Alaska Airlines and United Airlines have discovered loose door plug hardware on some of their Boeing 737 MAX 9 twinjets during preliminary inspections of grounded MAX 9s.

The two US carriers have been assessing certain MAX 9s as they prepare to meet the formal inspection requirements of the US FAA’s 6 January emergency airworthiness directive (EAD), which grounded for inspection any 737 MAX 9 aircraft “certificated in any category with a mid-cabin door plug installed”.

The EAD was issued after an Alaska MAX 9, operating as Flight 1282, experienced a mid-air blowout of a left side, mid-cabin door plug at roughly 16,320 ft, resulting in rapid decompression in the cabin and forcing the aircraft to return to Oregon’s Portland International for an emergency landing.

“As our maintenance technicians began preparing our 737-9 MAX fleet for inspections, they accessed the area in question. Initial reports from our technicians indicate some loose hardware was visible on some aircraft,” Alaska Airlines revealed in a statement yesterday evening.

Fellow US-based MAX 9 operator United is also reporting apparent loose door plug hardware on board some of its MAX 9s.

“Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening,” stated United, after aviation news publication The Air Current broke the story.

United said on social media that it has expanded inspections to include all its MAX 9 aircraft. “Flights operated by a 737 MAX 8 are not impacted,” added United.

Given what has been learned, airlines affected by the EAD are now waiting for authorities and the manufacturer to define the necessary inspection procedures for the safe and reliable return to operation of these aircraft.

Probing what happened on board Flight 1282, the US National Transportation Safety Board has recovered the left-side, mid-cabin door plug and its investigators are currently examining the plug, which will be sent to the NTSB Materials Laboratory in Washington DC for further examination. They have been unable to locate four bolts associated with securing the door plug.

The NTSB is also examining previous pressurization warnings regarding the Alaska MAX 9 in question, which prompted the carrier to restrict the aircraft from flying long-haul over water. “The components on this door plug tell a story,” said NTSB chair Jennifer Homendy during a press briefing last night. She declined to speculate on the cause of the event.

But given that Alaska and United have found loose door plug hardware on board MAX 9s in their fleets, a journalist asked Homendy: Does the MAX need to be looked at more closely?

“Well, first of all, we’re really focused on this aircraft. However, we are not shy about going broader than just this aircraft,” responded Homendy. “We need to first and foremost figure out what happened here on this aircraft. If we have a bigger system-wide or fleet issue, we will issue an urgent safety recommendation to push for change. With that said, we are aware of the reports that are coming back from the inspections from United and Alaska and Boeing.

“Our team is collecting that information and there will be some follow-up from the inspections, but right now, we’re focused on this one. But we can go broader at any time. We don’t have to finish the investigation to issue an urgent safety recommendation. We can do that anytime, and we have done that for entire fleets before.”

None of the passengers or crew members on board Flight 1282 were seriously injured, though the NTSB has acknowledged that those on board likely suffered psychological injury from the harrowing event.

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Featured image credited to Jason Rabinowitz