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On February 7, 2008, at 1435 Hawaiian standard time, a McDonnell Douglas 369E helicopter, N698DM, sustained substantial damage during a forced landing, subsequent to a loss of engine power shortly after departure from Hilo International Airport, Hilo, Hawaii. The helicopter was registered to K&S Helicopters and operated by Tropical Helicopters of Kailua Kona, Hawaii. The on-demand air tour sightseeing flight was operated under the provisions of Title 14, Code of Federal Regulations Part 135, when the accident occurred. The commercial pilot and four passengers aboard the helicopter received minor injuries. Visual meteorological conditions prevailed, and company visual flight rules (VFR) flight-following procedures were in effect. The round-robin sightseeing flight originated about 1420 from Hilo. The helicopter sustained substantial damage to the fuselage during the landing sequence.
During a telephone conversation with the National Transportation Safety Board investigator-in-charge (IIC), and subsequent written report, a representative from the operator stated that approximately 5 minutes after takeoff, at an estimated altitude between 1,800 and 2,000 feet mean sea level (MSL) the pilot activated the fuel start pump (boost pump) and initiated a 180-degree turn back to the airport after observing the illumination of the fuel bypass caution light. The pilot reported that he heard 2-3 low pitched "poofing sounds" during the turn with simultaneous "yawing of the aircraft." The pilot reduced power, entered an autorotation and initiated an off-airport landing. He reported that during the final stages of the autorotation, during the flare, the engine "flamed out." The helicopter touched down in tall dense grass, settled, and rolled over onto its left side.
The helicopter sustained structural damage to the fuselage, tail assembly and rotor head.
The accident helicopter, a McDonnell Douglas (MD) 369E, was manufactured in 1989. The single-engine helicopter was powered by an Allison/Rolls-Royce turbo shaft engine rated at 420 horsepower.
The helicopter was maintained in accordance with the manufacture’s airworthiness inspection program. Review of the maintenance records revealed that the helicopter’s most recent airframe and engine inspection (100-hour) was completed on January 19, 2008, at 9,118 total flight hours, engine and airframe (66 flight hours prior to the accident).
No open maintenance discrepancies were noted prior to the accident flight.
WRECKAGE AND IMPACT INFORMATION
The helicopter came to rest on flat open terrain covered by dense grass approximately six miles west of the departure airport. The helicopter was intact and all major components were identified at the accident site prior to recovery efforts. The tail boom, tail rotor gearbox and tail rotor assembly were in place and remained attached to the airframe. The main rotor system was in place, however, two of the four main rotor blades were separated from the main rotor head.
Federal Aviation Administration (FAA) Inspectors reported that fuel leak was observed during the on site inspection of the helicopter. Examination of the engine assembly revealed that the fuel leak originated from the fuel filter pressure differential switch. Inspectors reported that differential switch was in place and the associated fuel lines, bushing and adapter fitting were intact and appeared to be undamaged.
Subsequent to the preliminary inspection, the engine was removed from the airframe and shipped to the manufacturer for additional examination and testing.
TESTS AND RESEARCH
Visual inspection of the engine assembly was conducted at the manufacturer's facility by representatives from NTSB, Rolls-Royce and Boeing. During the inspection it was noted that one of the fuel filter pressure differential switch set screws, and associated safety wire, was missing. The two remaining screws were in place. One of the screws was fixed, the other finger tight. The corresponding safety wire was attached to the remaining set screws; however, the distal end of the safety wire leading to the missing set screw was severed.
At the completion of the inspection the engine was secured in a production test cell and evaluated. During the initial test, the cell fuel delivery system was turned on and the engine was motored to approximately 16 percent N1 (compressor speed). A considerable fuel leak, which originated from the fuel filter pressure differential switch, was noted and the test was terminated.
The original fuel filter pressure differential switch was removed and replaced with an exemplar switch. A second start sequence was initiated. The engine started and ran continuously without interruption throughout a multitude of steady state and transient power settings, to include rapid accelerations and decelerations. All engine parameters were found to be within factory specifications throughout the testing.
Engine testing was accomplished utilizing both the test cell fuel delivery system and engine driven fuel pump.
The fuel filter pressure differential switch, part number 369H8144-3, consists of, in part, of a diaphragm enclosed by a two-piece housing. The housing is secured by the three above mentioned set screws.
The accident helicopter utilizes a non-pressure boosted fuel system for normal operation. The engine fuel pump generates a vacuum at its inlet to lift the fuel from the tank to the engine fuel delivery system. According to a representative from the engine manufacturer, the MD369E utilizes a non-pressure boosted fuel system for normal operation. The engine fuel pump generates a vacuum at its inlet to lift the fuel from the tank and overcome line pressure drops. In field operation a supply line air leak may create a non-homogeneous two-phase flow condition of fuel and air creating a fluctuating output supply to the metering unit.
Maintenance records provided by the operator indicated that the helicopter's fuel system components were inspected during the 100-hour inspection interval completed on January 19. The maintenance "check sheet" corresponding with the inspection indicated that the engine's fuel system components, to include fuel lines and connections, were inspected for leaks or damage, and no anomalies were noted.
The helicopter maintenance manual CSP-HMI-2, Basic Handbook of Maintenance Instructions, Servicing and Maintenance, Chapter 28-00-00, Fuel System, has multiple warnings that "Air in the fuel system will cause a power reduction or flameout."