Aviation experts raise questions about 737 Max ‘door plug’ design

By Curt Devine, CNN

(CNN) — In the aftermath of last week’s Alaska Airlines in-flight emergency, some aviation experts are questioning the structural design of the section of the Boeing 737 Max 9 that blew off the plane.

On that January 5 flight, a “door plug” – a portion of the plane’s fuselage the manufacturer can put in place instead of an emergency exit door – detached from the plane and was later discovered in an Oregon backyard.

In interviews with CNN, some experts argued that if that door plug were designed to be larger than the opening it covers and installed inside the plane, the force of the pressurized air in the passenger cabin would force the plug against the plane’s interior frame and a situation such as the one on the Alaska Airlines flight could have been avoided. However, such a design could have added costs and practical disadvantages, some said.

“It doesn’t make sense to me why they would do it that way and not have it installed from the inside, where it literally cannot come out unless there is a structural failure in the airframe,” said David Soucie, a former FAA safety inspector and CNN analyst. “Historically, since we have had pressurized airplanes, emergency exits are designed to come inward… so why would they have not done the same thing with this plug?”

Soucie said the design of the door plug on the Max 9 may provide some advantages, such as making the plane more readily accessible for maintenance.

Robert Ditchey, an aviation consultant, agreed that if the plug were designed to be larger than the hole in the fuselage and installed inside the cabin, this incident likely would not have happened.

“It should have been installed from the inside to the outside, not the other way around,” Ditchey said of the plug, adding that the exterior installation raises the prospect that the bolts used to secure the plug could have failed.

“You could have missing bolts,” he said. “They could have had the wrong bolts, or they could have over-torqued the bolts thereby breaking them, or not torqued them enough.”

John Goglia, a former member of the National Transportation Safety Board, said that while a different design could theoretically prevent a problem like this from happening, he described this design as adequate so long as proper installation and quality controls are followed.

“Could you design a plug door for this airplane that would prevent it from going out, the answer is yes you could. It just is going to cost you money and time,” he said. “Was this design adequate is the real question, and if it was installed properly, we wouldn’t be talking about it.”

“There really is not a big issue with having this door in there, if it was secured right,” Goglia said.

A Boeing spokesperson declined to comment on the door-plug design, citing an active investigation by the National Transportation Safety Board. In a staff-wide meeting Tuesday, Boeing’s CEO attributed the incident to a “mistake,” which he did not identify. A company source told CNN that Boeing believes “the mistake in question” was introduced in the aircraft’s manufacturing supply chain.

Boeing CEO David Calhoun told CNBC in an interview that aired Wednesday that he is “confident” in the Federal Aviation Administration’s ongoing work with airlines to “inspect each and every one of the airplanes” and make “certain that they’re in conformance with our design, which is a proven design.”

Federal investigators have determined the components that may have been involved in the door plug coming loose, but they have not yet determined why it blew out, CNN reported Monday. The door plug is typically held in place by stop fittings and has a set of bolts that prevent the door from moving and potentially flying off the plane. Somehow, the plug on the Alaska Airlines flight moved, NTSB’s Clint Crookshanks explained at a news conference Monday night.

The FAA on Saturday ordered most Boeing 737 Max 9 aircraft to be temporarily grounded as the cause of the incident is investigated. The order applies to about 171 planes.

Former Transportation Inspector General Mary Schiavo, a CNN analyst, said door plugs have long been used in aviation and, in addition to the Max 9, have been used on planes converted to freighters. But she also questioned this particular design.

She said this door plug used on some Max 9s is “designed not to be one of these wedge-doors, that when pressurized it can’t be opened … it’s a door that has to be opened from the outside during some inspections.”

“That’s the part of the design that to me seems flawed,” Schiavo said.

Schiavo said she thinks Friday’s incident will lead to additional door-plug inspection protocols for airlines.

Separately, a federal lawsuit filed last year by investors in Spirit AeroSystems – the supplier that makes the fuselage of Boeing’s 737 Max jets – accused the company of “widespread and sustained quality failures” in its products.

The suit states that quality failures, which have allegedly ranged from debris in products to missing fasteners and peeling paint, led Boeing to place Spirit on probation from around 2018 to at least 2021. The suit, which was previously reported by the publication The Lever, does not specifically mention door plugs.

The suit claims that “constant quality failures resulted in part from Spirit’s culture which prioritized production numbers and short-term financial outcomes over product quality, and Spirit’s related failure to hire sufficient personnel to deliver quality products at the rates demanded by Spirit and its customers including Boeing.”

The suit further states that a former Spirit AeroSystems employee who worked as a quality manager and inspector and who was not named in the suit wrote an ethics complaint to the company in 2022 that described an “excessive amount of defects” in products. The former employee believes “Spirit treats moving products down the line as more important than quality,” according to the suit.

A spokesperson for Spirit AeroSystems, Joe Buccino, said in a statement that “Spirit strongly disagrees with the assertions made by plaintiffs in the amended complaint and intends to vigorously defend against the claims. Spirit will not comment further as to the pending litigation.”

A Boeing spokesperson declined comment on that suit.

In April, Spirit AeroSystems identified a production issue on the aft fuselage section of certain 737 models. “This is not an immediate safety of flight issue. We have processes in place to address these of types of production issues upon identification, which we are following,” the company then said in a statement.

In August, the company disclosed improper holes drilled on the “aft pressure bulkhead” on some models of the 737 fuselage. Both Spirit and Boeing said in statements that the issue was determined not to be an immediate flight-safety concern.

In December, Boeing asked airlines to inspect their 737 Max jets for a potential loose bolt in the rudder system after a potential problem was discovered. Boeing said a plane with a missing bolt was fixed but wanted all Max planes in service checked.

Boeing has faced scrutiny since two fatal crashes involving 737 Max 8’s– one in Indonesia in October 2018 and the other in Ethiopia in March 2019 – that killed 346 people. A safety system called MCAS used in those planes was linked to both crashes.

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