Originally Posted by
ST Dog
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.
If a power wire loses its insulation and a control wire loses its insulation and then something forces the conductors of the two wires together it is the same input as if a trim switch was used and happens downstream of the cutout switches, so it would not be stopped by opening the control wire enable switches on the console.
Eventually the way to handle this will be by avoiding simple on/off signals and require complex cryptographically signed handshakes between all control components. Recall the German plane that had the stick wired backwards? Easy-peasy. Put an accelerometer in the stick to compare it's movement to the control movement when the plane is on the ground. The aircraft company nearly killed everyone on the plane to save a couple of bucks in such a sensor.
A weak example of this communications protocol is used on cars for some functions with CANBus, where shorting power to a signal wire just disrupts every device on the network, so it's not a great drop-in solution for aviation. It does mean that a single wire can go to multiple devices to supply power and a single wire can carry comms, allowing a lamp holder to report to the car's computer that the lamp is not taking any current and is burned out; each component can self-test at startup and report the condition without having to have complicated test wiring on top of the function wiring. In aircraft the control could be over fiber-optic lines, avoiding short circuits, with a wireless option in case of major damage and local power for each actuator - nope, wait, that would be batteries and those are bad. Anyway, solving this sort of problem to 100% reliability is not easy.