Bonanza 35
Maintenance Airframe; Maintenance Powerplant
Maintenance Avionics 25; Maintenance Lead Technician 25
Aircraft Equipment Problem Critical
General Maintenance Action
Chart Or Publication; Procedure
Removed both the left and right magnetos for replacement due to component time. Both magnetos were functioning fine and passed the run-up RPM drop test. Rivet and cam assembly on one magneto was missing a rivet head; which had sheared off and dropped down inside the engine accessory case. This impulse coupling was covered by recurring Airworthiness Directive and Service Bulletin inspections. It had been inspected several times in the past and was in compliance with all inspection criteria. The other magneto had the newer snap ring style impulse coupling. The broken rivet head was found in the scavenge oil sump and there was no damage to the engine accessory drive section. Recommend immediate replacement of all riveted impulse couplings be made mandatory by AD. They can fail without warning; even when they pass the required 100 hour inspection.
The reporter indicated that even with the 100 flight hour periodic inspections; the rivet had failed since the last check.