What happened
On June 6, 2015, a Robin DR221, registration F-BOZS, was performing a local instructional flight near Saint-Croll-l’Ecole. During the final stages of the base leg for landing on runway 29 left, the instructor noted that the aircraft was flying too low on the glide path and instructed the student pilot to increase power. Despite several attempts to adjust the throttle, the engine RPM failed to increase.
Recognizing a potential engine failure, the instructor notified air traffic control and took control of the aircraft. The crew performed a forced landing in a field of tall crops. During the landing roll, the aircraft overturned onto its back, trapping the crew inside until firefighters arrived to extricate them.
The investigation
Investigators examined the wreckage and the site to determine why the engine failed to respond to power inputs. During the emergency response, firefighters used water to mitigate fire risks; however, the pressure from the hoses forced debris and water through the air intake, contaminating the engine's intake duct, cylinder heads, and carburetor.
Further examination of the engine and its components revealed contamination by organic sludge and debris. Additionally, analysis of the fuel sampled from the fuel pump showed that the 100LL Avgas had been contaminated with organic matter, resulting in a greenish discoloration. However, the exact source of this contamination could not be confirmed, as the collection container used for the sample may have introduced the pollutants.
Findings
- The engine failed to increase RPM despite throttle adjustments.
- The engine and its internal components were contaminated with organic matter.
- The specific cause of the engine's inability to increase power could not be determined due to the contamination of the engine components by debris during the firefighting efforts and the uncertainty regarding the fuel sample's integrity.