What happened
On October 6, 2014, a Reims Aviation F150G, registration HB-CSM, departed Langenthal airport for Môtiers with one passenger on board. During the flight, while flying below the Bern TMA and approaching the destination, the pilot began a descent near Noiraigue. At this stage, the engine began to lose power without any prior warning or unusual noises. Attempts to rectify the issue by adjusting the power lever or applying carburetor heat were unsuccessful.
Realizing the aircraft could not maintain altitude to reach the intended airfield, the pilot initiated a turn to perform an emergency landing in a valley between Travers and Noiraube. During the final approach, the aircraft struck several treetops before making a hard impact with the ground. The aircraft sustained heavy damage, but the pilot and passenger were able to exit the aircraft with only one person seriously injured.
The investigation
The investigation focused on the fuel levels and the accuracy of the aircraft's monitoring systems. Post-accident inspections revealed that only 7.2 liters of fuel remained in total across both wing tanks, with the left tank containing a maximum of 5 liters and the right tank containing 3 liters. This was below the unuseable fuel limit of 6.5 liters per tank specified in the aircraft flight manual.
Investigators analyzed the reliability of the manual dipstick measurements and the cockpit's electrical fuel gauges. Testing demonstrated that while dipstick measurements were relatively stable at higher fuel volumes, they became highly inaccurate as levels dropped. Specifically, when simulating the estimated fuel levels present at takeoff, the dipstick readings varied significantly depending on aircraft movement. Furthermore, the investigation found that the electrical fuel quantity indicators in the cockpit provided highly inaccurate readings, failing to reflect the actual low fuel state.
Findings
- The engine power loss was most likely caused by fuel starvation.
- The cockpit's electrical fuel gauges were faulty and provided misleading information regarding the remaining fuel.
- Manual fuel measurements using a dipstick are prone to significant error when fuel levels are low or when the aircraft is in motion.