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

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United Airlines has found loose bolts and other parts on 737 Max 9 plug doors as it inspects its fleet of Boeing jets following the Friday rapid depressurization aboard an Alaska Airlines jet of the same make, according to three people familiar with the findings.

The discrepant bolts and other parts on the plug doors have been found on at least five aircraft, one of the people told The Air Current.

A Boeing representative did not immediately respond to a request for comment. A spokeswoman for the Federal Aviation Administration declined to comment on the United findings, and pointed to the agency’s earlier statements.

United confirmed the findings in a statement, saying, “Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug – for example, bolts that needed additional tightening. These findings will be remedied by our Tech Ops team to safely return the aircraft to service.”

The findings aboard the five United aircraft will likely significantly widen the fall-out from the grounding, intensifying the focus on Boeing and Spirit AeroSystems. The pair together is responsible for the assembly, installation and quality checks of the aircraft structure.

Later Monday, Alaska Airlines confirmed it, too, had found issues with the plug doors on its grounded jets. “As our maintenance technicians began preparing our 737-9 Max fleet for inspections, they accessed the area in question. Initial reports from our technicians indicate some loose hardware was visible on some aircraft,” the company wrote.

Related: Inside the strained union of Boeing and Spirit AeroSystems

Across the five United aircraft, there is little consistency in the locations of the errant parts, according to documents reviewed by The Air Current. In one instance, United found that the bolts that affix the lower hinge of the plug door were not fully seated, and that the washers on the bolts could “spin”. 

Another aircraft was found to have loose bolts on the upper forward guide fitting on the plug and another on the forward guide roller that is an attachment point to the fuselage on the door frame. Additionally, screws that are part of the lower hinge bracket at the bottom of the plug on another aircraft were found to not be fully screwed in.

The five aircraft were delivered to United between November 2022 and September 2023, according to ch-aviation, and would likely not have been through a heavy maintenance C check that occurs every 4,000 to 6,000 hours or two to three years.

United is the only other U.S. airline to fly the 737 Max 9 and is the largest operator with 79 in its fleet. Alaska operates 65.

Boeing earlier Monday confirmed that it had formally distributed a Multi-Operator Message (MOM) to 737 Max 9 operators, prescribing the specific inspection criteria that would satisfy the emergency airworthiness directive issued by the FAA on January 6, a day after the Alaska accident.

Alaska on Monday morning said that before inspections could begin, the FAA would need to approve operators’ inspection processes to ensure compliance, and that the airline would need to develop detailed inspection instructions and processes for its maintenance technicians to follow. Alaska said that it canceled roughly 140 flights for Monday due to the 737 Max 9 grounding.

NTSB investigators announced late Sunday that they had recovered the plug that violently separated from the Alaska flight at 16,000 feet. No one was hurt in the accident. 

The board has not released any specific findings related to the door or said whether the accident was in any way caused by discrepancies in the plug installation.

Elan Head and Howard Slutsken also contributed to this article.

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