An Ilyushin Il-18E, operating on a flight for Bulair, collided with terrain near Karnobat, Bulgaria. Of the 89 persons on board, 47 were killed. The aircraft operated on a flight from Dresden to Bourgas. The flight encountered poor weather conditions with extensive thunderstorms, brief showers and low clouds. During the flight the aircraft deviated from the airway and approved flight levels, apparently to avoid these thunderstorms. After passing the RUS NDB at 19:18 hours at a 5400m a continuous descent was made, passing over the TRG VOR at 19:24 hours to proceed to Lyulyakovo descending to 2400m. However the aircraft descended to 1300m. After passing Lyulyakovo at 19:30 hours the flight was conducted below clouds with the cloud base lowering. At about 19:35 hours, the aircraft was observed over the northern suburbs of Karnobet flying at a height of 50-150 m. Shortly thereafter it collided wish the ground. PROBABLE CAUSE: Deviation from the prescribed flight rules in an effort to accomplish a visual approach in adverse weather conditions.