(NEWSnet/AP) — 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 issued a preliminary report on the Jan. 5 incident Tuesday.
The report included a photo from Boeing, which worked on the panel, which is called a door plug. In the photo, three of the four bolts that prevent the panel from moving upward are missing. The location of the fourth bolt is obscured.
[Related: Alaska Airlines Begins Flying Boeing Max 9 Jetliners For First Time Since Blowout]
The investigators said the lack of certain damage around the panel indicates that all four bolts were missing before the plane took off from Portland, Oregon.
No one was seriously injured although air pressure was lost in the blow out, and the crew made an emergency landing upon returning to Portland.
Without the bolts, nothing prevented the panel from sliding upward and detaching from “stop pads” that secured it to the airframe.
The preliminary report said the door plug, installed by supplier Spirit AeroSystems, arrived at Boeing’s factory near Seattle with five damaged rivets around the plug. A Boeing crew replaced the damaged rivets, which required them to remove the four bolts to open the plug.
The NTSB did not declare a probable cause for the accident – that will come at the end of an investigation that could last a year or longer.
Investigators said they were still trying to determine who authorized the Boeing crew to open and reinstall the door plug.
The Federal Aviation Administration is investigating whether Boeing and its suppliers followed proper safety procedures in manufacturing parts for the Max. The FAA has barred Boeing from speeding up production of 737s until the agency is satisfied about quality issues.
FAA Administrator Michael Whitaker said Tuesday that his agency is about halfway through a six-week audit of manufacturing processes at Boeing and its key supplier on the Max, Spirit AeroSystems.
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