The IFR positioning flight from Lille, France to Wroclaw, Poland departed Lille at 12:40 hours GMT and was cleared to climb to FL160 on a heading to “Silly”. At 12:42 hours the pilot contacted Brussels ACC and reported he was at 3000 feet, still climbing and estimating “Silly” at 12:49 hours. At 12:53 hours he reported aver “Silly” at FL120, and was cleared to climb and maintain FL130. The flight was heading for “Gatta”, estimating to be there at 13:00. Around 12:56 a French Caravelle which had just passed “Gatta” requested permission to make a detour to avoid an area of intense storm activity. The Viscount after passing “Gatta”, continued to “Olno”. Before reaching that beacon, the flight descended, disintegrated and crashed. PROBABLE CAUSE: “No evidence was found to explain why the aircraft left its cruising level. The overall atmospheric conditions in the vicinity of Jeuk and the circumstances of the accident were such that it was assumed that the pilot lost control of the aircraft when entering a cumulonimbus. It is possible and even highly probable that turbulence was a determining factor in the accident.”