History of Flight
On April 9, 2021, at about 1012 central daylight time, a Eurocopter EC120B helicopter, registration N421PB, was substantially damaged during a hard landing in a residential backyard near LeRoy, Kansas. The pilot and passenger were not injured. The flight was conducted as a personal flight under 14 Code of Federal Regulations Part 91.
The pilot reported departing from Philip Billard Municipal Airport (TOP), Topeka, Kansas, en route to Claremore Regional Airport (GCM), Claremore, Oklahoma, with about 90 gallons of fuel. Approximately 35 minutes after departure, while cruising at 2,000 ft, the low rotor speed warning horn sounded, but no cockpit caution or warning lights illuminated. The pilot observed that the engine's free turbine speed and main rotor speed had dropped. He reduced collective input and moved the cyclic aft to stabilize rotor speed, maintaining about 70% collective as indicated on the first limit indicator (FLI). Despite this, rotor speed continued to decrease, and the pilot headed toward an open field. As rotor speed further decayed, he initiated an autorotation before reaching the intended field and landed in a backyard.
The passenger, also a helicopter-rated pilot, corroborated the sequence. He noted that the low rotor speed horn sounded briefly at first, then after 2–3 minutes became continuous. The pilot maintained altitude while decelerating from 100 knots to 85 knots. During the landing, the passenger perceived a high descent rate; after touchdown, the engine continued running, and the pilot manually shut off the throttle and fuel.
Aircraft and Engine Information
According to logbook documentation, the helicopter had accumulated 52.4 hours in the 14 months before the accident. During that period, the engine's injection manifolds were replaced once, and the oil, main fuel, and fuel control unit (FCU) filters were replaced twice. The most recent filter change occurred about six weeks before the accident, after which the helicopter flew 2.9 hours.
Wreckage and Impact
The helicopter came to rest upright in a residential backyard. The landing gear skids were deformed outward. The aft tailboom showed buckling and crushing near the fenestron, consistent with tailboom ground contact during landing. Flight control and engine control continuity were confirmed. The engine's main fuel filter delta-p bypass indicator was found in the popped position.
Fuel samples from the refueling truck were uncontaminated and appeared as normal Jet-A fuel.
Postaccident Engine Testing and Findings
The engine was removed and tested in a test cell. With all original filters installed, the first start triggered a fuel filter pre-clogging warning and the bypass indicator popped. Under load, the free turbine speed drooped and the gas generator could not accelerate above 83%. The main fuel filter showed significant discoloration but no visible debris.
After replacing the main fuel filter, the engine restarted without pre-clogging or bypass indications. However, under load, the same symptoms recurred: free turbine droop, gas generator limited to 83%, and oscillation. Further troubleshooting involved swapping components; the fuel injection manifolds were identified as the cause of the power loss.
Bench testing revealed that the left fuel injection manifold flowed within design limits (average 21.3 L/hr, limits 21.2 ± 0.8 L/hr; max deviation 0.6 L/hr, limit 1.6 L/hr). The right manifold flowed significantly below limits at an average of 4.3 L/hr. The right manifold houses a filter at its inlet that protects both manifolds. After replacing that filter, the right manifold tested within limits (average 20.9 L/hr). Microscopic examination of the removed filter showed contamination covering about 75% of the screen.
The filter contamination was sent to the NTSB Materials Laboratory for Fourier Transform Infrared (FTIR) spectrometry. The spectrum matched cellulose, commonly found in plant fibers such as paper and cotton. Comparisons with known samples of white paper and white cotton showed strong matches.
The FCU was disassembled; the fuel drained from it was contaminated, and similar particles were found throughout the unit. Particles from the main fuel filter resembled those from the FCU. Laboratory analysis characterized the contamination as flakes and spheres up to 0.5 mm and 0.2 mm, respectively, but could not determine the material's origin or overall composition.
Additional Information
A few months before the accident, the passenger, an aviation mechanic, was contracted by the helicopter's maintainer to replace the helicopter's windshields. On the day before the accident, the maintainer flew a Pilatus PC12 to Dallas to retrieve the passenger and replacement windows, then continued to Topeka due to weather. The passenger stayed at the pilot's residence, intending to examine the windshields and accompany the pilot to Claremore the next day.
The passenger stated he was unaware of any unresolved maintenance issues. However, an engine manufacturer field representative reported that the passenger had contacted him the day before the accident about investigating a fuel filter bypass indication and illuminated fuel filter light on the helicopter. The passenger had instructed the maintainer to replace the fuel filter, which was done. The maintainer reportedly found no evidence of fuel system contamination. After the accident, the passenger told the representative that the maintainer said the issues were resolved, so the passenger did not troubleshoot further.
When interviewed, the maintainer denied any ongoing fuel system issues, stating that fuel filters were replaced routinely and that the helicopter was never fueled from non-standard sources.