Federal aviation investigators urged closer inspection of engine mounts on hundreds of Boeing 737 aircraft to detect possible cracking after an engine fell off a USAir jet during flight earlier this month. The National Transportation Safety Board said the engine separated from the plane shortly after it departed the Philadelphia airport because of cracking caused by metal fatigue in one of the three bolts holding the engine in place. When the rear bolt separated, the two forward bolts separated because of stress, as did a support cable, the investigators said. The pilot returned to Philadelphia and made a safe landing. No one was injured in the Dec. 5 incident. Safety board officials asked the Federal Aviation Administration to direct airlines to check the aft cone attachment bolt on all Boeing 737 model 100 and model 200 aircraft.