History of Flight
On 19 April 2005 at 1100 UTC, a Cessna F152 (registration G-BHFC) was conducting training flights near Hardwick Airfield, Norfolk. The aircraft had completed two stalls without incident as part of a Private Pilot's Licence renewal. During application of full power for a third stall recovery at an altitude of 3,000 feet, the pilot noticed severe airframe vibration emanating from the engine. The pilot issued a MAYDAY call to Norwich Approach and began searching for a suitable forced landing site. Shortly thereafter, the pilot observed pieces of the engine departing through the cowling; moments later, a loud bang occurred and the propeller ceased rotating. The pilot successfully executed a forced landing at Hardwick, a disused American wartime airfield, with no further damage to the aircraft and no injuries to the pilot or the student.
Engine Description
The Lycoming O-235-L2C engine is a flat-four configuration with a single camshaft carrying six cams that drive eight followers. Cam numbers 2 and 5 each drive two followers; the remaining cams drive one follower each. At the time of the incident, the engine had accumulated approximately 2,600 hours, operating on an extension beyond its nominal 2,400-hour life.
Engine Examination
The engine was removed and sent to the AAIB in Farnborough for a detailed strip examination. Significant damage was found, most notably to cylinder No. 4 (aft left), which had failed around its base where it attached to the crankcase. The fracture surfaces were heavily damaged, precluding detailed examination. All four cylinders exhibited pitting corrosion, but the size and pitch of the cooling fins prevented accurate assessment of its extent.
Piston No. 4 was extensively damaged below the center line of the gudgeon pin; the piston had seized inside its cylinder, and the connecting rod had fractured approximately halfway along its length. The upper portion of the connecting rod and the gudgeon pin were found in the oil sump. The remaining part of the connecting rod, still attached to the crankshaft, was severely twisted due to abnormal loads. Cylinder No. 3 also sustained some damage near its base.
The crankcase was cracked over approximately 6 cm in length and up to 2 cm wide. Extensive internal damage covered an area of about 5 by 15 cm, where much of the crankcase structure forming the oil sump 'roof' had broken away. Both the crack and internal damage were located opposite cylinder No. 4 and were likely caused by the detached portion of the No. 4 connecting rod.
Inspection of the camshaft revealed that two of the six cams were significantly worn, with corresponding cam followers showing pitting and wear. The height-to-width ratios for cam Nos. 1, 3, 4, and 5 ranged from 1.30 to 1.32, while cam Nos. 2 and 6 had ratios of 1.15 and 1.20, respectively. Although such wear was unlikely to have been a major factor in the engine failure, it probably caused some loss of engine performance due to reduced valve lift.
Conclusion
The investigation concluded that the most likely cause of the engine failure was the failure of cylinder No. 4 due to pitting corrosion induced fatigue cracking around the base of the cylinder. This allowed the cylinder to break free, subsequently causing all the observed damage. The French DGAC had reported a significant number of cylinder barrel failures on Lycoming O-235 engines, including the -L2C mark, initiated by corrosion pits near the cylinder base. In response, Textron Lycoming issued Service Instruction No. 1504 in January 2001, recommending replacement of cylinder assemblies with new ones having improved corrosion resistance.