What happened
On August 27, 2024, a student pilot was performing circuit training at Żerniki airfield (EPZE) in a Cessna 152, registration SP-KCC. After completing several successful solo circuits, the student began the takeoff for a fifth circuit when they noticed a loss of power, irregular engine operation, and an unusual engine sound. The aircraft failed to climb above 400–450 ft AGL, and the engine ceased functioning approximately five seconds later.
The student maintained a safe landing speed of approximately 70 knots. Following instructions from an instructor observing from the ground, the student performed an emergency landing in a nearby field. During the landing attempt, the student executed a left turn and applied maximum braking to avoid an obstacle. Upon touchdown, the aircraft traveled approximately 80 meters before a nose-over occurred, resulting in damage to the wings, propeller, vertical stabilizer, and rudder.
The investigation
The PKBWL conducted a detailed examination of the Lycoming engine. The investigation included testing the spark plugs, checking cylinder pressure, and measuring valve clearances. The inspection revealed that all cylinders exhibited pressure differences significantly below the manufacturer's minimum requirements. Furthermore, the investigation found that several intake and exhaust valves had excessively small clearances, with all intake valves being below the minimum limit.
Engine teardown revealed significant carbon buildup and evidence of extreme overheating, particularly on the intake valve of cylinder number 4. The investigation also noted the presence of water-oil mixture under the valve covers, likely due to high humidity during storage, though this was not considered a primary cause of the failure.
Findings
- The primary cause of the engine failure was a sequence of events initiated by insufficient valve clearances.
- Small valve clearances allowed exhaust gases to leak into the intake manifold, which leaned out the fuel-air mixture.
- This lean mixture, combined with high operating temperatures, caused a rapid increase in combustion temperatures.
- The resulting heat led to further valve leakage and even smaller clearances, eventually causing the engine to stall.
- The nose-over during the emergency landing was caused by the uneven terrain of the landing site and the application of maximum braking while turning.
- Contributing factors included high ambient temperatures (25°C) and the high frequency of short-duration training flights, which may have contributed to gradual valve overheating.