The letter also notes a different instance in which senior managers at the F.A.A. broke with their own employees’ assessment and allowed Boeing to remove lightning protection from part of the 787 Dreamliner.

The episodes suggest that “the F.A.A.’s safety and technical experts are being circumvented or sidelined while the interests of Boeing are being elevated by F.A.A. senior management,” Mr. DeFazio and Mr. Larsen wrote.

As the Max was being built, engineers at the F.A.A. who were responsible for approving the plane grew concerned about its new engines, which were bigger and more complex than on previous 737s. They worried that if the engines broke apart during a flight, there was a greater risk that the chunks would sever the cables, which control the plane’s rudder. The F.A.A. employees wanted the company to make changes to avoid a potentially catastrophic failure.

But Boeing resisted the change, arguing that it was highly unlikely that an engine would explode midair and damage the cables. Against the advice of six F.A.A. specialists, senior leaders at the regulator decided not to force the company to redesign the cables. An F.A.A. employee later submitted an anonymous complaint about the incident to a safety board inside the agency.

“During meetings regarding this issue the cost to Boeing to upgrade the design was discussed,” the engineer wrote in the complaint, which was reviewed by The Times. “The comment was made that there may be better places for Boeing to spend their safety dollars.”