What happened
On July 11, 1999, a Hoffmann H 36 Dimma motorized glider, registration PH-780, was involved in a fatal accident at Joigny Aerodrome. The aircraft, operated by a club, was performing a travel flight after arriving from Montargis.
During the takeoff roll on runway 08, witnesses observed the aircraft clear an electric cable by only a few meters before proceeding at a very low altitude over a nearby forest. As the aircraft flew over the trees, it began to oscillate on its roll axis and subsequently struck the trees, resulting in an immediate post-impact fire. The accident resulted in two fatalities, involving the pilot and one passenger, and the total destruction of the aircraft.
The investigation
Investigators examined the wreckage and the flight configuration to determine why the aircraft failed to maintain altitude during the initial climb. The examination of the propeller revealed that the blades were set to a "high pitch" setting. This was found to be in direct contradiction to the aircraft's flight manual, which mandates that all takeoffs, landings, and go-arounds must be performed using a "low pitch" setting.
Findings
While the aircraft was able to lift off from the runway due to ground effect, the inappropriate propeller blade pitch setting prevented the engine from providing sufficient thrust to accelerate the aircraft into its first flight regime. This lack of acceleration was particularly critical as the aircraft attempted to clear an electric line.
Several environmental factors likely exacerbated the situation. The high ambient temperature of 26 °C reduced the available engine power, and the presence of possible downdraughts above the forest increased the power required to maintain flight, ultimately leading to the loss of control.