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

"For Boeing ( BA ), such fines are easily absorbed as the

cost of doing business, not a meaningful deterrent to dangerous behavior," Blumenthal wrote in a letter to FAA Administrator Bryan Bedford. "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 found hundreds of quality system violations at the planemaker's 737 factory in Renton, Washington, and Boeing ( BA ) subcontractor Spirit AeroSystems' ( SPR ) 737 fuselage factory in Wichita, Kansas.