Cessna 402/402C/B379 Businessliner/Utiliner
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Aircraft Aircraft Damaged; Flight Crew Returned To Gate; General Maintenance Action
Aircraft
During taxi-out I noticed the left engine RPM was slightly lower than a normal engine taxi (800 RPM - 1;000 RPM) instead the engine was around 700 - 600 RPM. All other engine indications nominal. During the before takeoff flow during the magneto check I noticed a drop of nearly 200 in the normal mag check time. I continued with the rest of the mags and elected to return to the left engine left mag switch and recheck it. When I did; the engine RPM dropped dramatically in less than a second. I attempted to engage the magneto switch again to save the engine but my finger slipped when attempting to reactivate the switch and was unable to switch it back on prior to the left engine falling below 500 RPM at which point a loud metal sheering/snapping/cracking noise came out of what I perceived to be the interior of the engine. After conducting a gate return and securing the aircraft contacting Operations; Station Manager and Contract Maintenance a logbook entry was created and the aircraft was downed. Myself and my First Officer did a post flight inspection and we could not see any exterior aircraft damage. Cause: Engine magneto failure causing further engine damage.Suggestions: Re-engaging the magneto switch within a faster time period to try and reduce engine damage due to engine magneto failure during engine run-up procedure.
C402 Captain reported experiencing a magneto failure during taxi-out resulting in engine damage and eventual return to gate.