What happened
On 19 April 2005, a Cessna 1t52, registration G-BHFC, was conducting a training flight near Hardwick Airfield, Norfolk. The flight was part of a Private Pilot's Licence renewal procedure. After successfully completing two stalls, the pilot applied full power to initiate a third stall recovery at an altitude of approximately 3,000 feet. During this maneuver, the pilot experienced intense airframe vibrations originating from the engine.
Following a MAYDAY call to Norwich Approach, the pilot observed engine components being ejected through the cowling. A loud bang was heard, and the propeller ceased rotation. The pilot managed to perform a forced landing at Hardwick, a disused wartime airfield. There were no injuries to the pilot or the passenger, though the aircraft sustained significant engine damage and minor airframe damage.
The investigation
The AAIB conducted a detailed strip examination of the Lycoming O-235-L2C engine. The investigation revealed that the engine had been operating at approximately 2,600 hours, which was an extension beyond its nominal 2,400-hour life.
Examination of the engine components showed that cylinder No 4 had failed at its attachment point to the crankcase. The piston within this cylinder had seized, and the connecting rod had fractured halfway along its length. The remaining portion of the connecting rod, still attached to the crankshaft, had suffered severe twisting due to abnormal loads. This failure caused a crack in the crankcase and destroyed much of the oil sump structure. Additionally, while the camshaft showed some wear on two cams, this was not considered a primary factor in the failure.
Findings
- The primary cause of the engine failure was the failure of cylinder No 4, which was triggered by fatigue cracking around the cylinder base caused by pitting corrosion.
- The detachment of the cylinder led to the subsequent destruction of the connecting rod and extensive internal damage to the crankcase.
- All four cylinders showed evidence of pitting corrosion on the cooling fins.
- The engine was operating beyond its recommended service life of 2,400 hours.