Originally Posted by
CurtainTwitcher
Boeing made a deliberate design decision to NOT compare AoA signals or do any other processing of the data. The reasons has been fully documented on pprune.org and the Wall Street Journal, search for Rick Ludtke.
The bullet points are:
- Comparing AoA signals could generate a warning.
- AoA warning would be outside an iPad training path for 737NG--->737MAX and could possibly involve a simulator session.
- A simulator session would incur a $1 million penalty per aircraft for Southwest airlines.
- Boeing choose to categorise an uncommanded MCAS runaway as only "major" and not hazardous
- Therefore Boeing choose not compare AoA
The design failures between a single faulty AoA sensor and continuous MCAS activation have not been fully revealed.
Unfortunately, the design change from the original MCAS to the accident MCAS variant resulted in the hidden trim rate change, where the MCAS rate at high speed was much greater than the trim rate using the normal trim system by the pickle switches. That alone altered the potential impact of a failure, and if it had been evaluated in the sim it is reasonable to expect that the analysis would have increased the categorisation above major.