“We had a customer who refused to leave the aircraft,” he said. “We have a number of customers on board that aircraft, and they want to get to their destination on time and safely, and we want to work to get them there.

“Since that customer refused to leave the aircraft, we had to call” the police, and they came on board, he said.

The Chicago Department of Aviation said in a statement on Monday that the incident “was not in accordance with our standard operating procedure” and that an officer had been placed on leave pending a review of the matter. The department declined to identify the officer.

Airlines routinely sell tickets to more people than the plane can seat, counting on several people not to arrive. When there are not enough no-shows, airlines first try to offer rewards to customers willing to reschedule their plans, usually in the form of travel vouchers, gift cards or cash. The arrangement — which is usually negotiated before passengers board the plane — can be lucrative to flexible travelers and is crucial for airlines to maximize profit.

“A bakery doesn’t want to have a lot of extra pastries at the end of the day they have to throw out,” said Seth Kaplan, managing partner at Airline Weekly, an industry publication. “To an airline, an empty seat is basically the same thing as stale bread. It’s something they can never sell again.”

But involuntarily bumping passengers is rare. In 2016, United involuntarily denied boarding to 3,765 of its more than 86 million passengers on oversold flights, according to the Transportation Department. An additional 62,895 people voluntarily gave up their seats.



The event on Sunday was the second social media stir for United in two weeks. In March, two girls were barred from a flight because they were wearing leggings, which the company said violated its dress code for a benefit for United employees and their dependents. Critics called the policy sexist and overbearing.