Originally Posted by
Swiss51
So, what you say is that - if the FAA is correctly trying to get back control over the certification, and the wiring is found to be changed based on the findings - the wiring in all NG's would need to be changed as well?
The system was changed for the Max, so the wiring should have been part of the new stab trim system safety analysis for the Max. It either wasn't examined or the issue was missed. At this point, because the design was already approved and the FAA did not revoke the type certificate, what's driving the FAA is apparently a determination that an unsafe condition exists warranting corrective action under 14 CFR part 39 (the part under which airworthiness directives are issued). To address the issue, Boeing has to propose a design change. That design change is required by 14 CFR part 21 to comply with the airworthiness requirements, including 25.1309 and 25.1707. A TC holder can't obtain approval of a non-compliant design change other than via an exemption. You can bet the FAA won't be issuing any exemptions from the system safety requirements for the stab trim system.
If the lack of wire separation and the consequences of that lack of separation are similar for the NG, you would normally expect a similar finding that an unsafe condition warranting AD action exists on those airplanes as well. However, at this point top FAA management is likely making the decisions rather than staff engineers following the normal processes, so it's hard to predict what the FAA will decide for the NG wiring. The normal FAA process would call for an AD if the wiring is found to have the potential for a single fault condition to be catastrophic. The Boeing lobbying will likely be based on a probability argument.