What happened
On April 9, 2021, a Eurocopter EC120B helicopter, registration N421PB, was performing a personal cross-country flight from Topeka, Kansas, to Claremore, Oklahoma. While cruising at 2,000 feet, the pilot heard the low rotor speed warning horn. The pilot reported that the engine’s free turbine speed and the main rotor speed had "drooped," and although he attempted to stabilize the rotor speed using the collective and cyclic controls, the rotor speed continued to decrease.
The pilot attempted to locate a suitable field for landing, but the decreasing rotor speed forced the aircraft into an autorotation before the intended landing area could be reached. The helicopter subsequently performed a hard landing in a residential backyard. The landing gear skids were deformed, and the tailboom and fenestron sustained damage from impacting the ground. There were no injuries to the pilot or the passenger.
The investigation
Post-accident examination of the engine revealed that the main fuel filter's delta-p bypass indicator had popped. During subsequent engine testing on a test bench, the engine exhibited a fuel filter pre-clogging indication upon start, and the physical bypass indicator popped again when load was applied. Testing showed the gas generator could not accelerate above 83% and the free turbine speed drooped under load.
Investigators identified the cause as a contaminated fuel injection manifold filter. A microscope examination of the filter revealed contamination obscuring approximately 75% of the screen. Laboratory analysis using FTIR spectrometry determined the contamination was consistent with cellulose, matching the spectra of white paper and white cotton. Similar contamination was found throughout the engine's fuel control unit (FCU).
While the maintainer stated there were no ongoing fuel system issues, a manufacturer field representative noted that the passenger had contacted him the previous day to discuss a trip to investigate a reported fuel filter bypass indication and illuminated fuel filter light on the aircraft. Although the maintainer had replaced the fuel filter, no evidence of contamination was observed in the fuel cell or the main fuel filter during that maintenance.
Findings
- The contamination of the fuel injection manifold filter, likely introduced during maintenance, restricted fuel flow to the gas generator.
- The restricted fuel flow caused a partial loss of engine power during cruise flight.
- The pilot's delayed entry into an autorotation resulted in insufficient main rotor speed and an excessive descent rate during the landing.