A Boeing employees works outside of the cockpit of a Boeing 737 MAX 8 airplane in the company's factory in Renton, Washington. Image Credit: GETTY IMAGES /AFP

The Federal Aviation Administration said Wednesday it is launching a formal investigation into the Boeing 737 MAX 9 after a cabin panel blew off an Alaska Airlines flight while in mid-air 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 US 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.” 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 did not immediately respond to a request for comment. Its shares were down 1.6 per cent on 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.

'Quality control issue'

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.

US 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.