What happened
On August 17, 2016, at approximately 11:55 local time, a pilot was conducting a local flight departing from Sallanches Mont-Blanc aerodrome in an Apollo Fox ultralight (registration 74-AAA). Shortly after takeoff from runway 35, while the aircraft was climbing on a crosswind leg, the engine ceased operation. An eyewitness observed the aircraft enter a dive toward the ground. Although the engine was heard to restart just before impact, the aircraft struck a field north of the aerodrome with high energy and a steep nose-down angle. The pilot was killed, and the aircraft was destroyed.
The investigation
The investigation focused on the aircraft's mechanical state and the fuel system configuration. Investigators found that the engine was delivering power at the moment of impact. However, analysis of the fuel from both wing tanks revealed significant amounts of dissolved water and gums.
Significant unauthorized modifications to the fuel system were identified. The original composite tanks had been replaced with aluminum alloy tanks, and the system had been altered to include new low-level alarms, additional filters, and a second fuel valve per line. Notably, the fireproof fuel shut-off valve between the collector tank and the engine had been removed. The status of the fuel valves at the time of the engine failure could not be definitively confirmed due to the severity of the impact.
Findings
Several contributing factors were identified regarding the engine failure and the subsequent crash:
- Fuel contamination, specifically the presence of water and gums, may have caused carburetor issues or filter clogging.
- The complexity of the modified fuel system, featuring multiple valves per line, may have caused confusion during fuel selection.
- Moderate icing was a possibility given the ambient temperature conditions.
Ultimately, the accident was caused by the pilot's focus on managing the engine failure at the expense of maintaining flight parameters. The low altitude at which the stall occurred left insufficient time to recover control. The investigation noted that maintaining a glide path to seek an emergency landing site or deploying the onboard ballistic parachute might have mitigated the fatal outcome.