WASHINGTON – The Transportation Department’s in-house watchdog said Thursday it isn’t clear why air traffic controller errors soared by 53 percent last year, but inexperience could explain it in large part.

Inspector General Calvin Scovel told a Senate Appropriations subcommittee the Federal Aviation Administration has placed large numbers of inexperienced controllers at some of the nation’s busiest and most complex air traffic facilities.

The agency plans to hire 11,000 new controllers by 2019 to make up for a wave of retirements, he said. Many of the more than 15,000 controllers now working for the FAA were hired following President Ronald Reagan’s firing of striking controllers three decades ago, and are now eligible for retirement.

Scovel said that as of March, 25 percent of controllers were in training compared to 15 percent in 2004. At some complex facilities, the share is even higher, he added.