A single-engine airplane approaching Ontario International Airport struck high-voltage electric wires and skidded nearly 500 feet along the lines before coming to a halt, dangling precariously, upside down, 100 feet above the ground for more than four hours with its pilot and passenger trapped in the cabin. Shortly before midnight, the two men--neither of whom appeared to be seriously injured--were finally rescued, one at a time, by Ontario firefighters using a snorkel-ladder device after Southern California Edison Co. workers stabilized the aircraft with wires attached to cranes. The two 220,000-volt transmission lines in which the Cessna 172 was snared about two miles east of the airport were shut down shortly after the mishap, but rescuers were forced to move cautiously because the plane was hanging by only one landing gear and reportedly had a full fuel tank that could have exploded if it fell. While the Cessna's occupants waited, rescuers communicated with them via bullhorn. The two men, identified by authorities as pilot Dean Platt and passenger Ed Washburn, signaled back with a flashlight. The aircraft had taken off from Fullerton Municipal Airport and, authorities said, had asked for emergency landing instructions at Ontario.