Background
The aircraft, a Piper J3C (registration HB-ONB), was originally equipped with a 65 hp Continental engine. In spring 1962, the owner commissioned Airbase GmbH in Zürich to replace the engine with a 90 hp Continental. The conversion required modifications, and the firm, lacking documentation, devised its own solution, including changes to the fuel system. The work was completed on 16 May 1962.
Pre-accident Events
On 15 May, Airbase notified the Swiss Air Office of the modification. The Air Office instructed the airfield chief at Altenrhein to conduct the inspection. The inspection on 16 May was interrupted and later concluded, allowing a test flight on 17 May. No anomalies were reported. After weighing, the aircraft was returned to Altenrhein. The outstanding inspection was deferred.
On 19 May, a flight instructor and his student arrived for water flying conversion training at Lake Constance. The instructor provided theoretical instruction before flights began.
Accident
During the 21st takeoff at 1924 MEZ, the engine unexpectedly stopped. The student, seeing a harbor wall ahead, initiated a turn, causing the aircraft to stall. It struck the water heavily and was severely damaged. Both occupants were uninjured.
Investigation
The investigation focused on the fuel system modifications. It was found that the fuel filter was no longer at the lowest point, and the fuel lines had upward bends above the connection points, which could trap air or affect fuel flow. The engine failure was attributed to these defects in the modified fuel system.