A United Airlines DC-10 carrying 234 passengers from San Francisco to Chicago made an emergency landing at O`Hare International Airport Wednesday after a wing panel fell off in flight. Matt Gonring, a United spokesman, said Flight 122 landed without other incident at 1:07 p.m. The 2-foot-by-6-foot left upper-wing panel came off over Dubuque, Ia., and struck an engine, damaging hydraulic lines that led to instruments that indicate whether the left landing gear was down and locked, Gonring said. Midair visual inspection by the pilot of another aircraft indicated that both landing gears were down.