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Old 16th Apr 2019, 12:42
  #4079 (permalink)  
737 Driver
Join Date: Apr 2019
Location: USA
Posts: 217
Originally Posted by FrequentSLF View Post
Not a pilot
still I wonder why the MAX needs a PRI and B/U cut off! Forget the label changing, what surprises me, is that on a new design the two switches are in connected basically in series, and either one is doing a cut off! Means the previous design was faulty? And in the new design was considered safermto have a back up cut off switch, just in case the primary fails? What kind of engineer will design a system with a single sensors, single computer, single motor to drive a flight control, and feels the need to add a back up cut off?
Am a 737 pilot.

Reason I was given was to provide redundancy in case of a rare instance of switch (or actually relay) welding. That phenomenon occurs when a relay remains in a set position for so long that it basically welds the contacts closed. I'm not an electrical engineer, so I don't know how to evaluate this statement, but it is not unlike the rationale for the split thumb switches on the yoke.

I suspect another unspoken reason was to maintain the look and feel of the 737NG switches. As long as there were two switches that were always used together, then Boeing could take the position that no additional training was required.
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