According to US investigators, the plane was allowed to fly even though warning lights were triggered on the previous three flights.
Alaska Airlines allowed the Boeing plane that suffered a blowout this week to fly despite warnings from a cabin pressure system.
The National Transportation Safety Board (NTSB), the United States' lead accident investigator, said Monday that warning lights had been activated on three flights on the brand-new Boeing 737 Max 9. Two of the alarms came on consecutive days before the plane suffered horrific fuselage damage on Friday.
The company stopped flying the plane across the Pacific Ocean to Hawaii because of the warnings, but maintained the land flight, the NTSB said.
On Friday, a plug covering a location for an emergency door ripped off the plane as it flew 4,800 meters (16,000 feet) over Oregon.
Jennifer Homendy, chairwoman of the NTSB, said maintenance crews had inspected the plane and cleared it for flight, but the airline decided not to use it for the long route to Hawaii over water so it could “return to an airport very quickly.” If this is the case, the warning light has reappeared.
Friday's flight flew from Oregon to Southern California and returned to Portland without serious injuries to the 171 passengers and six crew members.
Loose parts
United Airlines and Alaska Airlines said they found loose parts in several grounded Boeing 737 Max 9 planes, raising new concerns among industry experts about the passenger planes' manufacturing process.
The Federal Aviation Administration (FAA), which regulates the industry in the United States, grounded 171 Max 9 planes worldwide after the incident, forcing the Alaskan pilots to make an emergency landing.
United Airlines said Monday that in its initial inspections it found “screws that need to be tightened even further.”
Alaska said initial reports from its engineers indicated that “loose hardware” was visible on some aircraft as the company conducted inspections of its fleet.
With 79 Max 9 aircraft, United has the largest fleet of these aircraft. Alaska has 65 of these planes, while the rest are operated by Turkish Airlines, Panama's Copa Airlines and Aeromexico.
The mid-air incident has raised concerns about the Max 9's production process and quality control. The model is used by relatively few airlines, but the 737 Max family of planes has been plagued by controversy since the entire global fleet was grounded in March 2019 after two accidents in six months that killed 346 people.
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 safety and quality standards,” the US plane maker said. “We regret the impact this has had on our customers and their passengers.”
On Monday, the FAA announced that it has approved a roadmap for carriers to conduct inspections that include both left and right door plugs, components and fasteners. It said the planes would remain grounded until operators completed “enhanced inspections.”