N510PAHughes Helicopters HUGHES 369D · 1981

On November 3, 2015, at 1130 Pacific standard time, an MD Helicopters 369D, N510PA, was substantially damaged 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 was operating 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 because of 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.

According to Chevron Global Aviation's publication-Aviation Fuels Technical Review, naphthenic acid and its corresponding metal salts can be present as naturally occurring materials in the crude oil or as residual refinery treating materials. Refinery processing should remove all traces of these materials. However, in some refining processes, small amounts of the naphthenates can get carried through with the jet fuel.

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. Following a prior accident, MD Helicopters had issued Operational Safety Notice 2015-001, Helmet Communication Cord Connection that notes, "In the event of an accident an attached communication cord may impede the occupants' ability to egress from the aircraft…Use of an intermediate "pig-tail" communication cord can help to mitigate this safety hazard."

The National Transportation Safety Board investigator, the FAA representative, and representatives from MD Helicopters, Rolls-Royce, Boeing, and Olympic Air examined the helicopter following its recovery from the accident site.

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 the fuel bypass valve inlet port screen(the port used when the start pump is off) was covered with a brown, sponge-like 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 was submitted to the NTSB Materials Laboratory for testing and identification.

The debris from the fuel bypass valve inlet port screen was examined using Fourier-transform infrared spectroscopy (FTIR) with a diamond attenuated total reflectance (ATR) accessory in accordance to ASTM E1252-98 (American Society for Testing Materials E1252-98: Standard Practice for General Techniques for Obtaining Infrared Spectra for Qualitative Analysis and American Society for Testing Materials). The debris was then examined by scanning electron microscope (SEM) and quantitative energy dispersive x-ray spectroscopy (EDS) in accordance with ASTM E15081. The FTIR and EDS examinations indicated that the unknown material was consistent with potassium naphthenate, a surfactant.

On January 12, 2016, the start pump was examined and tested at Globe Motors Inc. in Dothan, Alabama. The start pump was tested on a test bench with the contaminant in place. The electrically driven centrifugal fuel pump was tested in the non-powered and powered state, the pump demonstrated intermittent fuel flow from the discharge port; the pressure drop was erratic, and did not meet the defined performance parameters. The contamination was removed from the fuel bypass valve inlet port screen, and the start pump was retested. In the powered state, the fuel flow was intermittent and did not meet defined performance parameters. Disassembly of the start pump revealed debris internal to the pump and centrifugal impeller.

According to the operator, the company first identified fuel contamination in a company helicopter engine fuel filter on October 7, 2015. The operator reported that samples were retrieved from every refueling vehicle in its ground fleet, and some growth was noticed in a vehicle fuel tank, but not in the filters. All fuel sources were then treated with Biobor, a micro-biocide used in fuel to destroy microbial growth. This fuel was then supplied to company helicopters. 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.

MD Helicopters, Inc. (MDHI) maintenance procedures at the time of the accident required that the fuel bypass valve inlet port screen be checked if the FUEL FILTER cockpit warning light was activated. Because the light had not activated, the operator did not check the start pump screens. Following the accident, the helicopter manufacturer revised their maintenance procedures to require that the screens be checked when fuel contamination is identified.

During the examination of the helicopter, the main rotor blades were examined and cracking of the blade root fitting sealant, between the root fitting and airfoil, was observed on one of the blades. Although unrelated to the accident circumstances, one of the inboard sections of the main rotor blade was submitted to the Materials Laboratory for a detailed examination due to a previous event involving a blade root fitting disbond and its inspection procedures.

MDHI Maintenance Manual No. CSP-HMI-2 requires 100 hour inspections of the upper and lower root fittings for "…cracked adhesive/paint around the periphery of the root fitting." If the condition is found, the root fittings are to be inspected for disbonding. The root fitting inspection involves loosening (not removing) the outmost bolt and attempting to insert a 0.004 inch thick Mylar shim in the adhesive bond line between the root fitting and the blade doubler. The disposition of the blade is determined by the ability or inability of inserting the shim.

For the accident main rotor blade, visual inspections found cracked or missing paint around the entire periphery of the lower root fitting. The paint was intact around the upper root fitting. Magnified examination of the periphery of the lower fitting and area of missing paint near the outboard tip of the fitting revealed that the bond line was visible, however no gap was visible. The remaining bond line was hidden from direct view by the paint. The outermost bolt was then loosened, and a gap became visible, but a 0.004 inch thick feeler gauge could not be inserted. During the loosening and probing, more paint flaked off exposing more of the bond line. It was also noted that in some locations a fillet formed by adhesive squeezed out during manufacture partially hid portions of the bond line gap. Following loosening and removal of all bolts, the gap enlarged. In this condition, the 0.004 inch thick feeler gauge would easily slide into the exposed portions of the gap. At the two outermost bolt locations, the gauge would penetrate all the way to the bolt holes (about 1 inch). At the outermost bolt, the gap was estimated to be between 0.012 inch and 0.014 inch wide. Following these observations, MDHI and the blade manufacturer stated they would revise the main rotor blade inspection procedures to address the findings from this investigation.