What happened
On September 23, 2012, a Cessna 206, registration F-GBBY, departed from Saint-Dié-des-Vosges aerodrome for a skydiving flight. During the 12th flight of the day, as the pilot reached approximately 300 feet and retracted the flaps, the engine failed to provide sufficient power for the climb. The pilot reported that the flight controls felt "soft" and that the aircraft appeared to sink whenever a left turn was attempted.
After attempting to switch fuel tanks and adjusting the propeller to full fine pitch without success, the pilot declared a "MAYDAY." Due to the terrain along the intended flight path, the pilot performed a forced landing in a field. During the landing roll, the nose gear sank into the soft earth, causing the aircraft to capsize onto its back. The accident resulted in two injuries and the total destruction of the aircraft.
The investigation
Investigators examined the Continental IO-520-F engine and found that the air-fuel mixture filter was clogged with heterogeneous deposits. While the upstream and downstream fuel system filters were clean, three of the engine's six injectors were blocked. Following the cleaning of this specific filter, the engine performed normally during bench testing.
Regarding the flight controls, the investigation focused on the Navomatic 300A autopilot system. Although the owner had previously notified the pilot via email that the autopilot was inoperable, the pilot had not tested it. Evidence showed that the pilot had inadvertently activated the autopilot while retracting the flaps. Because the system uses an electronic clutch rather than a mechanical one, the pilot did not feel any physical resistance in the controls, which led to the perception of ineffective ailerons during turns.
Findings
- The primary cause of the power loss was the obstruction of the air-fuel mixture filter by deposits that had accumulated over a long period.
- The investigation suggests that the required cleaning of this filter was likely omitted during previous 100-hour maintenance inspections.
- The pilot's perception of heavy or ineffective control response was caused by the unintentional engagement of the autopilot in a mode attempting to maintain zero bank angle.