Missing Bolts Caused Boeing 737 Max 9 Door Plug to Blow Out Mid-Flight, Per NTSB Report

The NTSB indicates the missing bolts were removed during a repair procedure in September, and never replaced.

Federal transportation experts indicate the door plug that blew out of a Boeing 737 Max 9 during an Alaskan Airlines flight last month was missing four critical bolts that secure it in place.

A preliminary investigation report released by the National Transportation Safety Board (NTSB) into the January 5 incident indicates important parts that keep door plugs from moving were not reinstalled correctly after repairs were made to the aircraft during production, and may be responsible for the door plug blowing out mid-flight.

After the left mid exit door (MED) plug blew out of a Boeing 737-9 aircraft carrying nearly 200 passengers at about 16,000 feet, the U.S. Federal Aviation Administration (FAA) issued an Emergency Airworthiness Directive (AD) last month, grounding more than 170 planes and requiring them to undergo additional inspections before they could return to service.

Missing Bolts Caused Door Plug Blowout: NTSB

NTSB investigators examined the structural integrity of the aircraft body, flight deck, cabin interior, exits, and analyzed cabin communication, lighting, pressurization, and safety systems. They observed damage and deformities to the interior cabin but did not find any failures or malfunctions among the airplane or its systems.

In examining the left MED plug, investigators found it was missing the bolts that prevent the door from moving upwards and away from the plane, which have still not been recovered. They observed contact damage consistent with the MED plug pulling away from the upper forward stop location, but found little to no damage to the bolt holes, according to the report. This led investigators to determine the bolts were not there before the incident, allowing the door plug to blow out.

A review of the airplane’s manufacturing and maintenance records revealed the left MED plug had five damaged rivets to the frame, which Spirit AeroSystems replaced on September 19, 2023. The NTSB indicates the bolts were removed to open the MED plug during rivet repair.

Investigators found text message images showing the MED plug was opened to perform the repair, and another photo taken after the repair shows that it was reinstalled without retaining bolts in three visible locations.

The NTSB indicates the investigation is still ongoing and it will schedule interviews with both Boeing and Spirit AeroSystems personnel about the repairs to the left MED plug.

As a result of the incident, a Boeing 737 Max 9 class action lawsuit has already been filed by a group of seven passengers on the Alaska Airlines flight. One passenger said the pressure change was so violent it made their ears bleed.