What happened
On October 27, 1997, a Robin DR 400-120, registration F-BUPB, was conducting a local flight near Strasbourg, France. After approximately forty minutes of normal cruise flight at 1,800 feet, the pilot noticed the engine RPM dropping from 2,400 to 2,000. Suspecting a fuel delivery issue, the pilot activated the electric fuel pump and applied full throttle, but the engine power failed to recover.
Realizing the aircraft could not reach the nearest airfield, the pilot declared an emergency and planned an off-field landing to avoid populated areas. During the final approach, a tailwind caused the aircraft to overshoot the intended landing site. The aircraft struck a fence and came to rest after crossing a path and a ditch. The impact resulted in two injuries (the pilot and one passenger) and the total destruction of the aircraft.
The investigation
The investigation focused on the engine's performance degradation and the physical state of the fuel system. Investigators examined the engine wreckage and found that all spark plugs were blackened, with significant soot deposits on four of them. Upon dismantling the carburetor, investigators discovered water droplets on the intake butterfly valve.
An inspection of the fuel tanks showed no signs of water or impurities. Ground observers had also reported seeing black smoke emanating from the front of the aircraft during the descent, which is consistent with incomplete combustion caused by an overly rich fuel mixture.