A federal judge ordered the union for US Airways pilots to stop intentionally delaying flights, finding that it had engaged in a “concerted effort to disrupt the airline.”

US Airways saw its June on-time ranking drop to last place among big airlines as it struggled with delayed flights and a computer outage. In July the airline went to court, saying pilots were deliberately delaying flights.

The airline is the product of a 2005 merger between the old US Airways and America West. Pilots from those two airlines still fly under separate union contracts, and the airline said the slowdowns were concentrated among the so-called “East” pilots who came from the old US Airways.

Labor law bars aircrews from staging sick-outs, slowdowns or other otherwise disrupting an airline. The US Airline Pilots Association had strongly denied any intentional slowdown. And, in an Aug. 17 e-mail to pilots, USAPA President Michael Cleary told pilots, “To be crystal clear, you must not intentionally cause any delay where the purpose is to alter the status quo.”

But U.S. District Court Judge Robert J. Conrad Jr. in Charlotte, N.C., found Wednesday that the union was “engaged in an illegal slowdown.” Stickers left on airplanes appeared to encourage them to make sure flights were at least 16 minutes late. Pilots were accused of taxiing slowly. The airline said the rate of fatigue calls from East pilots more than doubled after May 1.

Conrad also found that pilots intentionally delayed completing online training by a May 31 deadline. The airline often schedules pilots for four-day trips, and they can’t start the trip if their training expires before a trip ends. Three days before the deadline, 897 pilots had not finished the training — all but one of them East pilots, according to the airline. The Federal Aviation Administration ended up granting a waiver allowing the airline to schedule the pilots who had not finished the training.