This story has been updated with a comment from American Airlines.

Pilots at American Airlines (AAL)  said the airline is so committed to making sure airplanes take off on time that in February it left 20,000 passengers stranded at the gate as the door closed on airplanes that had empty seats.

The number was provided to American pilots Thursday in a letter from Dan Carey, president of the Allied Pilots Association.

American, which operates about 6,500 daily flights, boarded about 14.4 million passengers in February.

The carrier's policy is that at D-10, which is 10 minutes before a scheduled departure, the jet bridge door must close. D-0 is the scheduled departure time.

Carey said management is committed to D0 departures "at all costs.

"APA has learned that management's D-0/D-10 (jet bridge door closing) binary measure of operational success led to nearly 20,000 standby passengers being left standing at the gate in February," Carey wrote.

"On almost 13,000 flights, standby passengers were denied boarding even though the aircraft had open seats," he said. "Up to 16% of all AA flights experienced this failure.

"While our union brother and sister gate agents are under extraordinary pressure to close the jet bridge and aircraft doors no matter what, we must do everything possible to ensure that no passengers are left behind while seats are empty," Carey wrote.

"Management's preoccupation with delays has turned into an obsession," he said, noting that the high number of passengers left behind in February reflects "a dramatic uptick in "pilot pushing," he said.

Carey's comments refer to a number, tracked internally by American, of standby passengers who do not board when there's an open seat.

American spokesman Matt Miller said the number includes all standby passengers, some of whom do not actually appear at the gate before a departure.

The number "is imprecise and not representative of reality because it reflects passengers on the electronic standby list, many of whom are not physically present at the gate area during the boarding process," Miller said. "This and many other factors cause any reporting on unaccommodated passengers to be significantly inflated."

The number is tracked as part of "policies and procedures in place to ensure standby and non-revenue passengers are accommodated when we have open seats," he said.

At Philadelphia International Airport, the percentage of American departures that left passengers behind in February was 16%, the highest for all the carrier's hubs, according to American's tracking, pilots said.

At Reagan National, Miami International and Dallas-Fort Worth International, 14% of departures left passengers behind. The percentage was 13% at LaGuardia; about 12% at Charlotte, Los Angeles International and Chicago O'Hare International; and about 10% at New York's John F. Kennedy International Airport, pilots said.

In a 2016 interview, Robert Isom, then American's chief operating officer and now president, said the airline is committed to safety, to the principle of captain's authority, and to on-time operations.

By holding a flight, even for a minute, "you may have handled one customer, but you may have delayed hundreds if not thousands of passengers," Isom said.

"We don't want to put that burden solely in our pilots' hands," he said. "There's no way anybody can know what has to happen with that aircraft throughout the day. It's a complex dance {involving} maintenance, customer service, the {operations} center, flight service, inflight, catering, customs -- a lot of different groups have to come together.

"What we try to do is to connect our pilots with ramp operations control then connect to {the operations center} so we have all the information coming together that no one person could keep straight, then give that feedback to the folks that need it," he said. "When things break down, if we leave a passenger off for some reason, we {typically} have someone who has already taken a look at that and taken care of that passenger."

As for captain's authority, "once the aircraft is loaded and ready to go, there has never been a question of captain's authority," Isom said. "That captain is in charge -- no ifs, ands or buts."

In the 12 months ended in January 2017, American ranked seventh among the 12 largest airlines with a 79.3% on-time performance, according to the U.S. Department of Transportation. However, American ranked third in both December and January, the most recent months reported.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.