Boeing plane was missing key bolts from door plug in Alaska Airlines incident: NTSB

The National Transportation Safety Board issued a preliminary report into last month's Alaska Airlines door plug blowout, saying evidence shows that four bolts were missing from the apparatus at the time of the incident.

In the wake of the incident, 171 Boeing 737 Max 9 aircraft, which had the same mid-cabin door plug, as the plane involved in the Alaska Airlines incident, were grounded, and Boeing has been under intense scrutiny from regulators since the explosive decompression on Jan. 5. The plug is referred to as an MED plug in the agency's report.

According to the NTSB, "two vertical movement arrestor bolts, two upper guide track bolts, forward lower hinge guide fitting, and forward lift assist spring were missing and have not been recovered."

The agency reports a lack of "damage patterns and absence of contact damage or deformation around holes" where the bolts should have been fastened. The "recovered aft lower hinge guide fitting indicate that the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads," the report says.

The NTSB's report further indicates that the bolts were removed to perform other repair work at the Boeing factory after the fuselage was delivered and were not replaced before the interior wall panels were reinstalled.

According to Flight Aware, the Max 9 involved in the Alaska Airlines incident took 151 flights before the Jan. 5 emergency. That number likely includes test flights and the delivery flight from Boeing to Alaska Airlines, which would not have had paying passengers onboard.

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The Federal Aviation Administration has stepped up oversight of Boeing's production processes and, in a hearing on Capitol Hill on Tuesday, administrator Mike Whitaker said he would continue to emphasize safety as agencies and airlines continue to scrutinize the manufacturer's operations.

Boeing said in a statement that it continues to cooperate with the NTSB and FAA investigations and will work to improve its manufacturing processes to make them safer.

“Whatever final conclusions are reached, Boeing is accountable for what happened. An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers. We are implementing a comprehensive plan to strengthen quality and the confidence of our stakeholders. It will take significant, demonstrated action and transparency at every turn – and that is where we are squarely focused," CEO David Calhoun said in the statement. “This added scrutiny – from ourselves, from our regulator and from our customers – will make us better. It’s that simple."

This article originally appeared on USA TODAY: Boeing plane missing key bolts from door plug in flight incident: NTSB