Bolts that helped secure a panel to the frame of a Boeing 737 Max 9 were missing before the panel blew off the Alaska Airlines plane last month, according to accident investigators.
The National Transportation Safety Board released a preliminary report on the Jan. 5 incident on Tuesday.
The report included a photo of Boeing working on the panel, called a door plug. In the photo, three of the four bolts that prevent the panel from moving up are missing. The location of the fourth bolt is unclear.
Investigators said the lack of some damage around the panel indicates that all four bolts were missing before the plane took off from Portland, Oregon.
Pilots were forced to make a harrowing emergency landing with a hole in the side of the plane.
Without the bolts, there was nothing to prevent the panel from sliding up and loosening from the “stop pads” that secured it to the airframe.
According to the preliminary report, the door plug, installed by supplier Spirit AeroSystems, arrived at the Boeing factory near Seattle with five damaged rivets around the plug. A Spirit crew replaced the damaged rivets, which required removing the four bolts to open the plug.
A text message between Boeing employees who finished work on the plane after the rivets were replaced included the photo showing the plug with missing bolts, the report said.
The NTSB has not released a probable cause for the accident — which comes at the end of an investigation that could last a year or more.
“Whatever final conclusions are reached, Boeing is responsible for what happened,” CEO David Calhoun said in a statement. “An event like this should not take place on an aircraft leaving our factory. We just have to do better for our customers and their passengers.”
Investigators said they were still trying to determine who allowed the Boeing crew to open and reinstall the door plug.
Safety experts say the accident could have been catastrophic if the Alaska plane had reached cruising altitude. The decompression in the cabin after the blowout would have been much stronger, and passengers and flight attendants might have been walking around instead of being strapped to their seats.
When Alaska and United Airlines began inspecting their other Max 9s, they reported finding loose hardware, including loose bolts in some door plugs.
The incident has raised questions about the quality of production at Boeing, starting with the deadly crashes of two Max 8 jets in 2018 and 2019 that killed 346 people.
The Federal Aviation Administration is investigating whether Boeing and its suppliers followed proper safety procedures when producing parts for the Max. The FAA has banned Boeing from accelerating production of 737s until the agency is satisfied with the quality problems.
FAA Administrator Michael Whitaker said Tuesday that his agency is about halfway through a six-week audit of production processes at Boeing and Spirit, the Max’s main supplier. He said the agency is faced with two questions: what’s wrong with the Max 9 and “what’s going on with production at Boeing?”
Spirit, which Boeing spun off as a separate company nearly two decades ago, said in a statement that it is reviewing the preliminary NTSB report and working with Boeing and regulators “to continuously improve our processes and meet the highest standards in the areas of safety, quality and reliability.”
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This story has been corrected to note that a Spirit crew, not a Boeing crew, repaired the rivets.