Fort Worth-based American Airlines is leaving tens of thousands of standby passengers at the gate to make sure its flights take off on time, according to figures from the Allied Pilots Association.

Nearly 20,000 passengers were blocked from boarding in February even though there were empty seats, Dan Carey, president of the Allied Pilots Association, said in a letter to U.S. pilots on Thursday.

“While our union brother and sister gate agents are under extraordinary pressure to close the jet bridge…