US investigators say the plane was allowed to fly despite triggered warning lights on previous three flights.
Alaska Airlines allowed the Boeing plane that had a mid-air blowout this week to fly despite warnings from a cabin pressurisation system.
The National Transportation Safety Board (NTSB), the United States’ chief accident investigator, said on Monday that warning lights were triggered on the brand-new Boeing 737 Max 9 on three flights. Two of the alarms came on consecutive days before the plane suffered a terrifying fuselage blowout on Friday.
The company stopped flying the aircraft over the Pacific Ocean to Hawaii due to the warnings, yet kept it flying over land, the NTSB said.
On Friday, a plug covering a spot left for an emergency door tore off the plane as it flew 4,800 metres (16,000 feet) above Oregon.
Jennifer Homendy, chair of the NTSB, said maintenance crews checked the plane and cleared it to fly, but the airline decided not to use it for the long route to Hawaii over water so that it “could return very quickly to an airport” if the warning light reappeared.
Friday’s flight was headed from Oregon to Southern California and made it back to Portland without serious injury to any of the 171 passengers and six crew members.
United Airlines and Alaska Airlines have said they found loose parts on multiple grounded Boeing 737 Max 9 aircraft, raising new concerns among industry experts about the manufacturing process for the passenger planes.
The Federal Aviation Administration (FAA), which regulates the industry in the United States, grounded 171 Max 9 planes worldwide after the incident, which forced the Alaskan pilots to make an emergency landing.
United Airlines said on Monday that it “found bolts that needed additional tightening”, in its initial inspections.
Alaska said early reports from its technicians indicated some “loose hardware” was visible on some aircraft when it conducted checks of its fleet.
With 79 Max 9 planes, United has the largest fleet of the aircraft in question. Alaska has 65 of the planes, while the remainder 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 for the Max 9. The model is used by relatively few airlines but the 737 Max family of aircraft has been dogged by controversy since the entire global fleet was grounded in March 2019 after two crashes in the space of six months killed 346 people.
Boeing said it was staying in close contact with Max 9 operators and would help customers address any findings during the latest inspections.
“We are committed to ensuring every Boeing airplane 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 approved a road map for carriers to complete inspections that include both left and right door plugs, components and fasteners. It said planes would remain grounded until operators complete the “enhanced inspections”.