United finds loose bolts on 737 Max 9 planes in wake of Alaska Airlines door plug incident

United Airlines said Monday that it has found loose bolts during inspections of its 737 Max 9 fleet in the wake of a door plug getting blown out of an Alaska Airlines plane over the weekend.

Monday evening Alaska Airlines said initial inspections of its 737 MAX 9 fleet revealed "some loose hardware" visible on some aircraft.

"Any findings will be fully addressed in a matter that satisfies our safety standards and FAA compliance," Alaska Airlines said.

"The safety of these aircraft is our priority and we will take the time and steps necessary to ensure their airworthiness, in close partnership with the FAA," the airline added.

United won't say how many planes had loose bolts.

"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," United said in a statement. "These findings will be remedied by our Tech Ops team to safely return the aircraft to service."

The Federal Aviation Administration grounded all 737 Max 9s in the wake of the Alaska Airlines incident.

"The FAA’s first priority is keeping the flying public safe," it said in a statement. "We have grounded the affected airplanes, and they will remain grounded until the FAA is satisfied that they are safe."

United said that due to the emergency inspections, it had to cancel 200 Max 9 flights on Monday and expects "significant cancellations" on Tuesday, as well.

In response to United's findings, Boeing issued a statement Monday evening, saying, “As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings. We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards. We regret the impact this has had on our customers and their passengers.”

The National Transportation Safety Board, which is investigating the incident involving the Alaska Airlines flight near Portland, Oregon, has yet to comment on the United Airlines announcement.

NTSB investigators recovered the door plug that fell off Alaska Airlines Flight 1282 on Sunday, giving them the key piece of evidence they are examining with a laboratory microscope.

The plug, measuring 26-by-46 inches and weighing 63 pounds, was discovered intact Sunday evening in the backyard of a Portland teacher's home, according to NTSB officials.

The part fell off the plane, a Boeing 737 Max 9, around 5:11 p.m. local time Friday as the aircraft with 171 passengers, including three babies and four unaccompanied minors, had climbed to 16,000 feet after taking off from Portland International Airport, according to the NTSB.

The door plug is used to seal unused exits on planes and, according to a diagram released by the NTSB, is attached to the plane with a series of bolts, cables, hinges and stop pads.

The midair incident caused an "explosive decompression" of the Alaska Airlines aircraft and prompted the flight crew to immediately return to Portland with a gaping hole in the side of the jet, according to the NTSB.

No one was injured in the incident. But NTSB Chair Jennifer Homendy told reporters that had the incident occurred at 30,000 feet, "We could have ended up with something so much more tragic."

During a news conference Sunday night, Homendy said the detachment of the door plug caused a violent and chaotic situation inside the plane's cabin and sucked the headrests, at least one tray table and two cell phones out the hole in the plane.

Homendy said the incident also blew open the cockpit door and sucked out a laminated quick reference checklist the pilots use.

"The laminated checklist flies out the door. The captain takes the quick reference handbook, hands it over to the first officer. They start going through their memory items," Homendy said.

She said the sudden decompression also pulled off the first officer's headset and a portion of the captain's headset.

"They put their [oxygen] masks on, turned on the speaker so they could communicate with folks back in the cabin," Homendy said.

She said the incident also caused damage to rows 1 through 33 on the plane.

The investigation also found three previous incidents on the Alaska Airlines plane where the auto pressurization fail light illuminated during flights on Dec. 7, 2023; Jan. 3 and Jan. 4, Homendy added.

"In these previous flights after the light illuminated, they flipped the switch to alt mode, which is normal. There's a backup. It was very benign. Nothing occurred," Homendy said.

She said it remains unclear if there is a correlation between the auto pressurization light illuminating and the door plug blowing out. She said Alaska Airlines prevented the aircraft from being flown to Hawaii over water and restricted it to overland use "so if some light did illuminate, it could return quickly to an airport."

Homendy said that while the cockpit voice recorder (CVR) was recovered from the plane, it contained no helpful data because it was programmed to reset and rerecord every two hours.

"So, we have nothing from the CVR," Homendy said.

She said investigators are closely examining the door plug and the frame it was blown out of, as well as all the components used to keep it in place to determine what caused the incident. She said the investigation also includes interviews with the six-member flight crew and passengers.