In the letter, Leach said major airlines have been shifting planes among their regional carriers. American Airlines ended its relationship with Trans States Airlines in December 2018.

The company looked at other options, beyond ending operations, he said, but none are “achievable in a realistic time frame.”

“In short, our expenses continue to rise, resulting in a balance sheet that is neither profitable nor sustainable,” Leach wrote.

Stephen Magoc, chair of the Department of Aviation Science at St. Louis University, said regional airlines face a number of challenges these days. Some are pressured by the major airlines’ desire to find the most cost-efficient agreements, and many find it difficult to recruit enough pilots.

Airlines are “losing pilots at a pretty good pace,” Magoc said. The general public is flying more, which translates into a demand for more aircraft, routes, pilots and mechanics, Magoc said.

“It’s just becoming a little bit more critical as time goes on,” Magoc said.