History of Flight
On November 3, 2015, at 1130 Pacific standard time, an MD Helicopters 369D, registration N510PA, sustained substantial damage during a forced landing following a loss of engine power near Sedro-Woolley, Washington. The commercial pilot, the sole occupant, was not injured. Olympic Air operated the helicopter as a 14 Code of Federal Regulations Part 133 external load flight. Visual meteorological conditions prevailed, and no flight plan was filed. The pilot departed from Arlington Municipal Airport, Arlington, Washington, about 0700.
According to the pilot, he was conducting longline operations using a 50-foot line to gather cedar pieces. After completing work at an initial jobsite, he flew to the second jobsite. Between jobs, the helicopter was refueled. Before beginning the second job, a safety briefing was conducted. The pilot then completed about 30 to 40 slings and, as he was positioning the helicopter to lift a load from a slope, the helicopter suddenly lost engine power and he entered an autorotation. The pilot attempted to land at the bottom of a hill due to the flat terrain, but the helicopter touched down on the slope, and the tailboom impacted the side of a hill, followed by the skids. The helicopter came to rest on its right side. Prior to the loss of engine power, the pilot did not receive any warning lights during the flights.
Post-Accident Egress
Following the accident, nearby workers tried to assist the pilot in evacuating the helicopter. He was unable to egress due to his headset cord. Once he removed his headset, he was able to egress from the helicopter. MD Helicopters had previously issued Operational Safety Notice 2015-001, Helmet Communication Cord Connection, which notes that an attached communication cord may impede occupants' ability to egress and that an intermediate "pig-tail" communication cord can help mitigate this hazard.
Fuel System Examination
The National Transportation Safety Board investigator, an FAA representative, and representatives from MD Helicopters, Rolls-Royce, Boeing, and Olympic Air examined the helicopter following its recovery. Examination of the airframe and engine revealed contaminants throughout the fuel system, including the engine fuel filter. All warning lights functioned normally. Removal of the start pump showed that the fuel bypass valve inlet port screen (used when the start pump is off) was covered with a brown, sponge-like debris. This debris was submitted to the NTSB Materials Laboratory for testing.
Using Fourier-transform infrared spectroscopy and scanning electron microscopy with energy dispersive x-ray spectroscopy, the debris was identified as potassium naphthenate, a surfactant. On January 12, 2016, the start pump was examined and tested at Globe Motors Inc. in Dothan, Alabama. With the contaminant in place, the pump demonstrated intermittent fuel flow from the discharge port, erratic pressure drop, and did not meet defined performance parameters. After contamination removal, the pump still exhibited intermittent fuel flow in the powered state. Disassembly of the start pump revealed debris internal to the pump and centrifugal impeller.
Maintenance History
According to the operator, company personnel first identified fuel contamination in a company helicopter engine fuel filter on October 7, 2015. Samples were retrieved from every refueling vehicle in its ground fleet; growth was noticed in one vehicle fuel tank but not in filters. All fuel sources were then treated with Biobor, a micro-biocide. For several years, the operator had been changing the engine fuel pump filter every 100 hours instead of the prescribed 300 hours due to contaminants. At the time of the accident, MD Helicopters maintenance procedures required that the fuel bypass valve inlet port screen be checked only if the FUEL FILTER cockpit warning light activated. Because the light had not activated, the operator did not check the start pump screens. Following the accident, the helicopter manufacturer revised its maintenance procedures to require that the screens be checked when fuel contamination is identified.
Main Rotor Blade Inspection
During examination of the helicopter, cracking of the blade root fitting sealant between the root fitting and airfoil was observed on one main rotor blade. Although unrelated to the accident circumstances, one inboard section of that blade was submitted to the Materials Laboratory for detailed examination due to a previous event involving blade root fitting disbond and its inspection procedures.
MD Helicopters Maintenance Manual No. CSP-HMI-2 requires 100-hour inspections of the upper and lower root fittings for cracked adhesive or paint around the periphery. If found, the root fittings are to be inspected for disbonding by loosening (not removing) the outermost bolt and attempting to insert a 0.004-inch-thick Mylar shim into the adhesive bond line. For the accident blade, visual inspections found cracked or missing paint around the entire periphery of the lower root fitting; paint was intact around the upper root fitting. Magnified examination of the lower fitting periphery revealed that the bond line was visible, but no gap was initially seen. After loosening the outermost bolt, a gap became visible, but a 0.004-inch feeler gauge could not be inserted. During loosening and probing, more paint flaked off. Following loosening and removal of all bolts, the gap enlarged. The 0.004-inch feeler gauge then slid easily into exposed portions of the gap. At the two outermost bolt locations, the gauge penetrated about 1 inch to the bolt holes. At the outermost bolt, the gap was estimated to be between 0.012 and 0.014 inch wide. Following these observations, MD Helicopters and the blade manufacturer stated they would revise the main rotor blade inspection procedures to address the findings from this investigation.
Additional Information
According to Chevron Global Aviation's publication "Aviation Fuels Technical Review," naphthenic acid and its corresponding metal salts can be naturally occurring in crude oil or residual refinery treating materials; refinery processing should remove all traces, but small amounts of naphthenates can carry through with jet fuel.