What happened
On September 30, 1997, a Rhönlerche II Motorlerche motorized glider, registration OH-230X, was performing a local flight near Liperi, Finland. The pilot had arrived at the airfield to inspect the airspeed indicator connections and decided to conduct a short 20-minute flight.
During the flight, while approaching runway 35, the engine stopped. At an altitude of approximately 150 meters and 1.2 km from the runway, the pilot attempted an emergency landing in a nearby field. However, the aircraft struck the tops of several pine trees approximately 100 meters before reaching the edge of the field. The aircraft became lodged in the trees at a height of about seven meters. The pilot escaped the wreckage without injury, though the aircraft sustained significant damage, including a destroyed left wing, a damaged right wing, a detached right landing gear, and a broken propeller blade.
The investigation
The investigation focused on why the engine failed and the accuracy of the fuel level indication. Technical examinations of the engine, ignition system, and valves revealed no mechanical defects. The investigation found that the fuel system was functional, but the fuel tank was empty at the time of the accident.
Investigators examined the newly installed fuel quantity gauge, a tube-type system. They discovered that the internal diameter of the gauge tube was only 3 mm, which was too narrow. This caused fuel to adhere to the tube walls, creating air pockets that artificially raised the visible fuel level. Furthermore, the use of a flashlight in a dark hangar and the pilot's use of reading glasses made it difficult to accurately observe the fuel level through the transparent plastic cover of the gauge.
Findings
- The engine failure was caused by fuel exhaustion.
- The pilot made an incorrect assessment of the remaining fuel quantity prior to takeoff.
- The fuel quantity gauge was difficult to read and prone to significant measurement errors due to its narrow 3 mm diameter.
- The pilot had limited experience with this specific new gauge, having flown only three missions with it before the accident.
- Environmental factors, including reflections from a flashlight and the pilot's use of corrective eyewear, contributed to the misreading of the fuel level.