WPR20LA191

Shortly after takeoff on an instructional flight, a helicopter was maneuvering at a low altitude when the engine rpm “skyrocketed,” and the engine lost total power; the flight instructor (who was at the controls) reported hearing “backfiring” at the time, although the engine was still running. The flight instructor then had to maneuver the helicopter to avoid nearby power lines, but the helicopter impacted terrain. The main rotor blades struck the tailboom, and the helicopter subsequently spun around about 180° before coming to rest. A postcrash fire then ensued, which consumed most of the helicopter. Additionally, the student pilot stated to a witness after the accident that the helicopter was losing power. An engine examination revealed that detonation had occurred in two of the four engine cylinders. (Detonation in a piston engine occurs when the fuel-air mixture in a cylinder detonates or explodes prematurely instead of being ignited by spark plugs and burning evenly and smoothly, as occurs with normal combustion.) Furthermore, damaged spark plugs were observed in one of the detonated cylinders as well as another cylinder. The combined effects of the detonation and damaged spark plugs likely caused the loss of engine power. Because the loss of power occurred at a low altitude, the flight instructor likely did not have enough altitude to adequately establish a steady-state autorotation to avoid the helicopter’s impact with terrain.

The Federal Aviation Administration’s Airplane Flying Handbook states the following about detonation: Detonation, as the name suggests, is an explosion of the fuel-air mixture inside the cylinder. During detonation, the fuel/air charge (or pockets within the charge) explodes rather than burns smoothly. Because of this explosion, the charge exerts a much higher force on the piston and cylinder, leading to increased noise, vibration, and cylinder head temperatures. The violence of detonation also causes a reduction in power. Mild detonation may increase engine wear, though some engines can operate with mild detonation regularly. However, severe detonation can cause engine failure in minutes. The handbook also stated, “excessive cylinder temperature can lead to detonation, which in turn can cause catastrophic engine failure.” Additionally, the Federal Aviation Administration Aviation Maintenance Technician Handbook—General stated that “detonation causes explosive burning of the fuel which creates an increased cylinder pressure, excessive cylinder head temperatures, and decreased engine performance.” According to the engine manufacturer, “severe or prolonged detonation can cause damage to the cylinder head and pistons. In some extreme cases, the connecting rod can be bent or broken, the cylinder head may crack or fail, or the piston ring lands may break.” The engine maintenance logbook revealed that the engine time since overhaul was 407.4 hours. The logbook showed that, during the annual inspection on July 15, 2019 (almost 1 year before the accident), the mechanic performing the inspection “cleaned [the spark] plugs.” The Enstrom height and velocity diagram revealed that the helicopter’s altitude and airspeed before the accident were not within the shaded regions, which represented areas, altitudes, and airspeeds to avoid during operation. On June 23, 2020, about 1701 Pacific daylight time, an Enstrom 280 FX helicopter, N280MM, was destroyed when it was involved in an accident near Hayden, Idaho. The student pilot was fatally injured, and the flight instructor was seriously injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor stated that, shortly after takeoff from Coeur d’Alene Airport, Coeur d’Alene, Idaho, he temporarily leveled off the helicopter at an altitude of about 100 ft above ground level and accelerated to more than 100 mph to ensure that all the helicopter systems and indications were properly functioning. Shortly thereafter, the engine’s rpm “skyrocketed,” and the engine lost total power; the flight instructor could hear “backfiring.” The helicopter then lost altitude, and the flight instructor maneuvered the helicopter to avoid nearby power lines. Subsequently, the low rotor rpm horn sounded; at that time, the engine was still running. As terrain approached, the flight instructor pulled the collective and increased the throttle, but the helicopter had norpm. Shortly afterward, the helicopter impacted terrain, the main rotor blades contacted the tailboom, and the helicopter spun about 180° before coming to rest. A postcrash fire ensued, which consumed most of the helicopter. According to witnesses, the helicopter was maneuvering at a low altitude before it crashed in an open field. Two witnesses stated that the helicopter was flying at or below the power line level. One witness stated that the tail of the helicopter was moving up and down and that the helicopter did not seem to be flying smoothly. The witness further stated that the helicopter’s tail swung around quickly before ground impact. One witness who provided assistance after the accident reported the student pilot stating that the helicopter was losing power and that the helicopter did not strike anything before ground impact. The Kootenai County Sheriff’s Office responded to the accident site, which was 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 on the ground that was about 45 ft 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. A postaccident examination of the helicopter revealed extensive thermal damage, which prevented a 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. An engine examination revealed no evidence of a catastrophic mechanical malfunction or failure and no anomalies with the transmission. When the top and bottom spark plugs were removed, the examination showed that the No. 3 cylinder bottom spark plug was missing part of its center electrode, and that the No. 4 cylinder bottom spark plug was missing its electrode. Furthermore, when all six cylinders were removed, the No. 2 cylinder piston was found to exhibit a sandblasted appearance consistent with detonation. Additionally, the No. 4 cylinder head and piston exhibited signatures and damage consistent with detonation.