Very little is known about the "potential electrical issue involving a component of the electrical power system, that requres sufficient ground path" , as advised to 16 Boeing 737 Max customers by Boeing Today .
Last year , amid B737 Max Re-certification exercise , Boeing realized the mistake during its intensive review of all the Max systems after the jet was grounded following the two Max crashes.
Reluctant to do the modification, Boeing showed the safety history of B737 NG aircrafts with similar wiring designs, but FAA was not convinced and asked Boeing to demonstrate the wiring standard compliance.
Similar wiring design in the B737 NG model was certified in the year 1997, so the EWIS (Electrical Wiring Interconnection System) regulation that came in the year 2007 does not apply.
Boeing B737 Max certification happened in March 2017 , hence EWIS regulation applies this time, but unfortunately Both Boeing engineers and FAA officials missed the implementation of this EWIS regulation, only to discover it later, during the re-evalution process due to world wide grounding caused by the crashes.
Electrical Wiring Interconnection System (EWIS) Requirements on transport category airplanes is in accordance with 14 CFR part 25, subpart H, sections 25.1701 through 25.1733 and sections H25.4 and H25.5 of Appendix H to part 2(14 CFR 25.1707).
During the FAA’s re-review of the B737 MAX horizontal stabilizer control system , it was revealed that the physical separation of the horizontal stabilizer trim-arm wiring and the horizontal stabilizer trimcontrol wiring does not meet the criteria specified in 14 CFR 25.1707.
Picture Source - FAA ( 12 areas of re-routing of wiring )
Certain wiring installations must have enough physical separation so that a wiring failure cannot create a hazard. (14 CFR 25.1707).
Since design changes must comply with FAA regulations, the FAA required changes to the wiring installation to meet the required physical separation between the horizontal stabilizer trim-arm wiring and the horizontal stabilizer trim-control wiring.
Re-routing of the stabilizer trim-arm wiring was made mandatory in 12 areas of the aircraft’s Electrical Equipment bay and Section 48 to prevent a potential simultaneous short circuit between the stabilizer arm and control wiring and another 28VDC wire.
So, the Re-certification added a clause for the 737 MAX design change, that includes physical separation of existing wires and/or routing of new wires to ensure compliance with the regulation.
.jpg)
