Boeing reinstalled the panel that later blew out of the 737 Max Jet

Nearly three weeks after a hole blew open in a Boeing 737 Max 9 during an Alaska Airlines flight, terrifying passengers, new details about the plane's production are intensifying investigations into Boeing's quality control practices.

About a month before the Max 9 was delivered to Alaska Airlines in October, workers at Boeing's plant in Renton, Washington, opened the panel that would blow off the plane's body and later reinstalled it, according to a person familiar with the case.

The workers opened the panel, known as a door plug, because work needed to be done on the rivets – which are often used to connect and secure parts in aircraft – said the person, who requested anonymity because the person is not authorized to speak publicly while the National Transportation Safety Board conducts an investigation.

The request to open the plug came from employees of Spirit AeroSystems, a supplier that makes the bodywork for the 737 Max in Wichita, Kan. After Boeing employees complied, Spirit employees stationed at Boeing's Renton plant repaired the rivets. Boeing employees then reinstalled the door.

An internal system that tracks maintenance work at the facility, where 737s are assembled, shows the request for maintenance but does not include information on whether the door plug was inspected after it was replaced, the person said.

The details could answer a crucial question about why the door plug came loose on Flight 1282 at 16,000 feet, forcing the pilots to make an emergency landing at Portland International Airport in Oregon minutes after takeoff on Jan. 5. where an emergency exit door would be if an airplane had more seats. To stay in place, the plug relies mainly on a pair of bolts on the top and another pair on the bottom, as well as metal pins and pads on the sides.

The Seattle Times reported earlier on Wednesday that Boeing had removed and reinstalled the door plug.

The FAA on Wednesday approved detailed instructions on how airlines should inspect the door plugs of about 170 grounded planes. The instructions tell airlines to retorque the fasteners on the door plug, check the plug's bolts and fittings, and repair any damage. Airlines can resume flying the jets after inspections are completed.

Also on Wednesday, Boeing CEO Dave Calhoun met privately with lawmakers in Congress on Wednesday. It was the second time in recent years that the company and its leaders had to answer for serious problems with its aircraft. In 2018 and 2019, two crashes involving the 737 Max 8 killed 346 people.

“The American flying public and Boeing line workers deserve a leadership culture at Boeing that puts safety over profit,” Washington state Sen. Maria Cantwell, the Democratic chair of the Commerce, Science and Transportation Committee, said Wednesday. She added that she would hold hearings “to investigate the root causes of these safety deficiencies.”

How the panel came to be installed at Boeing's factory will almost certainly be a focus of federal investigation. In addition to the NTSB, the FAA is investigating the incident and production practices at Boeing and Spirit.

Citing the open NTSB investigation, Boeing referred questions to the agency, which declined to comment. The FAA did not immediately respond to a request for comment on Boeing's handling of the door plug. A spokesperson for Spirit AeroSystems said the company “remained focused on the quality of every aircraft structure that leaves our facilities.”

John Cox, a former pilot for a safety consulting firm, said the new information about the door plug, if correct, would be an indication of “process failure” and raise questions about Boeing's entire production operation.

“Are there similar problems in areas other than the door?” he said. “You have to look at the entire assembly process.”

The FAA said Wednesday that it would not allow Boeing to expand production of new planes in the 737 Max series, a linchpin of the company's commercial aircraft business, until the agency was satisfied that quality control had improved.

Mr Calhoun suggested this month that a manufacturing error was responsible for the door plug burning out. But it was not clear whether the error, which Mr. Calhoun called a “quality lapse,” occurred at Boeing's Renton plant or at Spirit's Wichita plant, where the door plug was first installed.

The incident has raised new concerns about Boeing's quality control among investors, airline executives, pilots, passengers and others, in addition to regulators. Boeing's share price has fallen 14 percent since the hit.

In recent days, several airline executives have sharply criticized the company, a major supplier they rarely complain about publicly.

“I'm angry,” says Alaska Airlines CEO Ben Minicucci told NBC News Tuesday, adding that the airline found loose bolts on “many” of its Max 9s. “My question to Boeing is what they are going to do to improve their quality programs internally.”

Scott Kirby, CEO of United Airlines, told CNBC Tuesday that “the grounding of the Max 9 is probably the straw that broke the camel's back for us.” He also said he was concerned that Boeing would not be able to quickly deliver another 737 Max plane the airline had ordered, the Max 10. That model has not yet been certified by the FAA

“We're going to at least build a plan that doesn't include the Max 10,” Mr. Kirby said.

For now, Boeing remains in damage control mode. Mr Calhoun last week visited the Spirit AeroSystems factory – a factory that the aircraft manufacturer sold in 2005. And Boeing said this week that it planned to hold a “quality standdown” on Thursday, during which production, delivery and support teams would stop work to attend quality learning sessions.

The company said it planned to implement similar pauses at all of its commercial aircraft factories and manufacturing sites in the coming weeks.

James Glanz, Santul Nerkar And Bernhard Warner reporting contributed.