...About 10 To 15 Minutes From His Destination When The Airplane Sustained A Partial Loss Of Engine Power

Location: Big Delta, Alaska Accident Number: ANC23LA070
Date & Time: August 24, 2023, 16:00 Local Registration: N151RF
Aircraft: Oliver Robert K Oliver Series 6 Aircraft Damage: Substantial
Defining Event: Unknown or undetermined Injuries: 2 Minor Flight Conducted Under: Part 91: General aviation - Personal

Analysis: The pilot reported that he was about 10 to 15 minutes from his destination when the airplane sustained a partial loss of engine power. He pulled the carburetor heat on but power was not restored. He moved the throttle control from idle to full power and the engine would only increase to about 25% with full throttle. The pilot decided to make a forced landing along a treeless mountain ridgeline. During landing, a strong gust of wind lifted the airplane’s right wing and subsequently the left wing and engine impacted terrain and the airplane nosed over. 

The airplane sustained substantial damage to the fuselage and wings. After the accident, control continuity was established between the cockpit and engine. Although both carburetor bowls were empty of fuel, when disconnected the fuel line had fuel drain from both sides of the disconnection, consistent with fuel being fed to the engine at the time of operation. Thumb compression was established for all four cylinders. The top spark plugs all appeared to be normal; the lower spark plugs in the two right cylinders had oil in them, as did the forward right cylinder, which contained about a cup of oil. Small amounts of water were drained from the two left cylinders. The wreckage was stored unprotected outside on a trailer buried in snow and the engine was laying on it side which would account for the oil and water found in the engine. Postaccident examination of the engine revealed no evidence of any preimpact mechanical malfunctions or failures that would have precluded normal operation.

Although weather conditions were conducive to serious carburetor icing at glide power, the pilot reported application of carburetor heat after the power loss with no effect. Accordingly, the reason for the power loss could not be determined.

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this accident to be -- A partial loss of engine power for undetermined reasons.

FMI: www.ntsb.gov