Two passenger jets backed into each other while being pushed by "tugs" at San Francisco International Airport, but no one was injured, a Federal Aviation Administration official said Monday. The collision between a United Airlines 757 and a SkyWest commuter jet happened Sunday evening, FAA spokesman Ian Gregor said. One air traffic controller was directing both airplanes. The SkyWest plane was carrying 60 passengers, and the United plane had none. "It's a fairly common procedure, but obviously in this case, something went wrong and the planes ended up bumping into each other," Gregor said.