More than 100 of Boeing’s newer 737 MAX jets will soon return to the air, and new MAX deliveries will resume, after a five-week halt over a defect introduced by a minor design change in parts made at its Salt Lake City fabrication plant.

Boeing won approval from the Federal Aviation Administration late Wednesday for the fix to the jet’s latest electrical problem affecting MAXs built since early 2019, the federal safety agency confirmed.

The company said in a statement that “after gaining final approvals from the FAA, we have issued service bulletins for the affected fleet,” specifying modifications the airlines and Boeing must now make to return their airplanes to service.

The work on each plane will take no more than two to three days.

The problem arose because Boeing engineers based in the Puget Sound region adjusted the way two cabinets or racks that hold electronic equipment on the MAX flight deck are manufactured in Salt Lake.

In what the FAA later said was “a minor design change that did not require (regulatory) approval,” primer was added to the aluminum structure after holes were fully drilled, whereas previously the primer had been added before the final drilling.

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The primer coating on the interior of the racks — no longer pierced by the drilling of the holes — in some areas of the structure prevented full metal-to-metal contact, degrading the electrical grounding paths.

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This detail on the cause of the problem was first reported by the Wall Street Journal.

A former Boeing electrical engineer, who asked not to be named to protect his company pension, called this an “elementary mistake” that should have been flagged by engineering.

After its discovery in early April, the FAA ordered the affected MAXs grounded. Although no problems had surfaced in service, the electrical issue was deemed dangerous because it could potentially affect the operation of critical systems including engine ice protection.

That order affected 109 MAXs that have been delivered to airlines — including Alaska, American, Southwest and United — as well as about 350 still awaiting delivery.

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In investigating the origin of the problem, the FAA said it will also audit “Boeing’s process for making minor design changes across its product line.”

A person at Southwest working on implementing Boeing’s fix at that carrier detailed the manufacturing solution for the jets already built.

Inside the electronics rack, Boeing is adding multiple bonding jumpers (insulated wires) and metal bonding straps secured with bonding studs (screws that provide a clear path through bare metal) that seal connections between each shelf to the wall around it and from the bottom of the cabinet to the aircraft floor.

The person at Southwest called this “pretty straightforward.”

“They had a problem with electrical grounding and they added some grounding wires,” he said. “You’ve got a crew of two or three mechanics and it takes them a day or two to do it.”

Dwight Schaeffer, a retired Boeing senior manager who headed the company’s former avionics unit, said adding such grounding wires has become the standard way these electronics panels are designed.

More recently designed planes than the 737 “don’t use the rack as a ground path,” Schaeffer said. “They use these bonding straps instead.”

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Two people familiar with Boeing’s thinking said it is now considering what to do for future production of the MAX.

The person at Southwest, who spoke without permission from the airline, said that for future production planes, “they’re rethinking that design change and maybe go back to the way they used to do it.”

A latent problem emerges

The design change that caused the problem was introduced in early 2019, just months before MAXs were grounded worldwide after the second fatal crash in Ethiopia that investigators attributed largely to an entirely separate design flaw in the jet’s new flight control system.

The change to the electrical racks was so subtle that when they arrived from Salt Lake City to the Renton final assembly line for installation, they looked exactly as they had previously. Mechanics installed them and quality inspectors checked them as they had always done.

And the degradation of the electrical grounding didn’t show up immediately. The person at Southwest said the design change didn’t eliminate the necessary grounding path, just “made it not as good” as it should be.

Southwest had 34 MAXs flying at the time and on those planes “the electrical bond was good enough that we never had any symptoms,” he said. “The airplanes were working just fine.”

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With MAXs everywhere previously grounded for almost two years, no problems were noticed on any aircraft until early last month, when Boeing was preparing to deliver a MAX to Brazilian airline GOL.

During final functional testing on the ramp outside the Renton factory, a standby power control unit on the GOL airplane failed to operate. Boeing traced the problem to a disrupted electrical grounding path in the electronics panel right behind the first officer’s seat.

That panel opens up to a floor-to-ceiling aluminum cabinet with five shelves holding electronic boxes and circuit breakers. Further investigation revealed a similar lack of grounding in another such rack inside the main instrument panel in front of the pilots. Both had to be fixed.

It’s critical that all the electrical bonds in an airplane provide a continuous grounded electrical pathway through the metal airframe.

This ensures that any buildup of static electricity or a lightning strike will safely dissipate through the jet’s wingtips or tail. If a lightning strike were to travel through an airplane and hit an improperly grounded component, it could explode or start a fire.

Difficulty in achieving proper grounding of components has been an issue previously at Boeing.

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In 2018, an internal quality-control audit that monitored 1,200 “Bond and Ground” jobs on the 747, 767 and 777 airplane programs in Everett found that only 93 percent of the sampled jobs were properly performed, failing to meet the prescribed engineering target.

The FAA investigated that result and said Thursday it subsequently monitored the progress of Boeing’s corrective action at the Everett plant.

Scrambling for a fix

Preliminary information about the latest electrical grounding problem that was relayed to one airline last month suggested the problem arose after a switch from using rivets to another type of fastener in manufacturing the panels, a detail The Seattle Times reported after confirming with a person at Boeing.

However, this information proved incorrect. A different source who works on the panels clarified that they are still riveted together — a detail now corroborated by Boeing. The real issue was applying the primer after drilling the holes.

In the ensuing weeks, Boeing struggled to nail down the details of a precise solution. According to two people familiar with the course of events, after Boeing had already submitted a proposed fix to the FAA, it contacted the agency last Friday to ask it to hold for an adjustment.

Engineers intent on ensuring redundancy had determined that if one specific bonding strap broke, another strap that shared the grounding pathway wouldn’t be able to carry the full load. “They just made that strap bigger. They basically added a double strap to it,” said the person at Southwest.

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The FAA approved the precise modifications needed to fix the main instrument panel on Tuesday and those for the panel behind the first officer’s seat on Wednesday.

The renewed grounding of the MAX has vexed Boeing’s customers, killing the momentum from the jet’s return to service in December just as air travel domestically has begun to recover.

“It’s not how I thought 2021 was gonna go,” said the person at Southwest. “The timing’s unfortunate but, you know, it’s a problem, it’s been identified, we’ll fix it, and it’ll be fine.”

“Southwest has a summer schedule change on June 6,” he said. “We still have a hope that we’re going to get all our airplanes back to support that summer schedule.”

Correction: A previous version of this story misstated the location of the engineers responsible for the minor design change that caused the electrical grounding problem. They are based in the Puget Sound region.