In the wake of the electrical grounding problem that resulted in a five-week halt in deliveries of 737 MAXs and grounded more than 100 planes in service, Boeing technical leaders last week reminded engineers that design changes must be reviewed for their potential electrical impact.
An internal memo notified design engineers of two potential safety pitfalls when they introduce a new automated machining process that can inadvertently interfere with electrical grounding of components on airplanes.
“As many of you might have heard, we have recently discovered concerns with designs that leverage precision machining processes,” the memo stated.
Boeing said in a statement that the memo was sent to engineers “to reinforce that they have design guidance to prevent certain quality issues before they occur.”
On any airplane, it’s critical to provide a continuous grounded electrical pathway, so a lightning strike can safely travel through the metal airframe and dissipate through the wingtips or tail. If a lightning strike were to hit an improperly grounded component, it could explode or start a fire.
One mistake the memo warns engineers to avoid when making changes to a manufacturing process involves switching fasteners. The other bears on how a coat of primer is added to a metal structure. Mishandling of the latter issue led to the electrical problem on the MAX.
The memo points engineers to a Boeing Design Manual released March 18 specifically informing them that “requirements for electrical bonding and grounding will require a review by the appropriate program engineering functions.”
That manual was published three weeks before Boeing told airlines and the Federal Aviation Administration about the MAX problem in early April.
Boeing says the manual’s cautionary notification is unrelated to what happened on the MAX and the timing is coincidental.
A new production process
Last week’s memo was sent out by Patty Chang-Chien, Boeing’s chief engineer for electronic and electrical engineering, and Steve Chisholm, chief engineer for mechanical and structural engineering, to all engineers in their units as well as those in production engineering.
It relates to an automated manufacturing process — called Full Size Determinant Assembly — used widely in various industries to assemble structures without expensive holding fixtures, making production more efficient and cheaper.
In a more traditional process, a Boeing mechanic manually drills small pilot holes and slots the parts into a holding fixture. Once the parts are lined up and held rigidly in place, full-size holes are drilled.
This method then requires a time-consuming process of taking all the parts out of the fixture and checking that the holes are clean and properly sized, then reassembling the structure before finally fastening it together.
The key to Full Size Determinant Assembly is that automated machines drill clean, precise, “full-size” holes in advance of assembling the structure, and these holes then line up easily for insertion of fasteners without the need to take it apart.
The memo by Chang-Chien and Chisholm says this method “can be a powerful enabler for production efficiency.”
But engineers need to be careful in changing any manufacturing process to ensure proper metal-to-metal contact in the fastener holes to protect the necessary electrical grounding paths.
George Bullen, a leading industry expert on automation and advanced airframe manufacturing technology, said engineers should certainly know that. “It’s common throughout the industry that there has to be a grounding capability.”
He said a design engineer will lay out a specification that essentially says “I want that hole clean so that I’m grounded.” It’s then up to a manufacturing engineer designing the process to ensure that happens.
Warning engineers to pay attention
The first pitfall outlined in the memo arises if a switch is made from using rivets to less tightly fitting fasteners called “clearance fit fasteners.”
When rivets are squeezed in the fastening process, they expand to fill the hole and create good metal-to-metal contact. But with a clearance fit fastener, Bullen said there’s “a risk it will go in the hole and not seat completely and not firmly contact the sides of the hole.”
Boeing’s memo warns engineers that “allowing clearance fit fasteners in an application that previously utilized rivets that expand to fill fastener holes … may disrupt an electrical load path.”
The second pitfall with Full Size Determinant Assembly lies in the fact that the process reorders the steps in fabrication of the structure: With the holes drilled first, a coat of primer or some other finish to protect against corrosion is added after the drilling rather than before.
“This could result in the possibility of finish inside fastener holes,” the memo states. This again may prevent full metal-to-metal contact and “undermine the integrity of the intended electrical load path.”
After an early 2019 design change to make fabrication of that rack more efficient using Full Size Determinant Assembly, primer was put on after the holes were drilled — without a check that the electrical grounding was still secure.
Engineers discovered the problem when electrical glitches showed up on a MAX undergoing tests before delivery to Brazilian carrier GOL earlier this year. There had been no prior report of anything similar on previous planes. It took time to figure out the root cause.
Investigating such a quality issue and determining if it’s a safety concern “is exacting and time consuming,” Boeing said. “We give the technical teams the time they need to get it right.”
A total of 109 MAXs delivered after the design change were grounded for five weeks and Boeing stopped all new MAX deliveries for the same period until the FAA approved a fix that added multiple grounding wires to the electronics rack.
An FAA spokesperson said the federal safety agency “is looking into how Boeing made the design change and incorporated it into the assembly line.”
Last week’s Boeing memo warns engineers that either switching fasteners or changing the timing of applying primer “requires detailed review and careful consideration.”
All such changes, it states, must be approved by specialist electrical design engineers.
As part of its response to the two fatal crashes of the MAX that killed 346 people, Boeing last year began implementing a companywide “Safety Management System” — a formal, data-driven system to catch quality or design lapses and improve the safety of its products.
On December 9, the FAA issued an acceptance letter provisionally approving Boeing’s proposed safety system, with an initial performance evaluation scheduled for this summer.