(The Hill) – A glitch in an American Airlines platform for trading flights allowed pilots to drop thousands of trips throughout July on Friday, the airline’s pilots’ union told CNBC on Saturday. 

American Airlines said in a statement to The Hill that the technical glitch permitted certain trading transactions to be processed that should not have, but most flights that were affected have been reinstated. 

“We already have restored the vast majority of the affected trips and do not anticipate any operational impact because of this issue,” American said in the statement. 

The union, the Allied Pilots Association, told CNBC that up to 12,000 flights in July lacked a captain, first officer or both after the glitch allowed pilots to drop flights.

The union did not immediately return a request from The Hill for comment. 

Pilots often drop or pick up flights, but getting time off is more difficult during the summer and holidays as demand for air travel rises, CNBC reported. 

The Transportation Security Administration reported its busiest travel day on Friday since February 2020, before the COVID-19 pandemic. 

Thousands of delays and cancellations of U.S. flights have occurred over the past two days amid the start of the holiday weekend, according to the flight-tracking website FlightAware. Airlines have struggled to keep up with high demand as staffing issues continue and people increasingly return to normal activities after travel demand plunged earlier in the pandemic.

Dennis Tajer, a spokesman for the union and American Airlines pilot, told CNBC that the airline did not keep its IT system functioning properly, creating “uncertainty for passengers and pilots.” 

CNBC reported that American Airlines suspended access to the platform while it looks into the glitch. The airline said in an email to pilots on Saturday that it understands the platform is an important tool and is working as quickly as possible to investigate what happened.