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(CNN Business) —  

United Airlines CEO Oscar Munoz will step down from running the airline in May, the company announced Thursday.

Munoz will become chairman of the airline, and he will be succeeded as CEO by Scott Kirby. Kirby has been president of United since the airline wooed him away from rival American Airlines (AAL) three years ago.

Munoz is best known for being the face of the airline during the public relations debacle, when a passenger was dragged off an overbooked flight in April of 2017. In the face of outrage he testified before Congress that “we had a horrible failure.” The incident sparked changes in the way United and other airlines deal with the overbooking of flights.

“When I joined United as CEO, I laid out ambitious goals to build a new spirit of United by regaining the trust of our employees and customers — and I’m proud of how far we’ve come,” Munoz said. “With United in a stronger position than ever, now is the right time to begin the process of passing the baton to a new leader.”

Shares of United were slightly higher in premarket trading on the news. The stock has risen 54% during Munoz’s more-than four years on the job. It’s been a generally good time for airline stocks in an age of consolidation of the US industry, and a strong economy lifting travel demand and fares.

Munoz’s tenure at United started with another controversy, in which his predecessor Jeff Smisek and two other executives left in September 2015. The executives exited after the US government investigated the airline for doing favors with a state official who had authority over Newark Airport, where United has a major hub.

But five weeks after taking over as CEO, Munoz suffered a heart attack, and he had a heart transplant several months later. He returned to the job two months after the heart transplant.

Munoz had been expected to be named chairman in addition to CEO before the heart attack. but instead the airline kept the two jobs separate. The current chairman, Jane Garvey, will retire from the board in May 2020 when Munoz takes the new role.