Originally Posted by
737 Driver
Originally Posted by
Loose rivets 
Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.
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I have no doubt that this is true, but it is also largely irrelevant from a procedural viewpoint. The pilots don’t need to be able to read a wiring diagram and tell you all the things that happens when they throw the cutout switches. They just need to know when they need to throw the cutout switches - as in the case of the runaway stab trim procedure.
Unfortunately the AV herald "question" referenced above commingled the under floor cutout switches with the 2 center 'stab cutout' switches. If carefully parsed it might be correct but very hard to read.
Here are the relevant facts on the cutout switches:
The 737 NG the cutout switches:
left cutout kills all electric trim (protects against stuck pilot switches as well as some faults in motor module)
right disable automatic trim only.
On the 737 MAX either cutout switch disables all electric trim.
The 28V power to the main trim motor 115V 3 phase relay is in series through both switches. The av herald shows some function not identified secondary contacts that are probably status inputs to other systems.
The 737 MAX labels were changed to 'primary and 'backup', the changed functionality was not mentioned in at least some version of the 'i-pad conversion course.
One theory is that the ET pilots used the right switch and then discovered they had no manual electric trim, which they would have had on NG.
Others have posted that the runaway trim procedure was changed from first using the right switch only to using both a few years ago for unknown reasons.
No one has posted a rational sounding reason why the switch functions were changed.