Adds details from senator's letter in paragraphs 3-11

By David Shepardson

WASHINGTON, Sept 24 (Reuters) - Democratic U.S. Senator Richard Blumenthal said the Federal Aviation Administration's proposed $3.1 million fine against Boeing BA.N for a series of safety violations is inadequate and wants the agency to explain how it calculated the penalty.

"For Boeing, such fines are easily absorbed as the cost of doing business, not a meaningful deterrent to dangerous behavior," Blumenthal wrote in a letter sent on Tuesday and released on Wednesday to FAA Administrator Bryan Bedford, an appointee of Republican President Donald Trump. "Unless penalties rise to the level that forces the company to invest in real safety reforms, the risks to the flying public will persist."

The FAA and Boeing did not immediately respond to requests for comment.

Blumenthal is the top Democrat on a Senate committee that has investigated Boeing safety issues and chaired the panel when it looked into a January 2024 mid-air cabin blowout incident involving a new Alaska Airlines ALK.N 737 MAX airplane. The panel under Blumenthal released a report showing that Boeing whistleblowers had raised significant concerns about the company's manufacturing processes.

The FAA said it found hundreds of quality system violations at the planemaker's 737 factory in Renton, Washington, and the 737 fuselage factory of Boeing subcontractor Spirit AeroSystems SPR.N in Wichita, Kansas, from September 2023 through February 2024.

"If there was discretion in how penalties were calculated - or if reductions are likely to follow - then the FAA risks sending the message that systemic safety violations carry no serious consequences," Blumenthal wrote.

The Alaska Airlines incident, which involved a 737 MAX that was found to have been missing four key bolts, damaged Boeing's reputation and led to a grounding of the MAX 9 for two weeks and a production cap of 38 planes per month by the FAA that remains in place.

The FAA also has said Boeing presented two unairworthy aircraft to the agency for approval.

The FAA found that a Boeing employee pressured a co-worker who was performing tasks on behalf of the FAA to sign off on a 737 MAX so the company could meet its delivery schedule despite the fact the co-worker had determined that the aircraft did not comply with regulatory standards.

The Alaska Airlines incident prompted the U.S. Justice Department under Trump's Democratic predecessor Joe Biden to open a criminal investigation and declare that Boeing was not in compliance with a 2021 deferred prosecution agreement made after the company had misled the FAA during the 737 MAX regulatory certification process.

"The public deserves confidence that fines are not token gestures, but real enforcement tools," Blumenthal wrote.

(Reporting by David Shepardson; Editing by Mark Porter and Will Dunham)

((David.Shepardson@thomsonreuters.com; 2028988324;))