Amateur-built KOLB SPORT 600 N914NK accident near Roberts, Idaho

Probable cause

A total loss of engine power for reasons that could not be determined based on the available information.

NTSB analysis

The pilot reported that, after participating in a recreational fly-in event, he completed a successful engine run-up and the high-speed taxi test and departed the airstrip. As the airplane reached about 100 ft above ground level, the engine began losing power. The pilot initiated a slight right turn, followed by a left-turn teardrop maneuver. The pilot engaged the airplane's auxiliary fuel pump to maintain engine power, but the propeller stopped rotating a few seconds later. The airplane touched down about 200 ft south of the runway on sage brush, subsequently impacted a debris pile during the landing roll, nosed over, and then came to rest inverted, which resulted in substantial damage to the rudder. After the first taxi test, the pilot returned to the tie-down area and told the property owner that the engine had experienced possible vapor lock but added that he felt comfortable with the engine's ability to develop power after the taxi test. The pilot told the property owner that the engine had experienced vapor lock during previous flights.

During postaccident testing, the engine ran continuously despite momentary interruptions of power during the first three test runs. During two subsequent test runs, the engine power was advanced to full power, and it ran smoothly without interruptions; the cause of the power interruptions in the first three runs could not be determined. Postaccident examination of the engine revealed that the carburetor floats were 24% and 42% heavier than prescribed by the manufacturer's guidance. However, the heavier float weight likely would not have resulted in the in-flight loss of power due to the constant introduction of fuel through the fuel inlet at higher engine speeds.

The carburetor needle clips were found repositioned to the leanest (No. 1) setting, which the pilot had requested. Although the engine manufacturer recommended that the needle clip position remain in the No. 2 position, the engine ran continuously during the engine run, which was accomplished with both carburetor needle clips in the No. 1 position. Therefore, the carburetor needle clip adjustment likely did not contribute to the loss of power.

A representative of the engine manufacturer reported that this particular engine configuration was susceptible to vapor lock and that low fuel in either wing tank may allow air to be ingested, which may lead to vapor lock and fuel starvation. In this case, the engine was still hot from the previous flight and high-speed taxi, which could have increased the potential for heat-induced vapor lock. However, the accident pilot's prior encounters with vapor lock could not be verified, and the airplane was equipped with a fuel return line, which would have prevented the formation of vapor lock. Additionally, air ingestion was unlikely because the low quantity of fuel found in the fuel tank was likely due to postaccident fuel migration. Further, the airplane was equipped with a header tank, which also would have made air ingestion unlikely even if the left tank was empty in flight. Given that the engine did not display any anomalies during the postaccident engine run that would have precluded normal operation, the reason for the in-flight loss of power could not be determined.

Text from the NTSB. Open the NTSB docket for WPR17LA003 for the full record.