What happened
On August 3, 1963, a Bellanca "Crusair Senior" 14-13-3, registration HB-DUN, departed from Thun airfield for a private flight carrying the pilot and three passengers. Shortly after takeoff, while the pilot was performing a climbing left turn at approximately 80 mph, the engine RPM suddenly dropped from its operating speed to 1500 RPM.
As the aircraft began to lose altitude, the pilot executed a tight turn to avoid a line of trees located near the edge of the airfield. Despite the loss of power, the aircraft remained controllable, allowing the pilot to level the wings just before touchdown. The aircraft struck the ground with significant force, causing the landing gear to collapse. The aircraft slid approximately 80 meters before coming to a halt within the airfield boundaries. There were no injuries among the four occupants, though the aircraft sustained heavy damage.
The investigation
Investigators examined the engine, the fuel system, and the maintenance history of the aircraft. The investigation focused on the engine's performance during the flight and the recent maintenance performed by a co-owner, who also served as an aircraft inspector.
Findings revealed that while the fuel itself was clean and free of water, the engine exhibited irregular running at high RPMs on both ignition systems. The investigation also looked into recent repairs, including the replacement of magneto units and valve components, as well as undocumented work performed on the fuel pump and valve springs.
Findings
- The primary cause of the engine failure was a leak in the fuel pump, specifically caused by a cork gasket that had shifted inward and failed to provide an adequate seal, allowing air to enter the fuel system.
- A defect in one of the two ignition systems, specifically a faulty capacitor, may have contributed to the engine's irregular operation.
- The investigation identified inadequate maintenance as the underlying cause, noting that recent repairs were not performed according to standard practices and that several critical repairs, including work on the fuel pump and valve components, were not properly documented in the aircraft's technical logs.