Bell OH-58A N613SS accident near Blanchard, Idaho

Probable cause

Fuel contamination, which resulted in a loss of engine power. The source of the contamination could not be determined.

NTSB analysis

About 15 minutes into the flight, with the helicopter configured in a level attitude, the engine power reduced. The low-rotor alarm then sounded. In response, the pilot initiated an autorotation and performed a run-on landing in tall grass.

Postaccident examination of the engine revealed no evidence of any preimpact mechanical malfunctions or failures that would have precluded normal operation. The engine ran as expected when placed on a test stand. The helicopter’s fuel was contaminated and had a dark/cloudy appearance with suspended particulates. The fuel samples displayed very low oxidative stability, substantial gum formation, and particulate contamination, all of which likely resulted in the engine’s loss of power.

The pilot stored fuel for the helicopter in a 180-gallon aluminum tank with a filter positioned between the tank and a rubber fueling hose. When transferring fuel from the storage tank into the helicopter, the pilot did not filter fuel remaining in the hose between uses. The use of a large storage tank may have hastened fuel oxidation and gum formation. Also, fuel sitting in the hose may have degraded the hose, allowing contaminated fuel to enter the helicopter unfiltered.

Text from the NTSB. Open the NTSB docket for WPR23LA297 for the full record.