N95MS
McDonnell Douglas MD 369HS
The helicopter departed from a fishing vessel and lost partial engine power. The pilot stated he heard noise from the engine sounding like a compressor stall. The pilot regained power and attempted to return to the ship. On short final approach the helicopter lost power again and the pilot ditched the aircraft in the Pacific Ocean. The engine compressor section was examined and the compressor case plastic coating was found cracked with sections of the plastic missing, including the plastic sections between all of the fifth stage stator vanes. According to the engine manufacturer, loss of plastic can have an adverse effect on compressor performance. The engine manufacturer further indicated that corrosion was the most common cause of loss of the plastic coating. The missing plastic exceeded the quantity necessary to replace the compressor. The helicopter was immersed in salt water and it was not possible to determine the extent of corrosion on the compressor before the accident. The engine manufacturer specifies inspection of the compressor case should be made as necessary by the operating environment, but not to exceed every 300 hours.
HISTORY OF FLIGHT On March 11, 1997, at 1958 hours coordinated universal time, a McDonnell Douglas 369HS, N95MS, lost engine power while on approach to the fishing vessel M/V FETU, and collided with the surface of the Pacific ocean 1,756.4 nautical miles west-northwest of American Samoa. The helicopter was destroyed by postimpact salt water immersion. The certificated commercial pilot and mechanic were not injured. The helicopter was engaged in fish spotting operations when the accident occurred. Visual meteorological conditions prevailed at the time. The pilot reported that he had taken off from the fishing vessel and was climbing to 1,000 feet above sea level. About 2 to 3 miles from the ship the helicopter made an uncommanded yawing motion to the left. The pilot stated he heard noise from the engine ". . . sounding like a compressor stall." The pilot then contacted the fishing vessel and informed them he was returning. The helicopter lost power about 150 feet from the ship at 60 feet above sea level. The helicopter descended and the pilot turned right to avoid a collision with the side of the ship. The helicopter struck the water in the turn with the right skid/float. The helicopter rolled inverted and remained buoyant supported by the float type skids. ENGINE EXAMINATION The helicopter was recovered from the water by the fishing vessel. The engine was removed and treated with fresh water to prevent further damage from salt water emersion. The engine was shipped to the continental United States and examined by the Safety Board on May 5, 1997. Efforts were made to preserve the engine during shipment. The engine turbine section was separated from the gear box as well as the fuel control, governor, fuel pump, and bleed valve before shipment. The components were washed with fresh water and a light coating of oil was applied. The engine compressor section and gearbox, and the turbine section would not rotate by hand. The internal bearings were frozen. Bore scope examination of the turbine blades and compressor blades did not reveal any evidence of damage. The compressor case half was removed to expose the internal components of the compressor. There was no damage to the six stages of the axial compressor blades or stator vanes. The compressor case plastic coating was found cracked and sections of plastic were missing. A circumferential crack in the plastic was found in the third and fourth stage compressor blade path. Longitudinal cracks in the case plastic coating were found in the fourth stage compressor blade path emanating between the third and fourth stage stator vanes. Sections of plastic were missing between all of the fifth stage stator vanes. The area of the missing sections extended into the sixth stage compressor blade path. RESEARCH The engine manufacturer publishes standards and specifications for inspecting the compressor case plastic coating in its operation and maintenance manual. The manual states: "The frequency of inspection and interim repair of the compressor plastic case coating shall be determined by the judgment of the operator based on knowledge of the aircraft operating environment. Inspection time periods of Inspection Checklist, Table III-7, must not be exceeded." Table III-7 indicates the frequency of the inspection of the compressor case shall be made as necessary by the operating environment. The table states "In an erosive environment, inspect the case at least every 300 hours." According to the operator, the compressor had accrued 2,535.3 hours of operation since the last overhaul. The engine was last inspected during an 100-hour inspection in November, 1996, 60.5 flight hours before the accident. The manual also states: "Loss of plastic can have an adverse effect on compressor performance and is a justified reason for replacement of the case on low performance engines. If more than fifty percent of the plastic is missing between the vanes in stages 4 through 6, or if more than 20 percent of the plastic is missing in the blade path of stages 5 and 6, the case should be replaced." WRECKAGE RELEASE The wreckage was released to the representatives of the owner on May 8, 1997.
A loss of engine power due to a deteriorated compressor air seal, which resulted from inadequate maintenance inspection of the helicopter to detect corrosion in the compressor case while being operated in a marine environment.
Source: NTSB Aviation Accident Database
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