Originally Posted by
ST Dog
No the FAA is retroactively deciding that the old grandfathering rules don't apply to a certain case (the wire separation) and make that unchanged item meet the new standards.
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)). That rule is unchanged since the time of the NG certification in 1997. When you change the system, the whole system - not just the part you changed - has to be re-examined and found compliant because the system safety analysis regulations are system-level requirements. The wiring deficiencies should have been caught by a
competently performed failure modes and effects analysis, as would the effects of a failed high out of range AOA sensor. The EWIS requirements in 25.1707 introduced in 2007 are a red-herring in this case. While that rule is more prescriptive, 25.1309(b) effectively imposed the same requirement for adequate wire separation to prevent catastrophic events from wiring faults, and that requirement was identical for both the NG and Max programs.