A de Havilland Canada DHC-8-402 Q400 passenger plane sustained substantial damage in a landing accident at Busan-Gimhae (Pusan) International Airport (PUS), South Korea. Four of the 79 occupants suffered minor injuries. Jeju Air flight 502 departed Jeju (Cheju) International Airport (CJU) at 08:49 for a domestic flight to Busan. While the aircraft passing through an altitude of 9,500 feet, RUD CTRL caution lights and #2 RUD HYD caution lights illuminated. Accordingly, the pilots followed and carried out the procedures of QRH (Quick Reference Handbook). According to the pilots, #2 RUD HYD caution lights illuminated constantly during the flight; however, RUD CTRL caution lights illuminated intermittently. As it reached a cruising altitude of 15,000 feet, the pilots notified the condition of malfunctions and actions taken according to the QRH to a mechanic stationed at the Gimhae Airport. The pilots received a clearance from the Gimhae Approach Control for ILS runway 36L then circling approach to runway 18R. While turning downwind for the circling approach, the caution lights of Elevator Feel, RUD CTRL, Pitch Trim and other warning lights illuminated on the caution and warning lights panel. However, the pilots stated that they couldn’t recall all the caution lights illuminated at the time, and did not take any measures considering the illuminating lights as “nuisance.” The first officer who was Pilot Flying continued the circling approach, and aligned his aircraft with the runway 18R on the final approach course. Wind direction/speed was 130 degrees at 13 knots gusting to 18 knots, ceiling 4,000 feet, and it was mostly cloudy. From the point where the main landing gear touched down, the aircraft continued to drift left into the wind. Initially the pilots applied right rudder in an attempt to maintain runway center-line. Rather then apply left wing down, the control wheel input was toward the right. As the captain kicked onto the rudder pedal and said, “Why, why, Ah?” and the first officer replied, “it doesn’t respond.” As the nose landing gear touched down, and the pilots started to apply brakes. Then all of sudden, the deceleration rate dropped. The aircraft departed the left edge of the runway at 3,500 feet from the end of runway. The aircraft collided into a concrete drainage ditch, which is located 340 feet away from the centerline of the runway 18R and 4,600 feet away from the end of runway, and then it came to rest. When the aircraft stopped in the concrete drainage ditch, the pilots shut off the right engine, declared emergency to the air traffic control tower, and instructed the passengers to perform emergency evacuation. CAUSES: The Aviation and Railway Accident Investigation Board determines that the cause of the runway excursion of the Flight 502 was that the rudder failure was not recognized by the pilots during flight and as well as during landing roll. Contributing to this accident was that 1) the rudder was failing to respond to the pilots’ rudder pedal input and 2) After departing from the runway, no appropriate alternative measure was taken to control the aircraft direction.