Air Accidents Investigation Branch
Summary:
The number 3 cylinder and piston broke free from the engine causing engine failure during flight. A forced landing was carried out in a field resulting in significant damage to the aircraft but only minor injury to the passenger.
Examination of the engine crankcase found that the number 3 cylinders base studs had all failed in fatigue due to crack progression. When cylinder studs were replaced with new items on other engines of this type during overhaul or maintenance, some of the studs threads stripped before the required torque values could be achieved. Analysis revealed that the nuts used to fasten the cylinders were distorting and stripping the threads of the studs before reaching their required torque value or were failing at values just above the published maximum, leaving only a small safety margin. The investigation revealed that there was a mismatch of tensile strength between the nuts and studs.
Safety actions have been taken by the Type Certificate Holder to introduce a Service Bulletin to replace cylinder base studs during RR O-240 engine overhaul and carry out repetitive torque checks following their replacement. The cylinder base studs will be replaced with compatible alternative base studs which achieve consistent torque values above the maximum stated within the engine manuals.