Factbox-Five facts from US safety board report on Boeing MAX 9 door panel

FILE PHOTO: The fuselage plug area of Alaska Airlines Flight 1282 Boeing 737-9 MAX

(Reuters) - A door panel that flew off a Boeing 737 MAX 9 jet mid-flight on Jan. 5 appeared to be missing four key bolts, according to a preliminary report from the U.S. National Transportation Safety Board released on Tuesday.

Here are five key takeaways from the report.

BOLTS APPARENTLY MISSING

The 19-page NTSB report said four important bolts that played a role in holding the door plug in place appear to be missing and have not been recovered. Damage patterns and the lack of damage around holes associated with the missing bolts "indicate that the four bolts that prevent upward movement of the MED (door) plug were missing" before the door plug moved upward and blew out of the plane.

DAMAGED RIVETS

The report noted five damaged rivets next to the site of the door plug. The bolts in the door plug would have needed to be removed to repair the rivets. In a photo supplied by Boeing in the report, one can see the door plug after the rivets were repaired and just before interior material like insulation was restored. In three locations visible in the photo the key bolts were missing.

"The investigation continues to determine what manufacturing documents were used to authorize the opening and closing of the plug during the rivet rework," the report said.

SEAL FLUSHNESS

The door plug was manufactured by Spirit AeroSystems in Malaysia in March 2023 and then sent to its facility in Wichita, Kansas, in May where it was installed on the fuselage before being delivered to Boeing. During the building process it was noted that the door seal was off very slightly. But no action was required because Spirit determined that situation "was structurally and functionally acceptable."

SEATS 25A AND 26A DAMAGED

Inside the airplane investigators found damage in seat rows 25 ABC and 26 ABC, but otherwise no signs of any other failures or malfunction in the plane or any of its systems.

INVESTIGATION ONGOING

The report said Boeing and Spirit Aerosystems staff still need to be interviewed and that investigators will be looking at safety management systems as the investigation continues.

(Reporting by Chris Sanders in Washington; Editing by Matthew Lewis)