What happened
On 10 November 2019, a TECNAM P92-JS, registration F-HBSV, was conducting a flight instruction mission from Bellegarde-Vouvray to Chambéry Challes-Eaux. During the approach to runway 32, the student pilot was approximately 50 feet above the ground when the instructor initiated a go-around due to the aircraft being too high on the glide path.
Approximately ten seconds into the missed approach, while climbing through 250 feet, the instructor noted severe vibrations and a significant reduction in engine power. The instructor took control and attempted to use the carburettor heat to mitigate the issue, but the action provided no relief. To stabilize the aircraft and stop the vibrations, the power was adjusted, but the remaining thrust was insufficient to maintain level flight.
Unable to find a suitable emergency landing site ahead, the instructor performed a left-hand turn to return to the aerodrome, converging on runway 14. The aircraft touched down approximately 100 meters from the end of the runway, overran the paved surface, and entered the grass. While traversing the grass, the aircraft struck a rut, causing the nose gear to fail and bringing the aircraft to a stop. There were no fatalities or injuries.
The investigation
The investigation focused on the cause of the engine power loss and the functionality of the carburettor heating system. Investigators examined the engine's airbox and the duct responsible for delivering heated air from the exhaust manifold to the carburettors.
Technical examination revealed that the air duct, which consists of a rubber casing and a metal spiral, had broken off. The metal spiral showed signs of corrosion, and the resulting damage had torn the duct casing. While the exact timing of this failure could not be determined, investigators considered whether the break occurred prior to the flight, which would have prevented hot air from reaching the carburettors effectively.
Findings
- The primary cause of the power loss was likely carburettor icing facilitated by a malfunctioning heating system.
- The air duct supplying hot air to the carburettors was found to be broken due to a corroded metal spiral.
- This damage likely rendered the carburettor heat control ineffective during the descent phase, allowing ice to accumulate in the intake system.
- The engine power was insufficient to maintain level flight following the adjustment made to mitigate vibrations.
- The runway excursion and subsequent nose gear failure were caused by the aircraft overrunning the runway and striking a rut in the grass.