History of Flight
On June 23, 2020, at about 1701 Pacific daylight time, an Enstrom 280 FX helicopter, registration N280MM, was destroyed in an accident near Hayden, Idaho. The helicopter was conducting a Title 14 Code of Federal Regulations Part 91 instructional flight with a student pilot and a flight instructor aboard. The student pilot sustained fatal injuries, and the flight instructor was seriously injured.
The flight instructor reported that shortly after takeoff from Coeur d'Alene Airport, he temporarily leveled the helicopter at about 100 feet above ground level and accelerated to over 100 mph to verify system and indication functionality. Soon after, the engine RPM "skyrocketed" and total power loss occurred, accompanied by backfiring sounds. The helicopter began losing altitude, and the instructor maneuvered to avoid nearby power lines. The low rotor RPM horn activated while the engine was still running. As the ground approached, the instructor pulled the collective and increased throttle, but the helicopter had no RPM. The helicopter impacted terrain, the main rotor blades struck the tailboom, and the helicopter spun about 180 degrees before coming to rest. A postcrash fire ensued, consuming most of the helicopter.
Witnesses reported the helicopter maneuvering at low altitude before crashing in an open field. Two witnesses stated the helicopter was flying at or below power line level. One witness observed the tail moving up and down and the helicopter flying unevenly, and noted the tail swung rapidly before impact. A witness who provided postaccident assistance reported the student pilot stated the helicopter was losing power and did not strike any object before ground impact.
Aircraft Information
The engine maintenance logbook showed the engine time since overhaul was 407.4 hours. The logbook indicated that during the annual inspection on July 15, 2019, about a year before the accident, the mechanic cleaned the spark plugs.
The Enstrom height and velocity diagram indicated that the helicopter's altitude and airspeed before the accident were not within the shaded regions representing areas, altitudes, and airspeeds to avoid during operation.
Wreckage and Impact
The Kootenai County Sheriff's Office responded to the accident site in a field northwest of a road intersection. The Kootenai County Fire Department extinguished the postaccident fire. The wreckage was surrounded by a burnt area about 45 feet in diameter. The front section of the fuselage came to rest on a northeast heading. The tail rotor assembly was located west of the wreckage.
Postaccident examination revealed extensive thermal damage, preventing thorough examination of most systems and components. The tailboom was separated from the fuselage, and the horizontal stabilizer was separated from the tailboom. The engine also sustained thermal damage.
Engine examination found no evidence of catastrophic mechanical malfunction or failure, and no anomalies with the transmission. The top and bottom spark plugs were removed; the No. 3 cylinder bottom spark plug was missing part of its center electrode, and the No. 4 cylinder bottom spark plug was missing its electrode. When all six cylinders were removed, the No. 2 cylinder piston exhibited a sandblasted appearance consistent with detonation. The No. 4 cylinder head and piston showed signatures and damage consistent with detonation.
Additional Information
The Federal Aviation Administration's Airplane Flying Handbook states that detonation is an explosion of the fuel-air mixture inside the cylinder, where the charge explodes rather than burns smoothly. This causes higher force on the piston and cylinder, leading to increased noise, vibration, and cylinder head temperatures, as well as power reduction. Severe detonation can cause engine failure in minutes. The handbook also notes that excessive cylinder temperature can lead to detonation, which can cause catastrophic engine failure.
The FAA Aviation Maintenance Technician Handbook—General states that detonation causes explosive burning of fuel, increasing cylinder pressure, cylinder head temperatures, and decreasing engine performance. The engine manufacturer notes that severe or prolonged detonation can damage the cylinder head and pistons, and in extreme cases, bend or break the connecting rod, crack or fail the cylinder head, or break piston ring lands.