Boeing To Rewire All MAXes


The Seattle Times is reporting that Boeing will rewire the stabilator control systems and avionics bays of all 800 Boeing 737 MAX aircraft rather than battle the FAA for an exemption. During the post-grounding review of the aircraft, Boeing discovered that the wire bundle going to the motor that moves the stabilator didn’t have the required separation between low-voltage and high-voltage wires. The fear is that a so-called “hot short” in the higher voltage wire could send spurious signals to the motor over the low-voltage wires causing it to move the stabilator. It was subsequently discovered that noncompliant wiring was located in other areas of the aircraft. 

Boeing argued that the same wiring setup is used on previous model 737s, which have amassed 205 million hours without any issues. The NG models were certified before the current wiring standards were mandated. The FAA set the new standards after wiring problems were cited in the loss of a TWA Boeing 747 and a Swissair MD-11 in the late 1990s and wants the new standards met. It issued a statement last week that Boeing “must demonstrate compliance with all certification standards.”

Boeing threw in the towel over the weekend and will start with the aircraft it has in storage in Seattle and Moses Lake, Washington. Those that have already been delivered may be fixed when they get their new flight control software and are being readied for their return to service. That might cause some delays because the wiring rerouting will take about five days per airplane. In addition to the work in the tail, avionics boxes and shelving under the cabin have to be removed at 10 to 12 locations. “We will cap and stow a wire and reroute a new wire separated from the power wire,” an unnamed source told the Times.

The FAA has not yet issued an AD for the fix but it will most likely give operators a period of time to comply. The Times says some airlines may insist the wiring be fixed before they fly the planes again.

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    • It will affect safety. Negatively. A wire bundle that has been reworked in the aircraft is always less reliable than the original one that was built up on a loom and installed as a unit.

      With hundreds of millions of experience hours showing no failures, the FAA should have left this one alone. They had the data to support that. But they are choosing legal compliance over safety, because they don’t recognize the difference anymore.

      This action by the FAA is far more likely to cause a future incident than it is to prevent one, but as Scott says below, Boeing will get the blame when that happens so the FAA shrugs. The FAA isn’t concerned about safety, only compliance.

      • “We will cap and stow a wire and reroute a new wire separated from the power wire,”

        David, I Agree 100%. The current wiring is a known entity, but the rewiring and relocation of equipment can introduces it’s own set of new problems. Anytime you jiggle the wiring you can also introduce new failure modes.

        • You mean, anytime something is moved, something else is displaced causing a malfunction. Especially when inept technicians are in the mix. Well, by my experience in the biz, you are correct. Here’s where the dog comes in.

  1. With 205 million hours of flight experience, it isn’t broke, so why “fix” it.
    The FAA must be very careful making changes to demonstrated reliable systems, as new errors are created with every change. Thus, an exception may have been the more sensible approach. If a flight problem arises from these modifications, Boeing will get blamed for incorrect implementation, not the FAA, yet its the FAA that could have been wiser to grant the waiver in the first place.

  2. Hard to feel sorry for Boeing on this but it is actions like this and in the past that drive people nuts about dealing with the FAA. Different FSDO’s still have different interpretations on rules. What passes as ok or meeting the rules could later be found by another inspector and time as not meeting the standard. Until there is some consistency and standardization on what meets the standard or certification rules, the FAA will continue to drive everyone nuts trying to figure out what will meet what standard at what time.