What happened
On 1 August 2021, a Reims Cessna FRA150M, registration G-BDNR, was performing a training flight near the Humber Bridge when the engine began running roughly approximately 5 nautical miles from Retford Gamston Airport. During a carburettor heat check, the pilot observed the right engine cowling protruding outward. Shortly thereafter, the engine lost power and ceased operation.
The pilot transmitted a MAYDAY call and executed a forced landing in a field roughly 4.5 nautical miles north-northeast of the airport. While the aircraft touched down within the field, it struck a hedge and came to rest upside down. The pilot and one passenger were able to exit the aircraft without assistance; the passenger sustained one minor injury.
The investigation
Investigators discovered that the number 3 cylinder and piston had broken free from the engine and been ejected through the cowling during flight. A detailed examination of the engine crankcase revealed that the base studs for the number 3 cylinder, along with two through-studs, had failed. Metallurgical analysis showed that these failures were caused by crack progression due to high-cycle fatigue.
Further research into engine maintenance practices revealed that during the overhaul of RR O-240 engines, replacement studs were frequently stripping their threads before reaching the required torque. Testing showed that the nuts used for fastening had a higher tensile strength than the studs, creating a mismatch of tensile strength that caused the nuts to fracture the thread crests of the studs during installation. This resulted in a loss of torque and a dangerously small safety margin.
Findings
- The engine failure was caused by the number 3 cylinder and piston detaching from the engine.
- The primary cause of the stud failure was fatigue crack progression.
- A mismatch in tensile strength between the cylinder base nuts and the studs led to thread stripping during maintenance.
- The stripping of threads prevented the studs from being tightened to the required torque values, leaving a minimal safety margin.