Engine Maintenance Introduced FOD that Caused an EC120 Power Loss (N421PB)
The pilot and passenger were were uninjured.
The Accident Flight
…while in cruise flight at 2,000 ft, he heard the low rotor speed warning horn but no cockpit caution/warning lights illuminated.
The…engine’s free turbine speed and the main rotor speed were “drooped”, and he reduced collective and moved the cyclic control aft to stabilize the main rotor speed. He was able to maintain main rotor speed with about “70%” collective input as indicated on the first limit indicator (FLI).
The helicopter decelerated from 100 knots to 85 knots while the pilot maintained the helicopter’s altitude. The pilot and the passenger [also a qualified pilot] began looking for suitable fields to land in, but about 2-3 minutes after the initial main rotor low speed warning horn, the horn resumed a sustained tone for the remainder of the flight.
The pilot stated that the main rotor speed continued to decrease as he flew toward an open field for a landing, but he eventually entered an autorotation before reaching the intended field because of the decreasing main rotor speed.
[T]he pilot entered a right turn and made an immediate landing in a residential backyard.
The helicopter had a high descent rate when it touched down. The landing gear skids were deformed outward and the tail boom and fenestron exhibited buckling/crushing consistent with the tailboom impacting the ground.
Testing confirmed the pilot’s report of the engine being unable to maintain free turbine speed at 100% while under increased loads.
Further examination and testing revealed a contaminated fuel injection manifold fuel filter, which restricted the amount of fuel that could be delivered to the engine’s gas generator. A laboratory examination determined the contamination was consistent with a cellulose material.
Cellulose is found in natural plant fibers. The spectra of known samples of white paper and white cotton were…compared to the unknown sample and exhibited strong matches.
Similar contamination was found in engine’s main fuel filter and fuel control unit (FCU) during disassembly.
NTSB concluded that:
Based on the known information, the cellulose contamination found in the fuel injector manifold filter was likely introduced during associated fuel system maintenance at an undetermined date.
While the contamination source was not determined by NTSB it distinct possibility the cellulose was from some form of cleaning cloth or paper towel. This material is often not controlled with the same rigour as tools. for example, yet can also cause damage an in-flight emergencies.
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