Dan Carey, president of the Allied Pilots Association, said Thursday that nearly 20,000 passengers were denied boarding in February even though there were empty seats. 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 immediately comment.
(David Koenig - Associated Press / ABC News)