On the return leg from a training flight to the Humber Bridge, the number 3 cylinder and piston broke free from the engine, which caused engine failure during the flight. The pilot carried out a forced landing in a field, which resulted in significant damage to the aircraft and minor injuries to the passenger on board.
An examination of the engine crankcase found that the number 3 cylinder’s studs had failed due to crack progression in fatigue. When the studs were replaced during overhaul a number of the studs’ threads stripped before the required torque values could be achieved. This issue was also identified on a number of other engines of this type. Analysis revealed that the nuts used to fasten the cylinders were distorting and stripping the stud threads before reaching the required torque value or, were failing at values just above the required torque.
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