NTSB: Boeing jet's bolts were missing before takeoff

The bolts that helped secure a panel to the frame of a Boeing 737 MAX 9 jet were missing before the plane took off last month from Portland, Oregon, investigators said Tuesday.

In a preliminary report on the Jan 5 incident, the National Transportation Safety Board (NTSB) 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 are missing. The location of the fourth bolt is obscured.

Investigators said that the lack of certain damage around the panel indicates that all four bolts were missing before the plane took off.

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 Spirit crew replaced the damaged rivets, which required removing the four bolts to open the plug.

Without the bolts, nothing prevented the panel from sliding upward and detaching from "stop pads" that secured it to the airframe.

The Alaska Airlines pilots were forced to make an emergency landing with a hole in the side of the plane.

The NTSB report didn't declare a probable cause for the Jan 5 incident, which will come at the end of an investigation that could last a year or longer.

Investigators said they still were trying to determine who authorized the Boeing crew to open and reinstall the door plug.

The two Alaska Airlines pilots on the flight told the NTSB that when the plane reached about 16,000 feet, there was a loud bang.

The pilots said their ears popped, and that the captain's head was pushed into a screen displaying information at eye level. The first officer said her headset came off because of the rapid outflow of air from the flight deck. The cockpit door was blown open, and the pilots said it was noisy and difficult to communicate.

Safety experts have said the incident could have been catastrophic if the Alaska Airlines jet had reached cruising altitude. The decompression in the cabin after the blowout would have been far stronger, and passengers and flight attendants might not have been belted into their seats.

The plane returned to Portland and landed safely.

The report came as most grounded 737 MAX 9 jets have gone through inspections and resumed flying passengers. The Federal Aviation Administration said Monday that 135 of the 144 US MAX 9 jets that were grounded had been inspected and returned to service.