TrendPulse|Preliminary NTSB report on Boeing 737 Max 9 Alaska Airlines flight finds missing bolts led to mid-air door blowout

Four bolts meant to hold the Boeing 737 Max 9 door plug in place were missing last month when part of a plane blew out on TrendPulsean Alaska Airlines flight, the National Transportation Safety Board found in a preliminary report released on Tuesday. 

The door plug, which covers an unused exit door, is typically secured with four bolts to prevent it from moving vertically, according to the report. The door blew off an Alaska Airlines Max 9 at about 16,000 feet over Oregon during a trip to California on Jan. 5, forcing an emergency landing. 

The incident led the FAA to order a temporary global grounding of Boeing 737 Max 9 planes for "immediate inspections."

The agency said in the report that "four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads."

What happened on the Alaska Airlines flight?

The Jan. 5 flight from Portland, Oregon, to Ontario, California, was minutes into its journey when the door plug blew off.

The plane, carrying 174 passengers and six crew members, made an emergency landing back in Portland. Several people suffered minor injuries, but no one was seriously hurt. Passengers from the January flight filed a class-action lawsuit against Boeing. The suit alleges that "the event physically injured some passengers and emotionally traumatized most if not all aboard." Alaska Airlines was not named as a defendant in the lawsuit.

According to Tuesday's NTSB report, the crew said the preflight inspection, engine start, taxi, takeoff and departure climb were all normal. The situation changed minutes later. 

"The captain said that, while climbing through about 16,000 ft, there was a loud bang," according to the NTSB report. "The flight crew said their ears popped, and the captain said his head was pushed into the heads-up display (HUD) and his headset was pushed up, nearly falling off his head. The [first officer] said her headset was completely removed due to the rapid outflow of air from the flight deck. Both flight crew said they immediately donned their oxygen masks. They added that the flight deck door was blown open and that it was very noisy and difficult to communicate."

Investigation into what went wrong

Alaska Airlines and United Airlines canceled flights on Boeing 737 Max 9 planes for days as investigators looked over planes. In the days after the door blew out, United Airlines and Alaska Airlines said they found loose hardware on door plugs on several of their grounded Boeing 737 Max 9 planes.

The FAA is also conducting an investigation into Boeing's 737 Max 9 aircraft. The agency in January said the probe will examine whether Boeing "failed to ensure" whether the jet conformed to its design and whether its aircraft "were in a condition for safe operation in compliance with FAA regulations." 

"This incident should have never happened and it cannot happen again," the FAA said Tuesday. "The FAA is continuing to support the National Transportation Safety Board's investigation into the Jan. 5 door plug incident."

A Boeing spokesperson said the company is reviewing the NTSB's findings and will cooperate with NTSB and FAA investigations. 

"Whatever final conclusions are reached, Boeing is accountable for what happened," Boeing President and CEO Dave Calhoun said. "An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers. We are implementing a comprehensive plan to strengthen quality and the confidence of our stakeholders. It will take significant, demonstrated action and transparency at every turn – and that is where we are squarely focused."

The company spokesperson said Boeing had implemented a control plan to make sure mid-exit door plugs are installed according to specifications.

An Alaska Airlines spokesperson said the company remained in close contact with federal investigators. 

"Safety is always our top priority. As this investigation moves forward, we have full confidence in the safety of our operation and aircraft," the airline said.

Spirit AeroSystems, the manufacturer of the blown-out door plug, was the focus of a class-action lawsuit filed in December of last year. The company faces allegations that its products had "sustained quality failures" and "frequently contained defects." Spirit AeroSystems had been a manufacturing unit of Boeing until it was spun off in 2005.

"As we review the NTSB's preliminary report, we remain focused on working closely with Boeing and our regulators on continuous improvement in our processes and meeting the highest standards of safety, quality and reliability," the company said in a Tuesday statement.

Past investigations into Boeing 737 planes

Boeing previously had problems with the 737 Max 8, a different version of the plane than the one flown in the January Alaska Airlines incident. In 2018, a Lion Air flight on a Boeing 737 Max 8 plane crashed into the ocean. The following year, an Ethiopian Airlines plane of the same model crashed shortly after takeoff. More than 300 people died in the two crashes. The jets were grounded in March 2019. The Boeing 737 Max was allowed to return to service late in 2020.

In April, Boeing paused 737 Max production over an issue with aircraft parts.

FAA chief Mike Whitaker on Tuesday told a House panel that his agency would step up inspections of Boeing.

"We will have more boots on the ground closely scrutinizing and monitoring production and manufacturing activities," Whitaker told the aviation subcommittee of the House Transportation and Infrastructure Committee. "Boeing employees are encouraged to use our hotline to report any safety concerns."

Kathryn Krupnik contributed reporting.

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• Alaska Airlines

Aliza Chasan

Aliza Chasan is a digital producer at 60 Minutes and CBSNews.com. She has previously written for outlets including PIX11 News, The New York Daily News, Inside Edition and DNAinfo. Aliza covers trending news, often focusing on crime and politics.

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