Accident Narrative
On July 3, 2020, at 1920 central daylight time, an Enstrom 280FX helicopter, registration N282SH, was destroyed when it struck terrain about one mile south of Des Moines International Airport, Des Moines, Iowa. The private pilot, who was the only occupant, sustained minor injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Pilot's Account
The pilot reported that during final approach, both rotor speed and engine speed decreased below the green arc, and a low-speed alarm sounded. The rotor and engine speeds remained synchronized as they decreased. The pilot added throttle but observed no increase in either speed. Believing the engine had lost power, he initiated an autorotation by lowering the collective and pushing forward on the collective. The helicopter came to rest on its right side in an area of tall grass, which became blighted from fuel spillage. The pilot noted that he did not execute the autorotation perfectly and that the outcome could have been substantially improved with better execution.
Postaccident Examination
Examination of the flight controls and drive train to the main rotor blades and tail rotor blades confirmed continuity of both systems. The helicopter was destroyed by impact forces, which damaged the fuselage structure, main rotor blades, and tail rotor blades.
Engine Information
The helicopter was powered by a Lycoming HIO-360-F1AD engine, a factory rebuild from January 2020 with approximately 23 hours since rebuild. The engine was equipped with a turbocharger installed under Supplemental Type Certificate (STC) SE484GL. The STC was issued to Enstrom Helicopter Corporation, and the engine was purchased with the turbocharger installed by Lycoming. The normal rated power for the turbocharged engine was 225 horsepower at 3,050 rpm, compared to 190 horsepower at the same rpm without the turbocharger. Specifications for the turbocharger-equipped engine, such as power versus engine speed, fuel pump pressure output, or engine timing, were not provided to the investigation.
Engine Run and Examination
The engine was removed from the airframe and shipped to Lycoming Engines for testing. Lycoming removed the turbocharger system for the test without the knowledge of the National Transportation Safety Board investigator-in-charge (NTSB IIC). The engine run consisted of four start attempts. During the first attempt, the engine did not start; additionally, the magneto grounding socket, a pressed fitting into the magneto housing, was missing. For the second attempt, the ground wire was wedged into the grounding socket. Factory seals on the magneto retaining nuts were absent. A check of magneto-to-engine timing revealed an advance of about 5° before top center (BTC). Once timing was adjusted to the engine data plate specifications, the engine started during a second attempt, with no further magneto issues.
After the second start, engine idle speed was 1,250 rpm, later adjusted to 1,150 rpm for subsequent tests. Fuel pressure from the fuel pump was 30.1 psi, above the Lycoming test specification of 18-28 psi; it was adjusted to 22 psi with the engine operating. The engine attained rated power but not the specified engine speed of 3,050 rpm during initial runs. It met specifications at 1,500, 1,800, and 2,200 rpm. A rich mixture was noted, and the injector idle/mixture lever stop was missing (later found on the test cell floor). After replacing the injector with a slave injector and adjusting propeller speed, the engine produced rated power at 3,050 rpm.
FAA Handbook Reference
The Federal Aviation Administration (FAA) Helicopter Flying Handbook states that for low inertia rotor systems, simultaneous application of aft cyclic, down collective, and alignment with the relative wind is critical during entry into an autorotation at a wide range of airspeeds, including cruise.