US investigators say the plane was allowed to fly despite warning lights being activated on the previous three flights.
Alaska Airlines allowed the Boeing plane that was involved in a mid-air explosion this week to fly despite warnings from the cabin pressure system.
Warning lights on the brand-new Boeing 737 MAX 9 were activated on three flights, the National Transportation Safety Board (NTSB), the nation's top accident investigator, said Monday. Two warnings came on consecutive days before the plane suffered a horrific fuselage explosion on Friday.
The National Transportation Safety Board said the company grounded the plane over the Pacific Ocean to Hawaii due to warnings, but kept it flying over land.
On Friday, a seal covering a space designated for the plane's emergency door ruptured while it was flying at an altitude of 4,800 meters (16,000 feet) over Oregon.
Jennifer Homendy, head of the National Transportation Safety Board, said maintenance crews checked the plane and cleared it to fly, but the airline decided not to use it on the long route to Hawaii over water so it could “return very quickly to the airport” if the plane crashed. The warning light came on again.
Friday's flight was heading from Oregon to Southern California and returned to Portland without any of the 171 passengers and six crew members being seriously injured.
United Airlines and Alaska Airlines said they found loose parts on several Boeing 737 MAX 9 planes, raising new concerns among industry experts about the passenger plane manufacturing process.
The Federal Aviation Administration (FAA), which regulates the industry in the United States, grounded 171 Max 9 planes worldwide after the accident, forcing pilots in Alaska to make an emergency landing.
United Airlines said on Monday that it “found bolts that required additional tightening” in its initial inspections.
Alaska said initial reports from its technicians indicated that some “loose devices” had appeared on some aircraft when it conducted inspections of its fleet.
With 79 Max 9 aircraft, United has the largest fleet of aircraft involved. Alaska has 65 aircraft, while the rest are operated by Turkish Airlines, Panama's Copa Airlines and Aeromexico.
The mid-air incident has raised concerns about the production process and quality control of the MAX 9. The model is used by relatively few airlines, but the 737 MAX family has been subject to controversy since the entire global fleet was grounded in March 2019. After two accidents within six months resulted in 346 people killed.
Boeing said it remains in close contact with MAX 9 operators and will help customers address any findings during recent inspections.
“We are committed to ensuring that every Boeing aircraft meets design specifications and the highest standards of safety and quality,” the US planemaker said. “We regret the impact this has had on our customers and their passengers.”
The Federal Aviation Administration (FAA) announced Monday that it has approved a roadmap for carriers to complete inspections that include left and right door plugs, components and fasteners. It said the planes would remain grounded until operators completed “enhanced inspections.”
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