What happened
On May 19, 1962, a Piper J3C (registration HB-ONB) was conducting water-based flight training on Lake Constance near Altenrhein. During the 21st takeoff of a training session, the aircraft's engine unexpectedly failed while the plane was in a climbing attitude. The student pilot, attempting to avoid a nearby harbor wall, initiated a sharp left turn. Due to the aircraft being significantly overweight, the maneuver caused the plane to stall and strike the water with its left wing and float. While the aircraft sustained heavy damage to the wings, propeller, and float, both the instructor and the student escaped without injury.
The investigation
Investigators examined the aircraft's recent modifications and the circumstances of the flight. The aircraft had recently undergone an engine upgrade, replacing a 65-ps motor with a 90-ps Continental engine. This modification, performed by a local maintenance company, required changes to the fuel system. The investigation also reviewed the inspection process conducted by the local airfield chief, who had approved the aircraft for flight following a brief check without reviewing technical documentation. Furthermore, the investigation looked into the aircraft's weight, finding that the takeoff weight of approximately 625 kg significantly exceeded the maximum allowable weight of 590 kg for the floatplane configuration.
Findings
- The primary cause of the engine failure was defective fuel line routing within the newly modified fuel system. The lines between the tank, filter, and carburetor contained upward-curving sections that allowed air pockets or fuel vapor to form. As fuel levels depleted to approximately 11 liters, these air pockets interrupted the fuel flow during the climb.
- The aircraft was operating in an overloaded state, which contributed to the loss of control during the recovery maneuver.
- The student pilot's abrupt turn, while a reaction to the engine failure and the proximity of the harbor wall, directly led to the stall.
- The inspection of the engine modification was insufficient, as the inspector failed to identify the risks posed by the improper fuel line installation.