Boeing announced on Tuesday that it was planning to introduce design modifications to prevent incidents such as mid-air cabin panel blowouts.
In January, the door plug of an Alaska Airlines 737 Max 9 blew off in flight, marking one of the company’s most significant crises.
The plane maker is working to introduce changes within a year and they will be introduced across the fleet, said Elizabeth Lund, Boeing's senior vice president for quality.
“They are working on some design changes that will allow the door plug to not be closed, if there is any issue, until it is firmly secured,” Ms Lund said during a National Transportation Safety Board hearing in Washington.
The US agency began a two-day hearing to investigate the cause behind the loss of the panel on the nearly new Boeing 737 Max 9 aircraft during a flight earlier this year.
The board has released 3,800 pages of information, including testimonies and interviews, on the accident.
It occurred on January 5, six minutes after the Alaska Airlines plane took off from Portland, Oregon, while flying at an altitude of about 4,800 metres.
No passengers or crew members were injured, and the plane safely returned to Portland. No one was seated next to the door plug.
Video from the authority showed a hole where the door panel had dislodged.
The incident forced airlines to re-examine plans to expand their fleets amid slower production and delayed deliveries.
The US Federal Aviation Administration also imposed curbs on Boeing 737 Max production after the incident and there have been investigations started into the safety and quality standards in the company's production process.
Boeing also faced questions about why it had not been using warning systems for door plugs, such as those found on standard doors, that issue alerts when doors are not fully secured.
Ms Lund said Boeing has introduced a bright blue and yellow sign on the door plug when it arrives at the factory, clearly marked with “do not open” in large letters.
She said that before the January incident, every 737 fuselage delivered to Boeing had defects.
“What we don't want is the really big defects that are impactful to the production system," Ms 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."
She said Boeing was producing Max planes in the twenties each month, fewer than the 38 it is allowed to make.
“We are working our way back up, but at one point I think we were as low as eight,” Ms Lund told the board.
Working like 'cockroaches' at Boeing plant
Workers involved with the Boeing aircraft that lost the door plug said they were under immense pressure to work quickly, which could lead to mistakes, testimony revealed on Tuesday.
“As far as the workload, I feel like we were trying to put out too much product, right … that’s how mistakes are made,” one of the Boeing workers said in the testimony.
"People try to work too fast. I mean, I can’t speak for anybody else, but we were busy. We were working a lot."
One worker mentioned that there was no specialised training provided for handling door plugs, as opposed to regular doors.
The fuselage of the plane was made by Boeing supplier Spirit AeroSystems, and it came with the four bolts in place, according to a testimony at the hearing.
But there were issues with the bolts near the door plug that required its removal to complete the work. Ms Lund said two Boeing employees potentially responsible for the opening of the door plug have been placed on paid administrative leave.
In February, the board reported that the door plug that came off appeared to be missing four key bolts. However, in March, Boeing said there were no documents to prove the removal of four missing bolts.
Communication between the Boeing and Spirit workers at the Boeing plant was poor, according to the testimony.
“Well, basically we are the cockroaches of the factory,” a Spirit employee told the board.
Last month, Boeing entered a definitive agreement to acquire supplier Spirit in an all-stock deal at an equity value of $4.7 billion after months of negotiations.
A flight attendant described the moment of terror when the door plug blew out.
“Just all of a sudden, there was just a really loud bang and lots of whooshing air, like the door burst open," the attendant told the inquiry.
"Masks came down. I saw the galley curtain get sucked towards the cabin."
A Boeing team leader told the agency that employees were not happy with the dismal work conditions and there was high staff turnover.
“This can be a stressful job … what the company wants and what we have the skills and capabilities to perform at the time, sometimes that doesn’t coincide," the leader said.
"And so, some people get disgruntled. They feel like they are being overworked.”
Not a 'PR campaign' for Boeing
Boeing develops, makes and services commercial aircraft, defence products and space systems for customers in more than 150 countries.
Frustrated with hearing latest advances instead of concrete reasons for the January incident, the board's chairwoman, Jennifer Homendy, said: “This is not a PR campaign for Boeing.
“What is very confusing for a lot of people who are watching, who are listening, is what was going on then. This is an investigation on what happened on January 5.”
She also raised concerns over Boeing's safety culture.
"The safety culture needs a lot of work … there is not a lot of trust [at Boeing], there is a lot of distrust within the workforce," Ms Homendy said.
Boeing has “a long way to go … just based on what I have looked at … on safety culture”, she added.
“Why does it take a serious tragedy, which could have been so much more serious, for change to occur?”
Boeing has also encouraged employees to submit safety concerns through an internal programme called “Speak Up”.
The company's share price closed 1.19 per cent down at $165.04 a share on Tuesday, giving the company a market capitalisation of $106.69 billion.
The stock is down 34.45 per cent since the start of the year.
The company also had a management shake-up in March amid heavy scrutiny from US regulators, with production scaled back as Boeing addressed safety and quality concerns.
Last month, it appointed aviation industry veteran and former Rockwell Collins boss Kelly Ortberg as its president and chief executive to turn around the troubled plane maker.
It reported a net loss of $1.4 billion between April and June, compared to a loss of $149 million in the same quarter last year. The company racked up debt of $57.9 billion at the end of June quarter, due to the issuance of new debt.
