What happened
On March 22, 1970, a Cessna 336 Skymaster, registration I-SICC, was conducting a flight from Turin to Colombier, with a subsequent planned leg to Bern. While climbing to 12,000 feet near Vevey, the pilot experienced the first signs of engine malfunction in the rear engine. The pilot subsequently diverted toward Sottens with the intention of reaching Lausanne.
During the flight, the rear engine failed completely. The pilot attempted to maintain flight by utilizing auxiliary fuel tanks and switching to main tanks, but the front engine also began to fail. The pilot attempted various maneuvers to restore power, including switching fuel sources and engaging pumps, but the front engine eventually lost power as well. Facing a total loss of power, the pilot performed an emergency landing in a large, open field near Savigny. The aircraft struck the heavy, melting snow, causing the nose gear to collapse and the left wing to strike the ground, resulting in heavy damage to the airframe.
The investigation
The investigation examined the aircraft's mechanical state, the pilot's actions, and the prevailing meteorological conditions. Investigators inspected the wreckage, including the engine components, fuel filters, and fuel tanks. The investigation also reviewed the pilot's flight logs, the aircraft's maintenance history, and weather data from the Payerne radiosonde.
Technical analysis of the fuel system revealed traces of water in the auxiliary fuel tanks. The investigation also noted that while the aircraft had been overweight at departure from Turin, the weight at the time of the accident was within legal limits. The investigation focused on why the fuel from the main tanks failed to reach the engines despite the pilot's attempts to switch sources.
Findings
- The primary cause of the accident was an emergency landing on unsuitable terrain.
- This landing was necessitated by a dual engine failure caused by a loss of fuel supply.
- It is highly probable that ice obstructed the fuel lines, likely caused by water in the auxiliary tanks being drawn into the system and freezing at altitude.
- High humidity levels (approximately 95%) and temperatures around -9°C at 12,000 feet provided the conditions necessary for such an obstruction.
- The pilot's choice of landing site was considered appropriate given the lack of other clear, obstacle-free areas in the immediate vicinity.