WASHINGTON >> Boeing said today it plans to make design changes to prevent a future mid-air cabin panel blowout like the one in an Alaska Airlines 737 MAX 9 flight in January that spun the planemaker into its second major crisis in recent years.

The National Transportation Safety Board and Boeing said officials still have not determined who removed and reinstalled that plane’s door plug during production.

NTSB completed the first of two days of hearings today that lasted nearly 10 hours into the mid-air emergency that badly damaged Boeing’s reputation, led to the MAX 9 grounding for two weeks, a ban by the Federal Aviation Administration on expanding production, a criminal investigation and the departure of several key executives.

Investigators have said the door plug in the new Alaska MAX 9 was missing four key bolts.

Boeing, which has vowed to make key quality improvement, faced extensive questions about the production of the accident MAX 9 and lack of paperwork documenting the removal of the door plug.

NTSB Chair Jennifer Homendy today criticized the planemaker’s safety culture, asking why it had not made improvements earlier and said it must takes steps to improve. “The safety culture needs a lot of work,” Homendy said.

Boeing’s senior vice president for quality Elizabeth Lund said the planemaker is working on design changes that it hopes to implement within the year and then to retrofit across the fleet.

“They are working on some design changes that will allow the door plug to not be closed if there’s any issue until it’s firmly secured,” Lund said.

Lund said two Boeing employees who were likely involved in the opening of the door plug have been placed on paid administrative leave.

The board also released 3,800 pages of factual reports and interviews from the ongoing investigation.

Boeing has said no paperwork exists to document the removal of four key missing bolts. Lund said Boeing has now put a bright blue and yellow sign on the door plug when it arrives at the factory that says in big letters: “Do not open” and adds a redundancy “to ensure that the plug is not inadvertently opened.”

A flight attendant described a moment of terror when the door plug blew out. “And then, just all of a sudden, there was just a really loud bang and lots of whooshing air, like the door burst open,” the flight attendant said. “Masks came down, I saw the galley curtain get sucked towards the cabin.”

Doug Ackerman, vice president of supplier quality for Boeing said Boeing has 1,200 active suppliers for its commercial airplanes and 200 supplier quality auditors.

Lund said today Boeing is still building “in the 20s” for monthly MAX production – far fewer MAXs than the 38 per month it is allowed to produce. “We are working our way back up. But at one point I think we were as low as eight,” Lund told the NTSB.

Last month, Boeing agreed to buy back Spirit AeroSystems , whose core plants it spun off in 2005, for $4.7 billion in stock.

The hearings are reviewing key issues, including 737 manufacturing and inspections, safety management and quality management systems, FAA oversight, and issues surrounding the opening and closing of the door plug.

FUSELAGE DEFECTS

Jonathan Arnold, Aviation Safety Inspector at the FAA, said a systemic issue he witnessed at Boeing’s factory was employees not following the instructions.

“That seems to be systemic where they deviate from their instructions. And typically, tool control is what I see most,” Arnold said.

Lund said before the Jan. 5 accident, every 737 fuselage delivered to Boeing by Spirit AeroSystems had defects. “What we don’t want is the really big defects that are impactful to the production system,” Lund said. “We were starting to see more and more of those kinds of issues, I will tell you, right around the time of the accident.”

Homendy at one point expressed frustration with Boeing. “The safety culture needs a lot of work (at Boeing),” she said. “There’s not a lot of trust, there’s a lot of distrust within the workforce.”

Boeing executive Carole Murray described various problems with fuselages coming from Spirit AeroSystems in the run-up to the accident. “We had defects. Sealant was one of our biggest defects that we had write-ups on,” she said. “We had multiple escapements around the window frame, skin defects.”

Michelle Delgado, a structures mechanic who worked as a contractor at Boeing and did the rework on the Alaska MAX 9 aircraft, told NTSB the workload is heavy and requires working long hours.

“In order for me to not have to deal with a worse situation tomorrow, I’d rather work a 12 to 13-hour shift to get it all done, for my sake, so I don’t have to deal with people the next day,” he told NTSB.

Also in June, the NTSB said Boeing violated investigation rules when Lund provided non-public information to media and speculated about possible causes.

Last month, Boeing agreed to plead guilty to a criminal fraud conspiracy charge and pay a fine of at least $243.6 million to resolve a Justice Department investigation into two 737 MAX fatal crashes.