DALLAS (AP) — The head of the pilots’ union said Thursday that American Airlines is leaving thousands of standby passengers at the gate to make sure that flights depart on time.

Nearly 20,000 passengers were denied boarding in February even though there were empty seats, Dan Carey, president of the Allied Pilots Association, said. He called it intolerable.

American carried more than 14 million passengers last month.

The airline has been trying to improve its on-time performance. It ranked eighth among the nation’s largest 12 airlines at on-time arrivals in 2016, but rose to third in January, the latest month for which government figures are available.

American did not dispute that some passengers and employees flying standby get left behind. But, spokesman Matt Miller said, the company figure cited by the union president is “significantly inflated” because it includes people who signed up for standby but weren’t in the gate area when the door shut.

ADVERTISEMENT

Carey said standby passengers are left behind on about one in six flights, and he blames the airline’s policy of closing the door from the gate area to the jet bridge 10 minutes before scheduled departure. He says gate agents are under extraordinary pressure to close the door no matter what, but that passengers should not be left behind if there are empty seats.