The Federal Aviation Administration announced Thursday that it has launched a formal investigation into the Boeing 737 MAX 9 after a cabin panel blew off an Alaska Airlines flight while in midair last week, forcing an emergency landing.
The FAA has grounded 171 Boeing jets installed with the same panel after the landing, most of which are operated by U.S. carriers Alaska Airlines and United Airlines, pending safety inspections.
The FAA said the Alaska Airlines MAX 9 incident "should have never happened and it cannot happen again. "The FAA asked Boeing to respond within 10 business days and tell the agency "the root cause" of the problem with the door plug and steps the company is taking to prevent a recurrence.
"We will cooperate fully and transparently with the FAA and the NTSB on their investigations," said Boeing, which is headquartered in Arlington, Virginia.
It told Boeing of the investigation in a letter Wednesday "to determine if Boeing failed to ensure completed products conformed to its approved design and were in a condition for safe operation in compliance with FAA regulations" and after learning of "additional discrepancies."
Boeing shares were down 1.6% Thursday.
Both Alaska and United said on Monday they had found loose parts on multiple grounded aircraft during preliminary checks, raising new concerns about how Boeing's best-selling jet family is manufactured.
The carriers still need revised inspection and maintenance instructions from Boeing that must be approved by the FAA before they can begin flying the planes again.
Boeing on Tuesday told staff the findings were being treated as a "quality control issue" and checks were under way at Boeing and supplier Spirit AeroSystems, Reuters reported previously.
Boeing CEO Dave Calhoun told CNBC on Wednesday that a "quality escape" was at issue in the MAX 9 cabin blowout. The Alaska Airlines flight had taken off from Portland, Oregon, and was flying at 16,000 feet when the panel tore off the plane, which had been in service for only eight weeks. Pilots returned the full jet to Portland, with only minor injuries suffered by people on board.
Boeing’s manufacturing practices "need to comply with the high safety standards they’re legally accountable to meet," the FAA added.
U.S. Transportation Secretary Pete Buttigieg declined to say on Wednesday when the FAA may allow the planes to resume flights but said it would only be when safe.
"The only consideration on the timeline is safety," Buttigieg told reporters. "Until it is ready, it is not ready. Nobody can or should be rushed in that process."