What happened
On July 5, 1969, a Piper PA-18 (registration HB-ORB) was performing a glider towing operation at the Eplatures airfield. While flying on a downwind leg at an altitude of approximately 80 meters, the engine suddenly failed. The pilot immediately released the tow rope, which was subsequently released by the glider crew.
Due to the low altitude and the presence of obstacles, including power lines and a roadway, the pilot was unable to return to the airfield. The pilot executed a 180-degree turn at low altitude to attempt an emergency landing in a nearby hay field. During the landing approach, the pilot maneuvered to avoid a stationary hay wagon. The aircraft made contact with the ground while in a skid, and the momentum caused the aircraft to capsize onto its back.
The investigation
The investigation focused on the cause of the engine failure and the condition of the aircraft's fuel system. While the pilot was uninjured, the aircraft sustained approximately 70% damage, including a deformed fuselage, damaged wings, and a buckled landing gear.
Investigators examined the fuel gauges and the fuel supply. Testing revealed that while the left tank contained sufficient fuel, the right tank was completely empty. The investigation specifically scrutinized the fuel gauge mechanism, which utilized a glass tube with a floating cork ball. Upon disassembly, investigators found that the small orifice in the copper tube, designed to dampen fuel fluctuations, was obstructed by a black, sticky substance.
Laboratory analysis by EMPA confirmed that this substance consisted primarily of corrosion products, likely precipitated by metal oxides and potentially influenced by fuel additives.
Findings
- The engine failure was caused by fuel exhaustion.
- The right fuel tank was empty, despite the gauge indicating a level of approximately one-quarter.
- The fuel gauge was non-functional because the indicator tube was blocked by a buildup of dark, adhesive corrosion products.
- The pilot relied on a faulty reading, which led to an insufficient fuel supply for the duration of the flight operations.