Originally Posted by
MemberBerry
Yeah, the fact that the cutout switches are not wired identically in series is a bit strange. It's almost like somebody told the person responsible for the design "you have 5 minute to finish the damn thing, stop messing around with it and let's make it final, we have a deadline.". Anyway, about what the connections through the cutout switches seem to do:
1. connections 2 - 3 are wired in series between the two cutout switches, and are connected to the rest of the circuit in such a way that either of the cutout switches would do two things when used:
- interrupt the power between the circuit breakers and the thumb switches, making the thumb switches inoperable;
- de-energize the relay that connects the 3 phase 115V AC power to the trim motor, basically disconnecting the motor from AC power.
This means just cutting connection 2 - 3 on any of the two switches is enough to disable both manual and automatic electric trim.
2. connections 5 - 6 are wired in series as well between the two switches, and it seems that, if any of the two cutout switches are used, it would interrupt a 28V signal to the FCC, probably indicating to the FCC that the cutout switches have been used. No idea what the FCC will do based on that information.
3. connections 8 - 9 are wired only on the primary cutout switch, and as you said they seem to connect the FCC to the trim motor in some way. No idea why they don't go through the backup cutout switch as well, and exactly what signal they carry between the FCC and the motor. It almost looks like somebody forgot to route that connection through the second cutout switch. I don't see any reason for that.
Thanks for the succinct description of the cutout switch circuit functions. The design is strange isn't it? Given that previous 737s had two switches with different functions it is a real departure to have two switches in series when system reliability claims for stab runaway wouldn't have demanded it. Clearly the overall function has to be the way it is in order to give MCAS authority over the pilots. A cynic might argue that one switch could achieve that but that Boeing added the Backup switch to make it look like it always used to. If you hadn't been told about this new functionality and your point of reference was the previous design, the Lion Air & Ethiopian cockpits would not have been good places to work it out.