What happened
On May 16, 2017, an AYRES S2R, registration F-GIEN, was performing agricultural spraying operations (mosquito control) over the Étang de Mauguio in France. The aircraft, operated by Société DELTA2, was flying at a very low altitude while approaching a treatment area near the Montpellier-Fréjert contrôleur zone.
During the approach, the pilot noticed the engine decelerating while flying over the pond. In an attempt to manage the emergency, the pilot activated the manual pump to prime the engine without success and released the chemical payload, which caused the aircraft to pitch up sharply. The pilot declared a MAYDAY on the Montpellier frequency.
Due to the low altitude, the aircraft struck the water with a high pitch attitude, causing it to nose-over and capsize. The cockpit became submerged, but the pilot managed to force open a side door with his feet and escape the sinking cockpit. The pilot was subsequently rescued by a fisherman in a nearby boat.
The investigation
The investigation focused on the cause of the engine failure and the recent maintenance performed on the aircraft. The aircraft had undergone a carburetor replacement the day before the accident due to high fuel consumption. This work was performed by the pilot, who holds a valid Part 66 maintenance license, and was validated by the aircraft owner, MIDAIR.
Technical examinations of the dashboard, fuel system, and engine did not immediately reveal the cause of the shutdown. However, specialized laboratory analysis of the carburetor revealed a defect in the float assembly. While the engine's pre-flight static run showed no issues, investigators looked into the recent overhaul of the carburetor components.
Findings
Investigators determined that the engine failure was caused by a leak in one of the carburetor floats. A non-homogeneous brazed seam between the two halves of the float made it permeable. As fuel entered the float, the weight caused the float-and-lever mechanism to drop, allowing an excessive amount of fuel to flow into the cylinders. This created an excessively rich mixture that prevented combustion, leading to engine choking and subsequent failure.
This latent defect was not detected during the carburetor's overhaul in November 2016, nor during the pilot's pre-flight engine tests, as the accumulation of fuel within the float was a progressive process.