Originally Posted by
Dave Therhino
This is not a changed product rule issue. The stab trim system had several changes for the Max, including changes to the motor, the motor control, the pilots' control switches, and MCAS. These changes, even without MCAS, required the system to be re-examined for compliance with the system safety analysis regulation (25.1309(b)).
Your the first to say this isn't due to new wire separation rules that went into effect after the NG was certified.
All the other comments/reports have said it was related to changes in the rules that make the separation used on the NG no longer compliant.
Per Boeing the wiring in the areas of concern wasn't changed so it wasn't looked at.
They changed the "logic" in the console (signals/power passing through the switches and relays), but the same wires/signals eventually run out of the cabin to the tail. I'd have to go back and find both diagrams but I don't remember any new wires/signals compared to the NG.
Now whether the changes that were made would lead to a re-examination of the wire bundling/routing from cockpit to tail I'm not clear.
My inclination is that no, the wiring would not be looked at as they weren't changing it.
The new SSA would not have looked at the prior unchanged aspects for the unchanged portions.
And I'm still not clear where the possibility of a runaway that can't be stopped with the cockpit switches comes from.
If anything there are now 2 switches in series that cut the signal to the motor vs the old setup with parallel paths in the cockpit.
But either way there was just one signal going back to the tail.
I guess I need to find the wiring diagrams again and look at it again. This wasn't the focus at the time I last looked.