While the aircraft was cruising at FL190, about three hours after leaving Cardiff-Rhoose Airport, for Palma de Mallorca Airport the captain noticed that no. 2 engine was de-synchronized, as compared with no’s 3 and 4, and its rpm were rapidly decreasing. He then heard a “thump”, and saw that the jet pipe temperature on no.2 engine was low. At this time, a sudden, and very rapid, increase in no. 2 engine rpm occurred; before propeller feathering action could be completed, the propeller broke away and punctured the fuselage, causing an explosive decompression. There was no fire but fire drill was carried out as a precaution. An emergency landing was made at Barcelona, without further incident. The puncture in the fuselage proved to be a gash about 5 to 6 ft . long and 6 ins. wide in line with the port forward toilet. It was later established that failure or a tooth in the reduction gear led to disruption or the reduction gear casing, and this in turn led to the shedding of the propeller.