Originally Posted by
737 Driver
I posted this yesterday morning, but I think it got buried awaiting moderator approval (hopefully I'll be off probation soon
)
Procedurally,
it does not matter what the switches do or how they are labeled. You could call the switches "Hank" and "Frank", and it does not matter. From Boeing's perspective, you don't have to know what these switches are connected to. Whether the NG or the MAX, you always cutoff BOTH switches when called for in the NNC.
It wasn't always so on the NG. A while back, the 737NG stab runaway procedure was changed so that BOTH cutout switches are always selected together, and we no longer try to isolate the offending circuit. I was kind of curious why the change, but I was simply told that Boeing thought this was a better to handle runaway trim. The conspiracy theorist in me now says that Boeing did this because they were looking down the road at the MAX certification and were looking for any opportunity to harmonize procedures.
Missed this when it first appeared:
Interesting info on the runaway procedure change, hard to see what advantage it would have other than keeping things simple at the cost of eliminating the possibility of using manual electrical trim.
And from a later post:
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.
While relays can get stuck (welded) it is much more likely to happen as it closes due to arcing while the contacts are bouncing. The length of time in the closed position is very unlikely to be a factor.
Toggle switches are very unlikely to get stuck, especially when infrequently operated, the contacts are positively driven, not relying on a spring to open as in the case of a relay or momentary contact switches such as thumb switches on the yoke.
The previous (NG) switches had multiple paths to disable trim with the left disconnecting the main power to the motor via a (single) relay, while the right (autopilot) disabled inputs from the autopilot. The max has 2 switches in series, either of which kills the main power via the same single relay.