In a letter obtained exclusively by CBS News, Democratic Sen. Tammy Duckworth of Illinois, chair of the Senate Subcommittee on Aviation Safety, demanded that the Federal Aviation Administration reject Boeing's request for a safety waiver on the so far uncertified 737 Max 7, the smallest of the four 737 Max variants.

"Boeing forfeited the benefit of the doubt long ago when it comes to trusting its promises about the safety of 737 MAX, and the FAA must reject its brazen request to cut corners in rushing yet another 737 MAX variant into service," she wrote in the letter sent late Wednesday to FAA Administrator Michael Whitaker.

The letter was penned on the same day that Boeing CEO Dave Calhoun traveled to Washington, D.C., to meet with legislators in the wake of an incident earlier this month in which the door panel of a 737 Max 9 blew off during an Alaska Airlines flight.

The FAA has grounded all 171 Boeing 737 Max 9 aircraft since the event, but announced Wednesday that it had cleared the way for the aircraft to return to service following a rigorous inspection and maintenance process.

Alaska Airlines said it expected to begin bringing its 737 Max 9 planes back into service on Friday, while United Airlines said its fleet would begin returning to service on Saturday.

The issue in Duckworth's letter centers around an anti-ice system on 737 Max engines that Boeing identified and self-reported to the FAA last year. The regulator approved Boeing's guidance to mitigate the problem on the existing fleet of Max aircraft while Boeing engineered a fix by May of 2026.

The FAA issued an Airworthiness Directive in August 2023 that it said "was prompted by a report indicating that use of engine anti-ice (EAI) in dry air for more than five minutes during certain environmental and operational conditions can cause overheating of the engine inlet inner barrel beyond the material design limit, resulting in failure of the engine inlet inner barrel and severe engine inlet cowl damage."

The FAA told airlines that pilots should limit the use of the anti-ice system to less than five minutes until Boeing's fix was available.

While the issue has never occurred in-flight, Boeing determined it was theoretically possible under specific weather conditions, and in a worst-case scenario, could result in components breaking off.