FAA launches probe into Boeing 737 MAX 9 door plugs

The Federal Aviation Administration is investigating whether the panel doors on Boeing's 737 MAX 9 jets, such as the one that flew off in the middle of an Alaska Airlines flight last week, match with approved design and manufacturing requirements.

In a statement Thursday, the FAA said its investigation will determine if Boeing failed to ensure that the "plug doors" installed on the 737 MAX 9 planes, which cover over an unused exit door, were safe and appropriate to use and whether they complied with FAA-approved design standards.

"Boeing’s manufacturing practices need to comply with the high safety standards they’re legally accountable to meet," the FAA said in an unsigned statement.

Boeing said Thursday it will cooperate with the FAA and National Transportation Safety Board, which is conducting a separate investigation into the mid-air incident.

Boeing CEO Dave Calhoun apologized for the incident on Wednesday, telling CNBC it should have never happened. “We’re going to want to know what broke down in our gauntlet of inspections, what broke down in our original work that allowed that escape to happen,” Calhoun said. Calhoun also told Boeing employees this week they will approach any investigation "with 100 percent complete transparency every step of the way."

The FAA sent a letter to Boeing Vice President of Total Quality Carole Murray on Wednesday giving the manufacturer 10 days to provide any evidence or explanations related to the probe. In that letter, John Piccola, director of the FAA's Integrated Certificate Management Division, wrote that "circumstances indicate that Boeing may have failed to ensure its completed products conformed to its approved design and were in a condition for safe operation in accordance with quality system inspection and test procedures."

The FAA investigation is not surprising considering that United Airlines and Alaska Airlines, the only U.S. airlines operating the MAX 9, found additional loose bolts on similar 737 MAX 9 door plugs earlier this week.

Lawmakers on Capitol Hill are calling for hearings and investigations into the incident, which forced an Alaska Airlines flight to turn around and perform an emergency landing after the door plug was ripped from the MAX 9's fuselage. No one was seriously injured in the incident.

United and Alaska's 171 MAX 9 planes remain grounded pending inspections and potential fixes as of Thursday, and dozens of flights have been canceled across both airlines. The FAA said Tuesday that inspections were paused as they await updated instructions from Boeing to operators for inspecting and maintaining the door plugs. The FAA said it will conduct a "thorough review" of Boeing's revised instructions before potentially allowing the planes to fly again.

United canceled 194 flights on Thursday and Alaska canceled 158, according to the flight tracking service FlightAware, though those totals include cancellations that are unrelated to the MAX 9 issue. Alaska said Wednesday that its MAX 9 fleet would remain grounded until at least Saturday.