Boeing has become a target of the Federal Aviation Administration’s investigation into the 5 January in-flight failure of an emergency exit door plug on an Alaska Airlines 737 Max 9.

The US regulator is evaluating Boeing’s “alleged non-compliance” with regulations related to inspection and testing of new aircraft, according to a 10 January letter from the FAA to the aircraft manufacturer.

The letter says evidence suggests the incident resulted from such failures by Boeing prior to delivering the jet to Alaska.

Alaska 737 Max 9 door plug

Source: National Transportation Safety Board

The Alaska Airlines 737 Max 9 suffered a rapid decompression during a 5 January 2024 flight when a plug in an unused emergency exit blew out

“Circumstances indicate that Boeing may have failed to ensure its completed products conformed to its approved design and were in a condition for safe operation in accordance with quality system inspection and test procedure,” says the letter. “The FAA is investigating this matter.”

FAA director of integrated certification management John Piccola signed the document, which is addressed to Boeing Commercial Airplanes vice-president of total quality Carole Murray.

It says regulations require Boeing to ensure each aircraft, prior to delivery, “conforms to its approved design and is in a condition for safe operation”.

Aerospace experts have already speculated that the incident resulted from a quality problem on the part of Boeing.

The Alaska 737 Max 9’s door plug blew out at around 16,000ft, leaving a gaping hole in the side of the jet. The pilots safely landed in Portland without serious injuries to passengers or crew.

Boeing delivered the jet new to Alaska on 31 October last year, according to Cirium data.

The FAA responded on 6 January to the incident by grounding 737 Max 9s with mid-cabin emergency door plugs, pending inspections. Two airlines – Alaska and United Airlines – already said they found loose door-plug hardware during inspections.

“After the incident, the FAA was notified of additional discrepancies on other Boeing 737-9 airplanes,” the FAA’s letter notes.

Alaska 737 Max 9 incident 5 January 2024

Source: National Transportation Safety Board

The FAA says circumstances suggest that inspection and testing shortcomings by Boeing may have resulted in the door plug’s failure

The door plugs are supposed to be secured by four bolts, according to the National Transportation Safety Board (NTSB), which is also investigating the incident.

The FAA’s letter gives Boeing 10 business days to submit information related to the event.

“Your response should contain the root cause of the encountered condition(s), products/articles affected, service impacts, the extent of any immediate/long-term action taken to correct and preclude its recurrence, and any mitigating circumstances which you believe may be relevant to this case,” the FAA’s letter says.

In response, Boeing says it “will cooperate fully and transparently with the FAA and the NTSB on their investigations”.

On 9 January, Boeing chief executive David Calhoun told employees that the company will “approach this, number one, acknowledging our mistake”.