What happened
On July 26, 1964, a Cessna 140, registration OO-REL, departed from Sion on a private flight toward Basel. The aircraft was carrying the pilot and the aircraft owner. While flying through the Rhône valley near Martigny, the engine began to experience increasingly frequent misfires.
In an attempt to manage the failure, the pilot engaged the carburetor heat, which briefly restored normal operation. However, the engine failed again shortly after. After attempting to check oil pressure and magnetos and performing a full-throttle test, the pilot realized the engine was deteriorating and attempted to return to Sion. As the aircraft passed over Vernayaz, the loss of altitude became critical, forcing the pilot to select the only available landing site: a flat field near the Rhône.
To avoid high-voltage power lines, the pilot performed a steep descent. During the landing, the aircraft struck the ground heavily, bounced, and veered into a small orchard. The landing gear caught in a furrow, causing the right wing to strike the ground and the aircraft to come to rest on its nose. There were no fatalities and no injuries to the occupants, though the aircraft sustained heavy damage.
The investigation
The investigation focused on the mechanical state of the Continental C-85-12F engine and the fuel system. Technical examinations revealed that while the spark plugs and magnetos were functional, the fuel strainer was heavily contaminated with debris, including rust and paint particles.
Further inspection of the carburetor identified a significant malfunction: the float needle was not regulating fuel levels correctly, allowing the fuel to rise 8 mm higher than the prescribed limit. Additionally, investigators noted several maintenance irregularities, including the use of a non-original fuel hose, a non-standard gasket in the carburetor, and an improperly fitted propeller hub that had been shimmed with a thin piece of metal. The engine components, including the crankcase and exhaust, were found to be in a dirty condition with significant rust on the oil dipstick and cap.
Findings
- The primary cause of the engine failure was likely a malfunctioning carburetor, potentially triggered by impurities from the contaminated fuel strainer.
- The engine had been subject to insufficient and poor-quality maintenance.
- The pilot's decision-making and execution of the forced landing were judged to be skillful and appropriate given the circumstances.