in keeping with Boeing’s fundamental design philosophy of retaining commonality with the 737NG" why on earth was it not used as a critical input to the MCAS activation logic ??
Very good point, and I believe the MCAS update will do exactly this. I suppose the answer to why this wasn't done in the first place resides in the thinking that led to the MCAS design team using only one AOA input. This probably arose from the fact that the MCAS is considered to be an ancillary function of the Speed Trim System (STS). The STS has been around since the 737NG was launched and it has always relied on just one AOA input. The difference is that the STS respects the control column trim cutout switches (this is different than the pedestal cutout switches) and MCAS does not. The control column cutout switches prevent the STS from trimming nose down when the control column is moved aft. Apparently no one connected the dots that by removing the control column cutout switches from the circuit, MCAS was now set up to malfunction in a novel and hazardous manner.