What happened
On November 2, 2008, a Socata MS 880 B Rallye (registration F-BVLD) was performing a flight training mission at Pontoise Aerodrome. During the second circuit of the airfield, while on the downwind leg for runway 05, the instructor noted a progressive decrease in engine RPM, followed by a total loss of engine power. Realizing the aircraft could not reach the runway, the instructor performed an emergency landing in a field approximately three kilometers from the aerodrome. There were no injuries and no damage to the aircraft.
The investigation
Investigators examined the fuel system to determine why the engine failed despite the pilot's attempt to manage fuel levels. During the pre-flight inspection, the instructor had noted that the left tank appeared empty while the right tank appeared full; consequently, the pilot had selected the right tank for the flight. Upon landing, the crew found the left tank was indeed empty, while the right tank remained nearly full. Subsequent engine tests confirmed that the engine would only run when the left tank was selected, and would fail when the right tank was engaged.
Further inspection focused on the fuel selector valve, which had recently undergone repairs on October 30, 2008, following a fuel leak. The investigation revealed that the selector mechanism—a T-shaped cylindrical body containing a cam and an axis—had been incorrectly reassembled. The internal plate was mounted on the axis with a 180-degree offset.
Findings
- The engine failure was caused by insufficient verification during a maintenance operation.
- The fuel selector valve was incorrectly installed, meaning that when the cockpit lever indicated the right tank was selected, the mechanism actually closed the fuel flow from the right tank and opened the left.
- The selector lacked a physical error-proofing mechanism (poka-yoke) to prevent incorrect assembly beyond the engraved numbers on the plate.