On returning from a dual flight to Montpellier Méditerranée, at night, the pilot in training performed a touch-and-go on runway 18 at Nîmes Garons airport. In the middle of the downwind leg on the second runway circuit, while configuring the aircraft for the approach, the instructor pulled the carburetor heat control.
At the end of the downwind leg, while the pilot was turning onto the base leg, the engine speed suddenly decreased. The instructor made an input on the power control and the engine speed rose again. As the aircraft was under the approach path with a speed which was too high, the instructor asked the pilot to level off and decrease power. When the power was reduced, the engine speed dropped abnormally and then the engine stopped.
The instructor took over the controls and took a converging path towards the runway approach lights while maintaining glide speed. He tried unsuccessfully to restart the engine. He was unable to reach the runway and made a forced landing in the runway approach lights.
Several times during the flight, the aircraft was operating in conditions which, in all likelihood, were conducive to severe carburetor icing.
During the second runway circuit, when levelling off at the beginning of the crosswind leg until the middle of the downwind leg, neither the pilot nor the instructor preventively activated the carburetor heat control. It was only pulled in the middle of the downwind leg.
It is highly likely that engine carburetor icing had begun before the instructor activated the carburetor heat control. Those two minutes without heating the carburetor could have been enough to cause the decreases in power experienced on the base leg, and then the engine shutdown.