By David Shepardson

WASHINGTON (Reuters) -A door panel that flew off a Boeing 737 MAX 9 jet mid-flight on Jan. 5 appeared to be missing four key bolts, according to a preliminary report from U.S. investigators that provided the first official look into how the frightening mishap took shape.

Lawmakers and the flying public have demanded answers to what caused the panel to fly off a brand-new Alaska Airlines-operated jet, in what has turned into a full-blown safety and reputational crisis for Boeing.

“Whatever final conclusions are reached, Boeing is accountable for what happened. An event like this must not happen on an airplane that leaves our factory,” Boeing CEO Dave Calhoun said in a statement. “We simply must do better for our customers and their passengers.”

The U.S. Federal Aviation Administration (FAA) grounded 171 of the Boeing 737 MAX 9 planes after the incident, most operated by U.S. carriers United Airlines and Alaska Airlines, for inspections. Those planes were cleared to return to service in late January and nearly all are flying again.

The U.S. National Transportation Safety Board (NTSB) report released on Tuesday focused on how the panel – fitted into this MAX 9 model in place of an optional exit – could have detached from the plane. The plug is held down by four bolts and then secured by “stop fittings” at 12 different locations along the side of the plug and the door frame.

Representative Rick Larsen, the top Democrat on the committee overseeing the FAA, said the “failure to re-install bolts on a safety-critical component of this 737 MAX 9 aircraft is a serious error that signals larger quality control lapses that must be corrected.”

The plug was manufactured by Spirit AeroSystems, a former Boeing subsidiary. The part was produced at Spirit’s facilities in Malaysia and delivered to its Wichita, Kansas, facility in May 2023. It arrived at Boeing’s assembly plant in Renton, Washington on Aug. 31.

The report shows the panel had to be removed at Boeing’s factory before being reinstalled. The initial findings released on Tuesday include photo evidence the bolts required to hold the plug in place appeared to be missing.

The report found the panel was first removed to repair rivet damage logged by Boeing workers on Sept. 1, a day after the panel arrived in Renton. Investigators are still trying to determine what documentation was used to authorize the opening and closing of the plug during the rivet repair.

The report raises questions about who initially installed the bolts and why the door’s opening at Renton to correct the rivets was not properly documented, said U.S. aviation safety expert John Cox.

“When was the last time those bolts were installed? Did Spirit not install them and then when Boeing opened it the guys didn’t realize that they didn’t have the bolts? Or did Boeing not install them? That is something that I don’t think we have an answer for yet.”

Senator Tammy Duckworth, who chairs an aviation subcommittee, questioned the lack of paperwork about the removal or replacement of bolts.

“It is a series of problems,” Duckworth said in an interview. “What is going on between the maintenance and inspection teams… Holy cow – nobody noticed?”

Boeing said it had “implemented a control plan to ensure all 737-9 mid-exit door plugs are installed according to specifications.”

The panel was found in a backyard in a suburb of Portland, Oregon, but the NTSB did not recover the bolts. The agency also did extensive tests and analysis to determine if they had been present before the crash or had come undone during the incident, it said.

A photo in the report shows three visible locations where bolts are missing, with the fourth location covered by insulation.

“Photo documentation obtained from Boeing shows evidence of the left-hand MED plug closed with no retention hardware (bolts) in the three visible locations,” the report said. MED is short for “mid exit door.”

BOEING UNDER PRESSURE

The incident has prompted regulators and lawmakers to ratchet up oversight of the jet manufacturer. The FAA in late January barred Boeing from expanding production of its 737 MAX planes due to the quality issues. That means it can continue producing MAX jets at its current rate, but it cannot increase that rate.

“I certainly agree that the current system is not working, because it’s not delivering safe aircraft,” FAA Administrator Mike Whitaker told lawmakers on Tuesday. “So we have to make changes to that.”

Boeing’s Calhoun bowed to lawmaker pressure to drop a request for a temporary exemption from design rules for a different MAX model, and more hearings in Washington will be held, Senate Commerce Committee Chair Maria Cantwell said on Tuesday.

“The NTSB’s preliminary report on the Alaska Airlines Flight 1282 accident underscores how important quality assurance is from manufacturers and how important quality control inspections from both manufacturers and the FAA are to the safety process,” she said.

The FAA is about halfway through a six-week audit of 737 MAX manufacturing, which is looking at all elements of production at Boeing and fuselage production at its supplier Spirit.

Spirit will invest in autonomous technology to limit any defects in its production of 737 fuselages, CEO Patrick Shanahan said on Tuesday following the company’s earnings.

Boeing shares closed up 1% on Tuesday. The stock has lost more than 20% of its value since the beginning of the year. Spirit shares ended up 5%.

(Reporting by David Shepardson in Washington and Allison Lampert in Montreal; Editing by David Gaffen, Nick Zieminski, Matthew Lewis and Jamie Freed)

---