DALLAS (AP) — The hiring sign is out at United Airlines. The airline said Wednesday that it plans to add 15,000 workers this year, and hired 7,000 in just the past four months. It’s part of a trend, as airlines benefit from a boom in travel. Airlines are trying to avoid a rerun of last year, when they were caught unprepared for a rapid recovery in air travel. Shortages of pilots and flight attendants contributed to a jump in the rates of canceled and delayed flights. United says it’s got enough pilots to operate its summer schedule.