What happened
On June 23, 1962, a Piper PA-18 (registration HB-OPH) was conducting a local flight near Bienne, Switzerland. After an initial flight from Basel-Mulhouse to Bienne earlier that day, the pilot commenced a local flight in the afternoon. While flying at approximately 3,000 feet near the French border, the engine began to sputter and eventually ceased operation. The pilot attempted to restart the engine using the starter, which briefly succeeded before the engine failed again.
Believing the aircraft was suffering from a mechanical engine failure, the pilot identified a suitable field near Leymen, France, for an emergency landing. The pilot executed the approach with flaps extended, maintaining sufficient airspeed to clear potential obstacles. The aircraft touched down in an oat field and traveled approximately 54 meters before the pilot encountered an unexpected one-meter drop in the terrain. Fearing the aircraft would crash into the depression, the pilot applied heavy braking, causing the aircraft to nose down and pitch upward into an inversion.
The investigation
The investigation focused on the cause of the engine stoppage and the sequence of events during the landing. Investigators examined the fuel system and the state of the aircraft's tanks. It was discovered that while the right fuel tank was full, the left tank was completely empty. The fuel selector valve had been set to the left tank, meaning the engine had been running solely on the remaining fuel in that tank until it was exhausted.
Calculations indicated that the pilot had flown for approximately 33 minutes during the local flight leg, consuming enough fuel to deplete the left tank. The investigation also reviewed the pilot's maneuvers during the emergency landing, noting that the sudden braking in response to the terrain drop was the immediate cause of the aircraft flipping over.
Findings
- The primary cause of the engine failure was fuel exhaustion in the left tank.
- The pilot failed to switch the fuel selector to the full right tank as the left tank depleted.
- The engine stoppage was not a mechanical failure but a lack of available fuel.
- The aircraft inversion was caused by the pilot's sudden braking and aggressive maneuvering when encountering an unforeseen terrain drop.
- The pilot's reaction to the terrain drop was influenced by an overestimation of the obstacle's height.