United Airlines said Monday it found loose bolts in the panels of its Boeing 737 Max 9 aircraft that resemble the part from the Alaska Airlines plane that exploded during a flight on Friday, adding to growing safety concerns about the Max 9.
The revelation came after reports that Alaska Airlines had been warned three times about cabin pressure problems on its aircraft. These warnings were significant enough that the airline decided that the plane could no longer be used for flights to Hawaii.
United Airlines said Monday it found loose screws in the panels, known as plugs in the industry, after it began taking out seats and sidewall liners to inspect the part this weekend. The plugs are placed where the emergency exit door would be if the plane had more seats.
A door seal suddenly flew off an Alaska Airlines plane Friday during cabin depressurization about 10 minutes after takeoff from Portland, Oregon, exposing passengers to high winds and forcing pilots to quickly return to the airport. The door stop, phones, toys and other personal items came out of the hole in the side of the plane and fell across the city.
Airlines have canceled hundreds of flights as they prepare to inspect nearly 200 planes that will be grounded until regulators and company officials decide they are safe. Alaska Airlines used 65 Max 9 planes, about 20 percent of its fleet, and United used 79 planes, more than any other airline and about 8 percent of its fleet, according to Cirium, an aviation data provider. Some passengers’ travel plans may be disrupted for several days.
The FAA sent instructions to airlines on Monday on how to conduct the inspections, though Alaska and United said they were waiting for additional approval from the FAA to begin.
Officials led by the National Transportation Safety Board are focusing, among other things, on plug installation and inspection.
“I think investigators will focus on the manufacturing process of this particular aircraft,” said Jeff Guzzetti, a former NTSB and FAA investigator. “How was this door plug installed or who installed it?”
The door was initially installed by Spirit AeroSystems, which makes the fuselage for the 737 Max and other planes. Investigators said they were looking into whether work had been carried out on or near the door since the plane entered service in November.
Jennifer Homendy, head of the National Transportation Safety Board, said investigators have a great deal of work to do, including examining the plug, which was recovered from a backyard near Portland. The board will also examine the seal that remained intact on the other side of the plane, interview the flight crew and passengers, review maintenance records and repair records, and conduct laboratory analyzes of parts of the plane.
Investigators may also look into whether the installation of wireless Internet equipment on the plane by the contractor, AAR, between November 27 and December 7 played any role in the pressurization problems, which arose after that work was completed. AAR said in a statement Monday that it “did not perform any work on or near any middle cabin exit door seal for that specific aircraft.”
Experts said that although no serious injuries were reported, the accident could have been more catastrophic, especially if the plane had been at a higher altitude. Ms Homendy said on Sunday night that the passengers included three children and four unaccompanied children aged between 5 and 17.
Ms. Homendy said in a brief interview on Monday that her team was reviewing the plane’s flight data recorder to try to determine whether the pressure warning light could be linked to the door plug. The plane has several backup systems in case one of the pressure systems fails.
“Maybe there was something wrong with the light or that other unit, but there are duplicates in the system,” Ms. Homendy said.
Kathleen Bangs, an aviation expert and former airline pilot, said she believed the investigation would reveal a faulty door plug because of the plane’s condition. Explosive decompression incidents typically occur in older aircraft that suffer from metal corrosion and fatigue, Ms. Bangs said. In this case, she added, the plane was almost new, indicating there was likely a problem with the door plug.
An explosion at more than 30,000 feet could have been catastrophic, said Anthony Brickhouse, a professor of aviation safety at Embry-Riddle Aeronautical University. “We could have been looking at a situation where more of the structure could have exploded, we could have been looking at a situation where occupants who were not properly strapped in could have exploded because the forces would have been so enormous.” He said.
Mr. Brickhouse, who previously investigated aviation accidents for the safety board, said decompression began to affect most commercial planes at about 8,000 feet. Failure to properly control the air entering and leaving the cabin can cause altitude sickness or hypoxia among passengers and crew.
The FAA says hypoxia, a condition that develops when the brain is deprived of oxygen, can occur on planes without proper decompression when they begin flying above 10,000 feet or experience rapid decompression. That’s why flight attendants ask passengers to use drop-down masks in case of rapid decompression, Mr. Brickhouse said.
The FAA said in a statement that the required inspections will focus on plugs, door components and fasteners.
“Our teams have been working diligently — with a comprehensive FAA review — to provide comprehensive technical instructions to operators to conduct required inspections,” said Stan Deal, CEO of Boeing’s Commercial Airplanes business, and Mike Delaney, chief aviation safety officer. In a message to staff of that unit on Monday.
Other airlines with Max 9 aircraft are located outside the United States, such as Copa Airlines Panama, Turkish Airlines and Icelandic Airlines. The European Union’s aviation safety agency announced Monday that Max 9 planes operating in Europe have not been grounded because they are of a different configuration.
The FAA previously said it would take four to eight hours to inspect each plane. Inspection of the nearly 200 Max 9 planes in the United States, according to the aviation agency, may take a few days.
Aviation regulators and Boeing said the inspections were only for the MAX 9 and not for other versions of the MAX. The Max 9, along with the more popular Max 8, was grounded for about two years after two Max 8 crashes in 2018 and 2019 killed 346 people.
Federal authorities investigating the accident are also looking into what led to pressure warnings on the damaged plane during three recent flights. Alaska Airlines workers reset the system, and the plane was returned to service, even though the airline has banned its use on flights to destinations such as Hawaii, Ms. Homendy said. She added that it is not yet clear whether the warnings are related to the incident that occurred on Friday.
Alaska said in a statement that it cannot answer many outstanding questions about the plane and what led to its explosion without safety board approval. The airline said it has asked the NTSB to share more information and will do so if allowed to do so. In such investigations, parties are usually restricted in what they can share publicly.
Boeing CEO Dave Calhoun plans to host a company-wide safety meeting on Tuesday to discuss the company’s response to the accident and reaffirm its commitment to safety. Boeing is still working on obtaining approval for the smaller MAX 7 and larger MAX 10 planes.
Boeing shares closed down about 8 percent on Monday, and Spirit AeroSystems shares closed down 11 percent.
J. Edward Moreno Contributed to reports.