Md Helicopter 369D N510PA accident near Sedro-Woolley, Washington

Probable cause

The loss of engine power due to fuel starvation as a result of naphthenate fuel contamination, which blocked the fuel flow through the start pump.

NTSB analysis

The commercial pilot reported that, while conducting longline operations, the helicopter lost engine power. The pilot entered the helicopter into an autorotation and attempted to land at the bottom of a hill on flat terrain, but the helicopter touched down on a slope; the tailboom, followed by the skids, impacted the side of a hill, and the helicopter then came to rest on its side.

During postaccident examination of the airframe and engine, debris was found throughout the fuel system. The start pump was removed, and the fuel bypass valve inlet port screen was found covered with a brown, spongelike debris. Normal operation is with the start pump off (except when using alternate fuel mixtures or emergency fuels). When the start pump is not in use, fuel passes through the fuel bypass valve inlet port screen. The debris located on the fuel pump bypass valve inlet port screen, throughout the inside of the pump, and embedded in the centrifugal pump prevented the pump from producing sufficient fuel flow, which starved the engine of fuel and resulted in the power loss. Although the operator reported that it monitored for fuel contamination in the accident helicopter and its other company helicopters in accordance with the helicopter manufacturer's maintenance procedures, these procedures did not require that the fuel bypass valve inlet port screen be checked unless a cockpit warning indication light was activated. The light had not activated in the accident helicopter; therefore, the operator had not checked the screen. Following the accident, the helicopter manufacturer revised its procedures to require that the screen be checked whenever fuel contamination was identified.

Testing of the debris was consistent with naphthenates, which are surfactants that reduce the surface tension between the fuel and free water and allow the two liquids to mix. Refinery processing should remove all traces of naphthenic acid and its corresponding metal salts; however, in some refining processes, small amounts of the naphthenates can get carried through with the jet fuel, which can lead to microbial growth in the fuel. About 1 month before the accident, the operator found microbial growth in company fuel and treated the fuel with a microbiocide to destroy biological growth. However, there is no evidence that the microbiocide used by the operator contributed to the dissolution of the naphthenates, and the reason for the separation of naphthenates from the fuel could not be determined.

Text from the NTSB. Open the NTSB docket for WPR16LA021 for the full record.