<p><strong><font color ="blue">TCM Magneto; S6LSC-204T; Broken Distributor Teeth; ATA 7414</font> </strong></p>
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<p><em>(This magneto sparks a Lycoming O540F1 on a Robinson R44 helicopter.) </em></p>
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<p>A helicopter mechanic describes this magneto difficulty. “The pilot noticed the R/H magneto produced excessive RPM drop after startup.” “Investigation found this magneto's distributor gear had 13 missing teeth (all pieces were recovered). I reassembled the unit with a new part <em>(distributor rotor P/N 10-357586). </em>The pilot flew for about an hour with no problems. The next morning the L/H magneto failed—five teeth were missing <em>(from the rotor; all recovered </em>). Both starts <em>(occurred) </em>in the morning, with a <em>(previous) </em>overnight temperature down to about 20 degrees F. This aircraft normally operates in the <em>( Arizona environment). (Speculation includes) </em>a remote possibility a small amount of dirt/oil/grease on the washer with the retaining clip may have impeded rotation in cold weather. I have not seen this problem on fixed wing aircraft that stay outside. <em>(It may be) </em>possible rotorcraft harmonics cause the nylon gear to have additional stress. Both <em>(described) </em>gears appeared to have the correct amount of TCM grease from the 500 hour inspection <em>(performed) </em>about 130 hours earlier.” </p>
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<p><em>“(I) </em>confirmed a magneto gear holding tool (sometimes used during stabbing a magneto to an engine) was not used <em>(and/or) </em>was not a cause <em>(of the defect) </em>since the damaged teeth clock angle was not near the ‘window' timing teeth. <em>(My) </em>recommendation <em>(to avoid this problem) </em>is to install a new gear at 1000.0 hours.” <em>(A search of the FAA Service Difficulty Reporting System data base revealed 25 entries for the full part number, 39 for the base number.) </em></p>