LEESBURG, VA, USA
N184CM
CRAIG N. MOEN RV-8
While in the traffic pattern, the engine backfired a few times. After landing, and while taxiing to the ramp, the pilot determined that the right ignition source was working intermittently. He was unable to replace the right ignition source and departed the following day. As the airplane made a normal descent into the final destination airport, the pilot discovered the right ignition source was "completely dead." The pilot had replaced both magnetos with a Dual Lightspeed Plasma II ignition system with Hall Effect Sensor Modules. After landing at the final destination airport, the pilot removed the back plate of the right Hall Effect sensor module and found that the timing rotor had fractured, and that the internal face of the back plate exhibited rotational scoring. Subsequent Safety Board examination revealed that the timing rotor had failed due to low-stress, high-cycle fatigue. According to the manufacturer, 400 timing rotors were in use, and there were no reported failures.
On October 22, 2002, about 1600 eastern standard time, a homebuilt RV-8, N184CM, experienced a failure of the right ignition system during descent into Leesburg Executive Airport (JYO), Leesburg, Virginia. The airplane landed uneventfully, and the certificated private pilot/builder/owner was not injured. No flight plan was filed for the flight that originated at Elizabethtown Airport, (4W1), Elizabethtown, North Carolina, about 1400. Visual meteorological conditions prevailed for the personal flight conducted under 14 CFR Part 91. According to the pilot, he was on a return flight from Florida, when he was forced to land at Elizabethtown due to weather. While in the traffic pattern the engine backfired a few times. After landing, and while taxiing to the ramp, he determined that the right ignition source was working intermittently. The pilot was unable to replace the right ignition source in Elizabethtown, and returned to Leesburg the following day. As the airplane made a normal descent into Leesburg, the pilot discovered the right ignition source was "completely dead." The pilot had replaced both magnetos with a Dual Lightspeed Plasma II ignition system with Hall Effect Sensor Modules. The pilot removed the back plate of the right Hall Effect sensor module and found the timing rotor had fractured, and the internal face of the back plate exhibited rotational scoring. The module had accrued 34 hours of operation. According to the manufacturer, 400 timing rotors were currently in use and there were no reported failures. The fractured timing rotor and back plate were examined by the Safety Board's Materials Laboratory. According to the Materials Laboratory factual report, visual examination of the rotor revealed that it fractured into two sections at the edge of the washer for the rotor attachment bolt. Significant post-fracture damage was observed on both rotor pieces. Visual and scanning electron microscope (SEM) examination of the lesser-damaged fracture surface on the larger piece of the rotor revealed that the fracture initiated adjacent to the washer edge (aft side) and propagated toward the forward side. Characteristics of a low-stress, high-cycle fatigue fracture mode were present. Examination of an etched cross-sectional mount through the fracture revealed that the component was made from clad aluminum. The core microstructure was consistent with 2024 aluminum. An overall view of the metallographic mount showed that material on the intact arm was significantly swaged out, adjacent to the washer. Microstructural damage as a result of rubbing/heating was also observed on the forward and aft surfaces adjacent to the bolt hole and on the bolt hole surface itself. Energy dispersive spectroscopy (EDS) analysis revealed that the outer clad layers were made of pure aluminum. EDS analysis of the aft side of the rotor in the area of circumferential scoring revealed the presence of cadmium, likely from the washer coating. Micro-hardness measurements in the core of the clad material revealed an average hardness of Brinell 55. The drawing requirements were for 2024 aluminum in the T3 condition, with a typical Brinell hardness of 120. The hardness value found was more typical of 2024-0. The pilot reported a total of 1,016 flight hours, of which, 60 hours were in make and model.
A low-stress, high-cycle fatigue fracture of the right ignition timing rotor.
Source: NTSB Aviation Accident Database
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