N918DKEnstrom F-28F

On March 27, 2013, about 1200 mountain daylight time, an Enstrom F-28F, N918DK, experienced a loss of engine power while in the traffic pattern at Joslin Field – Magic Valley Regional Airport (TWF), Twin Falls, Idaho. The pilot subsequently made a forced landing onto airport property. The student pilot was not injured. The helicopter's main rotor head and tailboom sustained substantial damage. The helicopter was registered to, and operated by, the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a solo flight. Visual meteorological conditions prevailed for the flight, and no flight plan had been filed. The flight originated from TWF at 1050.

The student pilot reported that during the fourth landing of the flight the engine started to sputter and lose RPM. The pilot lowered the collective and added power; the engine RPMs increased. When he raised the collective, the engine would lose power again. As the pilot turned toward the runway, the engine completely lost power and he auto rotated the helicopter to a field on airport property. During the landing, one of the helicopter's skids got stuck in the soft dirt and the helicopter rolled onto its left side.

During a postaccident examination of the airframe and engine by a Federal Aviation Administration Inspector (FAA) flight control and fuel system continuity was verified with no anomalies noted. The FAA Inspector removed the single drive dual magneto, ignition harness, and spark plugs. The Inspector opened the single drive dual magneto. It was noted that the right distributor gear was internally timed to the clockwise (CW) markings, and the left distributor gear was internally timed to the counter clockwise (CCW) markings, making a four-teeth difference in timing between the gears. The magneto was reassembled and the ignition components were sent to a magneto service shop for further examination.

During the follow up examination, the ignition harness was high tension tested and it responded normally. The spark plugs were examined and compared to the Champion Check-a-Plug chart; the plugs were noted to be between "WORN OUT – NORMAL" and "WORN OUT – SEVERE". Each plug was connected to an ohmmeter and only two spark plugs had ohms within the manufacturer's resistance range. The magneto was disassembled and it was noted that the magneto's magnet was installed properly and looked "almost brand new". The condensers were installed onto a test bench and did not leak. The magneto was timed, reassembled, and installed onto a test bench. The magneto operated normally, and a strong spark was obtained at various RPMs over a 9 mm gap. The magneto service representative reported that with a strong magnet, the magneto will operate normally with a CW configured right distributor gear. However, as the magnet ages and weakens, it may fail or misfire.

According to a magneto specialist at Continental Motors, the 'Continental Motors D-2000 and D-3000 series high tension ignition systems service support manual' dictates that both distributor gears should be timed to the CCW markings. The specialist also stated that a magneto configured improperly will result in faster wear on the internal components; however, the timing error was probably not noted during the operation of the engine.