What happened
The pilot was cruising at 1600 feet mean sea level above a cloud layer when the left engine experienced a sudden loss of power. Attempts to restart the engine were unsuccessful. The pilot feathered the propeller and diverted toward Tonopah, Nevada. However, the aircraft could not maintain altitude with only one engine operating.
The last reported weather at Tonopah indicated ceiling conditions of 400 feet broken and 800 feet overcast. Over the area, the pilot entered a descending spiral in instrument meteorological conditions. During this descent, icing conditions were encountered. The aircraft broke out of the clouds at approximately 400 feet above ground level at night. The pilot was unable to see the airport. Unable to maintain altitude, the pilot executed a forced landing with the landing gear retracted.
The investigation
A post-accident examination of the left engine revealed no evidence of preimpact failure or mechanical malfunction. Inspection of the left fuel selector bowl showed signs of microbial growth. Additionally, there was heavy microbial growth on the left fuel filter. Water and significant microbial growth were found in the left fuel tank.
Findings
The aircraft was equipped with STC SA117NW, which required the addition of anti-icing and anti-biological additives to the fuel. No such additives were used during the previous refueling. This lack of treatment allowed for the development of microbial contamination in the fuel system, leading to fuel starvation. The pilot was not injured.
Safety message
Operators must strictly adhere to Special Certificate of Airworthiness (STC) requirements for fuel additives. Failure to use required anti-icing and anti-biological agents can lead to severe fuel system blockages and engine failure.