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Undamaged plug exit on Alaska Max 9 had fasteners tightened during assembly

During assembly, Boeing personnel tightened six loose fasteners on a key structure around the right-hand plug exit directly across from the failed exit.

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Boeing personnel tightened six loose fasteners on the fuselage structure around the plug exit opposite the one that was later violently expelled from an Alaska Airlines 737 Max 9 at altitude last Friday, The Air Current has learned.

The work took place at Boeing’s Renton, Washington factory after the fuselage was shipped by rail from supplier Spirit AeroSystems’ factory in Wichita, Kansas, which supplies 70% of the 737’s parts to Boeing.

While the significance of this factory fix on the unaffected right hand door plug aboard the accident aircraft isn’t entirely clear, it confirms that Boeing staff gave direct attention to discrepant components on at least one side of the Alaska Airlines jet, N704AL, while it was being assembled.

Related: Essential reading to understand Boeing, Spirit AeroSystems and the 737 Max 9 crisis

The Federal Aviation Administration on Thursday morning said it had formally notified the plane maker that it had launched an investigation into the company to “determine if Boeing failed to ensure completed products conformed to its approved design and were in a condition for safe operation in compliance with FAA regulations.”

“The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service,” the U.S. aviation regulator said in a statement.

Boeing in a statement said, “We will cooperate fully and transparently with the FAA and the NTSB on their investigations.”

For the purpose of this report, TAC is deliberately using the term bolts and fasteners separately to delineate the location and purpose of the components we are discussing. In aircraft manufacturing, they are often used interchangeably.

Related: Inside the strained union of Boeing and Spirit AeroSystems

The loose fasteners were tightened on components known as the guide roller plates, which are mounted on each side of the upper fuselage structure surrounding the plug. That guide roller plate interfaces directly with the guide roller tracks on the plug exit. 

Exactly to what extent the loose fasteners did not meet specifications is not known by The Air Current, however, the fasteners were considered a defect in manufacturing that was resolved as the aircraft advanced through Boeing’s normal assembly process. It is not clear what work, if any, was done on the left side plug.

Each guide roller plate has a total of four fasteners, and six of those eight fasteners across both plates on the right side plug frame on the fuselage were found to be loose before being tightened. The 63-lb. plug, which is typically only opened for maintenance, must be translated upward around two inches before it can be folded out. A locking bolt on the plug structure runs underneath each guide fitting to prevent the plug from being lifted. Two more arrestor bolts are secured at the bottom of the plug, for a total of four, each secured with a cotter pin.

The NTSB said that its preliminary examination of the missing left-hand plug exit, recovered in suburban Portland, showed that its guide roller tracks were fractured. The NTSB said it is not yet known when or how the guide tracks were fractured during the accident, but noted that the four bolts that restrain the plug from vertical movement were missing. “We don’t know if there were bolts there, or if they are just missing and departed when the door plug departed,” said NTSB Chair Jennifer Homendy.

Determining what happened to those bolts remains a central question for investigators.

Homendy noted the board on January 8 examined the right hand door plug still attached to the aircraft and investigators “found no discrepancies. Everything was in place.”

“Our team is working through the NTSB investigative process. Right now, we are focused on the collection of perishable evidence on-scene in Portland and elsewhere. During the investigation the team will carefully and methodically go through all the records related to the case and follow any leads as appropriate,” said NTSB spokesman Eric Weiss.

Representatives from Boeing, Alaska Airlines and Spirit AeroSystems declined to comment. Parties to an active NTSB investigation (among which Spirit is now included) are severely limited in their communication by internationally agreed upon rules that govern aviation safety.

In at least one instance, United Airlines observed similarly loose fasteners on the upper roller guide plate on a 737 Max 9 as part of its own preliminary inspections of its grounded fleet. The 171 737 Max 9 aircraft with plug exits remain grounded pending inspections that the FAA, Boeing and the airlines are finalizing. 

Related: United finds loose bolts on plug doors during 737 Max 9 inspections

TAC reported Monday that United had found at least five aircraft with discrepancies around their plug exits and by Wednesday that that figure had risen to roughly 15 findings across its 737 Max 9 fleet, including some aircraft with more than one, according to a person familiar with the inspections. Alaska, too, has found 737 Max 9s with issues.

The plug is typically not removed by Boeing machinists after it is installed unless there is a rigging issue, a knowledgeable person tells TAC, though quality checks to ensure correct adjustment around the plug take place in Boeing’s Renton factory. 

Boeing CEO David Calhoun during an interview on CNBC Wednesday acknowledged responsibility for the “quality escape” the company believes caused the accident. A quality escape is manufacturing jargon for when something wasn’t built or repaired correctly. The quality has “escaped” during the prescribed engineering or manufacturing process. 

“We’re not going to point fingers,” he added with respect to Spirit AeroSystems. “Because, yes, it escaped their factory; but then it escaped ours too. So, we’re all in this together.”

The fuselage for N704AL arrived in Renton for final assembly in late August, progressing through Boeing’s assembly process that stretched through September. Each aircraft, including the one involved in the incident, undergoes what is called a standard high-pressure test to verify the cabin can be safely pressurized and verifying the safety of its structure. The aircraft underwent two Boeing test flights and two additional customer acceptance test flights prior to its Oct. 31 delivery to Alaska.

Write to Jon Ostrower at

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