What happened
On May 19, 2009, a single-seat Mignet "Pou du Ciel" ultralight, identified as 74-YG, departed from the unpaved runway 23 at Albertville aerodrome for a local flight. During the downwind leg of the circuit, the aircraft entered a steep dive and struck the ground in the residential neighborhood of Fronten/enex. A witness reported that the engine was producing power during the descent and that the aircraft followed a straight flight path. The impact resulted in the death of the pilot and the total destruction of the aircraft.
Prior to the accident, the pilot had acquired the aircraft only three days earlier, following test flights performed by the manufacturer. The pilot had spent the previous day preparing the aircraft for its first flight under his ownership.
The investigation
The investigation focused on the aerodynamic stability of the aircraft and the mechanical integrity of the flight controls. Investigators confirmed that the mechanical linkages for the flight controls remained continuous upon inspection of the wreckage. While the engine was functional, its exact power output at the moment of impact could not be determined. Medical examinations of the pilot revealed no physical abnormalities that could have impaired his ability to fly the aircraft.
Findings
The investigation identified the inappropriate application of the elevator control as the primary cause of the accident. Because the 'Pou du Ciel' design relies on varying the incidence of the forward wing to control pitch, the investigation explored several aerodynamic phenomena that could lead to such a loss of control:
- An aerodynamic phenomenon where an increase in the incidence of the forward wing creates a Venturi effect. This increases lift on the rear wing, causing a nose-down pitch. If the pilot attempts to correct this by pulling back on the controls, the reduction in the vertical gap between the wings can amplify the effect, leading to a self-reinforcing dive.
- A sudden forward movement of the elevator control could momentarily force the forward wing into a negative angle of incidence, causing an unrecoverable pitch-down motion.
- An improper center of gravity, specifically a center of gravity located too far aft, could necessitate excessive elevator deflection, making the aircraft susceptible to entering a negative angle of incidence.