BFO94LA155

The pilot reported that he was overflying wooded terrain at 1000 feet msl when the engine lost power. He autorotated to a hilly field and during the landing the rotor blade severed the tailboom. According to the pilot, the fuel low warning light did not activate before the power loss occurred. The examination of the helicopter revealed the fuel tanks were empty and the fuel emergency vent valve had malfunctioned. As a result, the fuel cell decavitated and collasped, holding the fuel quantity probe in a position to indicate a one-quarter tank amount of fuel.

On August 28, 1994, at 1415 eastern daylight time, a Hughes 369D helicopter, N5027P, lost total engine power and was landed in a field near Butler, Pennsylvania. The pilot, the sole occupant, was not injured. The helicopter was substantially damaged. Visual meteorological conditions prevailed at the time of the accident. The ferry flight originated from Gettysburg, Pennsylvania, and was destined for the New Castle Municipal Airport in New Castle, Pennsylvania. The flight was operated under 14 CFR Part 91.

The pilot reported that while in cruise flight at 1,000 feet above the ground, he noticed "...a shudder in [the] anti torque pedals." He stated shortly thereafter, he received visual and audible engine out warnings. He stated the helicopter was over a heavily treed forrest so he turned the helicopter 180 degrees to land in a nearby field. He reported, "While trying to make the field, I had to use all available rotor RPM (within power off RPM limits) which left me with little rotor inertia to cushion landing. Upon pulling pitch for landing, rotor struck tailboom, which sheared off tailboom approx. 1-2 feet in from HOR/vert stabilizer fin."

The pilot reported that the helicopter did not roll, and the terrain was "hilly." He also stated, "There was no caution or warning lights prior to engine out and fuel quantity indicated 75-80 Lbs. remaining." He said the low fuel light never illuminated.

On-scene examination of the helicopter revealed that there was no fuel present in the helicopter's two fuel tanks. No leaks were found throughout the fuel system.

The helicopter was removed from the accident site and brought to a storage facility for further examination. The fuel system was examined by the helicopter's operator and manufacturer under the supervision of the Federal Aviation Administration (FAA). The examination revealed the fuel emergency vent valve prevented all the fuel tank vents from operating. The FAA inspector who was at the examination also stated, "An undesigned air leak was present in the fuel system aft of the emergency vent valve. The leak could have been caused by the presence of a large vacuum in the tank."

The Field Service Representative from McDonnell Douglas Helicopter's (MDHC) present at the examination stated, "...based on his experience, the aircraft fuel cell probably collapsed due to a vacuum from the inoperative vent system." He further stated, "...the pilot would not have received a fuel low level indication since the lifted fuel cell wall would hold the fuel quantity probe arm at the 1/4 tank position until fuel exhaustion occurred."

The fuel emergency vent valve was disassembled and examined by MDHC representatives under the guidance of the FAA. A Memorandum from a MDHC representative stated, "Visual examination indicated that the 369H8116-31 weight was obstructing the lower portion of the vent tube. The complete assy (weight, ball, valve, etc.) was removed from the canister which contained it for a more detailed examination. The...weight separated from the 369H8118 ball seal.

The weight pin is retained at the ball with a retaining ring. The ring was not engaged in the weight pin groove but still attached to the ball due to the sealant. A closer (microscopic) examination showed some sealant material in the groove of the weight pin....The...weight was submitted to our quality people for a dimensional check....The weight pin groove was undersized...and not machined properly." (See attached reports for further details and diagrams).