Incident Overview
On March 27, 2013, at approximately 1200 mountain daylight time, an Enstrom F-28F helicopter, registration N918DK, lost engine power while in the traffic pattern at Joslin Field – Magic Valley Regional Airport (TWF) in Twin Falls, Idaho. The student pilot executed a forced landing onto airport property. The pilot was not injured. The helicopter's main rotor head and tailboom sustained substantial damage. The aircraft was registered to and operated by the pilot under 14 Code of Federal Regulations Part 91 as a solo flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight had originated from TWF at 1050.
Pilot Report
According to the student pilot, during the fourth landing of the flight, the engine began to sputter and lose RPM. The pilot lowered the collective and added power, which temporarily increased engine RPM. However, when he raised the collective, the engine lost power again. As the pilot turned toward the runway, the engine completely lost power, and he autorotated the helicopter to a field on airport property. During the landing, one of the skids became stuck in soft dirt, causing the helicopter to roll onto its left side.
Postaccident Examination
A Federal Aviation Administration (FAA) inspector conducted an examination of the airframe and engine. Flight control and fuel system continuity were verified with no anomalies noted. The inspector removed the single drive dual magneto, ignition harness, and spark plugs. Upon opening the magneto, it was observed that the right distributor gear was internally timed to the clockwise (CW) markings, while the left distributor gear was timed to the counterclockwise (CCW) markings, resulting in a four-tooth difference in timing between the gears. The magneto was reassembled and sent to a service shop for further examination.
Follow-up Examination
At the magneto service shop, the ignition harness was high tension tested and responded normally. The spark plugs were examined using the Champion Check-a-Plug chart and were found to be between "WORN OUT – NORMAL" and "WORN OUT – SEVERE." When each plug was connected to an ohmmeter, only two spark plugs had resistance within the manufacturer's specified range. The magneto was disassembled, revealing that the magnet was installed properly and appeared "almost brand new." The condensers were tested and did not leak. After timing and reassembly, the magneto was installed on a test bench and operated normally, producing a strong spark across a 9 mm gap at various RPMs. The magneto service representative noted that with a strong magnet, the magneto could operate normally with a CW-configured right distributor gear, but as the magnet ages and weakens, it may fail or misfire.
Manufacturer Guidance
According to a magneto specialist from Continental Motors, the Continental Motors D-2000 and D-3000 series high tension ignition systems service support manual specifies that both distributor gears should be timed to the CCW markings. The specialist further stated that an improperly configured magneto would result in faster wear on internal components, although the timing error likely was not noticed during engine operation.