What happened
On June 14, 1964, a pilot conducting a flight inspection for the Federal Office of Civil Aviation departed from Sion, bound for Bern, in a SAAB 91-D Safir, registration HB-DBL. Due to heavy cloud cover following a cold front, the pilot flew at a low altitude to avoid clouds. During the flight near Ecuvillens, the pilot noticed the airspeed dropping from 90 knots to approximately 80 knots, despite the engine appearing to maintain normal power.
As the aircraft approached Freiburg, the airspeed decreased further to 75 knots, and the pilot struggled to maintain altitude. Attempts to increase engine RPM via the propeller pitch control were unsuccessful, and the engine began to run unevenly, causing significant vibration throughout the instrument panel. Suspecting carburetor icing, the pilot activated the carburetor heat; however, the power loss worsened. After approximately 30 seconds of no improvement, the pilot deactivated the heat to regain power.
Unable to reach the intended destination in Bern due to the deteriorating engine performance, the pilot decided to perform an emergency landing at the small grass airfield in Schmitten. During the landing roll on the wet grass, the pilot applied the brakes but found them ineffective. In an attempt to steer the aircraft back onto the runway, the pilot veered to the right, causing the aircraft to slide into a corner marker consisting of two fuel drums. The aircraft came to a stop approximately 40 meters outside the airfield boundary.
The investigation
The investigation examined the engine's performance and the environmental conditions at the time of the accident. Investigators noted that the recent passage of a cold front had brought high humidity (90-95%) and low temperatures, creating conditions conducive to carburetor icing. The investigation also scrutinized the aircraft's instrumentation, finding that the carburetor temperature sensor was installed in an improper location, providing misleading readings to the pilot.
Upon disassembly of the engine, investigators discovered a mechanical defect in the intake system. A portion of the air/reversion filter located before the carburetor was severely damaged, with a piece approximately 2 cm in size found loose within the intake manifold. This loose fragment may have caused a partial blockage or turbulence in the intake stream.
Findings
- The primary cause of the accident was the inability to decelerate the aircraft on wet grass during the emergency landing, leading to a loss of directional control and a collision with airfield markings.
- The engine power loss was likely caused by either carburetor icing or a partial blockage of the intake manifold caused by a loose fragment of the air filter.
- The pilot's ability to diagnose the issue was hindered by an improperly positioned temperature sensor that provided inaccurate data regarding carburetor temperature.