What happened
On June 28, 2003, a Cessna 152F, registration G-BIOM, was performing a VFR flight from Santa Cruz Aerodrome to Montargil. During the return leg, while the aircraft was at approximately 1,000 feet and approaching the destination, the engine began to malfunction. The pilot reported that engine RPM fluctuated, dropping between 1,000 and 1,500 RPM below normal cruise levels.
Unable to maintain altitude or reach the runway, the pilot executed an emergency landing in a nearby agricultural field near Paradas, Torres Vedras. Although the field appeared clear from the air, a shallow drainage ditch, hidden by vegetation, ran perpendicular to the aircraft's landing path. After the initial touchdown, the nose gear struck the ditch and collapsed, causing the aircraft to capsize and come to rest in an inverted position.
The pilot and passenger both exited the aircraft uninjured.
The investigation
The GPIAAF investigation focused on the engine's performance degradation and the landing site conditions. Investigators examined the fuel system, ignition system, and engine compression. While the fuel was found to be clean and free of contamination, compression tests on cylinders #1 and #2 revealed significant leaks through the exhaust valves.
Upon opening the cylinders, investigators discovered solid deposits on the valve seats and stems. Further inspection of maintenance records revealed that the engine was being operated with 100LL Avgas, which has a higher lead content than the 80/86 octane fuel recommended by the manufacturer, Lycoming. The investigation also noted that the aircraft's maintenance program utilized 50-hour oil change intervals, whereas the manufacturer recommended 25-hour intervals when using higher-lead fuels to mitigate lead buildup.
Findings
- The primary cause of the accident was the nose gear collapse and subsequent capsize caused by the aircraft striking a hidden ditch during an emergency landing.
- The emergency landing was necessitated by a partial loss of engine power.
- The engine failure was caused by "valve sticking" in cylinders #1 and #2, resulting from lead deposits on the exhaust valves.
- The use of 100LL fuel, which contains more lead than the recommended 80/86 octane, contributed to the accumulation of these deposits.
- The maintenance program failed to implement necessary precautions for high-lead fuel usage, such as installing full-flow oil filters or reducing the oil change interval to the recommended 25 hours.