What happened
On April 17, 1967, a Piper PA-18 Super-Cub, registration HB-ORW, departed from Hausen a. Albis airfield for a glider towing operation. Shortly after takeoff, while climbing at an altitude of approximately 50 to 70 meters, the engine suddenly lost power.
The pilot attempted to manage the emergency but was unable to maintain sufficient airspeed. During the descent, the pilot focused on avoiding overhead power lines, which caused him to delay releasing the tow rope. The glider, HB-697, eventually released the cable, but the sudden change in pitch—combined with the aircraft's tail-heavy trim—caused the Piper PA-18 Super-Cub to enter an uncontrolled attitude. The aircraft struck a meadow approximately 1 km west of the runway at a steep angle. The pilot sustained serious injuries, and the aircraft was heavily damaged.
The investigation
Investigators examined the wreckage and the engine components to determine the cause of the power loss. The engine itself was found to be in good working order, with no mechanical defects or issues with fuel/oil quality identified during bench testing.
Upon inspecting the fuel system, investigators discovered a reddish foreign substance—likely a hardened sealant or adhesive—inside the rubber connection between the right wing tank and the metal fuel line. This substance had adhered to the inner wall of the hose, reducing the effective cross-section of the fuel flow by approximately 65.4%. This restriction meant that the fuel flow rate was reduced to only about one-third of its normal capacity.
Findings
- The primary cause of the engine failure was a lack of fuel delivery caused by a physical obstruction in the fuel line.
- The obstruction, a hardened plastic-like substance, significantly restricted the flow of fuel from the right tank.
- The crash was exacerbated by the aircraft's trim settings; the plane was trimmed in a tail-heavy configuration, which, when combined with the pilot's control inputs during the emergency, led to a loss of airspeed and an aerodynamic stall.
- The pilot's attempt to avoid power lines delayed the release of the glider, contributing to the unstable flight path.