During the exhaustive review of the 737 MAX systems demanded by safety regulators, Boeing has discovered a potential new wiring-design problem that could add delay to the company’s target of returning the jet to service around March.
And a separate manufacturing issue affecting MAXs built over the past year will require hours of repair work on a large number of jets to ensure the engines are fully protected from a lightning strike before they can fly again.
The impact of the potential design problem inside some wiring bundles is still unknown.
According to an insider familiar with the details, Boeing engineers discovered “a theoretical possibility” of an electrical short circuit in wires connected to the jet’s moveable horizontal tail — known as the stabilizer — that could cause the tail to swivel uncommanded by the pilot, pushing the nose down.
Although this potential fault in the wiring bundles is unrelated to the flight-control system that went wrong on the two crash flights in Indonesia and Ethiopia, Boeing said that as part of “a robust and thorough certification process to ensure a safe and compliant design,” it is analyzing the risk that it could produce a similar outcome to the crash scenarios.
“We identified this wire bundles issue as part of that rigorous process, and we are working with the FAA [Federal Aviation Administration] to perform the appropriate analysis,” said Boeing spokesman Gordon Johndroe. “It would be premature to speculate as to whether this analysis will lead to any design changes.”
The Boeing insider said it’s not yet clear if this is a real concern.
“Our current understanding is that analysis may show that the theoretical fault cannot occur in the specific way required, and that other protections already in place – ranging from shielding to insulation to circuit breakers – would prevent it from being possible,” the insider said.
He added that company engineers are currently working to complete their analysis of the issue. Boeing is sharing its findings with the FAA and figuring out what if anything must be done to address it.
Both the wiring issue and the lightning vulnerability were first reported Sunday by The New York Times.
Exploring all potential faults
During the review, Boeing’s engineering team identified a theoretical scenario in which three wires routed close together might cause an electrical short that could result in “a high-speed, continuous, horizontal stabilizer runaway,” the insider said.
During the original certification of the MAX, Boeing’s system safety assessment classified such a “runaway stabilizer”—in which the horizontal tail swivels without pilot input to push down the nose of the jet—as a “major” hazard, meaning a flight upset that would likely cause only minor injuries to those on board.
Given the outcome of the two crashes, when both flight crews failed to cope with a runaway stabilizer, the event was reassessed last summer as “catastrophic” — two hazard levels higher, signifying a risk of losing the plane. A catastrophic hazard classification requires a design such that no single failure could trigger the event. That led to re-examining every possible system failure and the discovery of the potential electrical short.
This fault is not related to the flawed flight control software—the Maneuvering Characteristics Augmentation System (MCAS)—that contributed to the MAX crashes.
If further analysis confirms that the wiring is a real concern, the fix would require additional separation of a short segment of two wire bundles, the Boeing insider said.
Adding more wire separation toward the tail of the airplane is difficult, because the structure narrows there and there is little extra room for maneuver.
Even as Boeing engineers try to figure out if the failure could really happen, they are also working to design the separation.
Manufacturing flaw left MAXs vulnerable to lightning
The separate MAX issue related to lightning protection is not a design but a manufacturing mistake.
On any jet aircraft, the two areas that draw the most lightning strikes are the nose of the airplane and the pods surrounding the engines that thrust out ahead of the wing. On the 737, the engine pods and the pylons that hold them to the wing, which are largely made of nonconducting carbon composite, have a metal foil just beneath the surface to safely disperse the current from such a strike.
Boeing said in a statement Monday that an incorrect manufacturing procedure used on some MAXs damaged the protective metal foil on two panels covering the engine pylons.
“On some airplanes built between February 2018 and June 2019, the protective foil inside the composite panels may have gaps,” Boeing said.
The pylons, or struts, holding the engines are covered on top by a couple of composite panels, resembling an elegant thumbnail resting on top of the engine pod. Boeing said operators of the affected jets will need to replace the two panels with new ones provided by the manufacturer.
Boeing is also asking all MAX operators to apply a sealant to establish a required electrical bond path, which connects metallic components to ensure that lightning-generated electricity can flow through without a gap that would risk creating a spark. Boeing will provide the parts to ensure the bond path works as intended, the statement said.
A person with knowledge of the details said replacement of the panels with the damaged protective foil could be accomplished in about five hours, while the work to establish the electrical bond path will take approximately 20 hours.
All affected MAXs will have this done before they are allowed to fly again, the person said.
People close to the FAA process say that late February or early March remains the earliest that the agency could clear the MAX to fly again. But a federal government official cautioned that as the meticulous audit of the MAX systems continues, these two new issues may not be the last to be discovered.
“There’s no guarantee they won’t find other issues that need to be addressed,” the official said.