# SDR 2026FA0000329

- Type: Service difficulty report (as reported; unverified)
- Difficulty date: May 7, 2026
- Submitted: May 7, 2026
- Operation: General aviation
- Aircraft: [N252AN](https://openairdocs.org/aircraft/N252AN), registered now
- Make and model: Mooney M20K
- Registry model: [Mooney M20K](https://openairdocs.org/model/mooney-m20k) (OpenAirDocs)
- Aircraft serial: 251118
- Engine: Cont TSIO520*
- System (JASC): [2434 · DC Generator-Alternator](https://openairdocs.org/jasc/2434)
- Part: Alternator
- Part number: ALU9522
- Part serial: A157523
- Condition: Destroyed
- Location: Engine
- Part time: 115 hours
- Stage of operation: NR
- How discovered: V
- Nature of condition: J
- Precautionary procedure: D
- FAA source: FAA Service Difficulty Reporting System (https://sdrs.faa.gov/)
- Page: https://openairdocs.org/sdr/2026FA0000329

## Discrepancy

ALTERNATOR DRIVE COUPLING DESTROYED CAUSING SUBSEQUENT FAILURE OF ENGINE'S DRIVE GEAR P/N 657984.  ALL FOUR BOLTS P/N 649205 SHEARED OFF OR THREADS STRIPPED FROM SHANKS CAUSING SEPARATION OF GEAR FROM CRANKSHAFT.  LOCK PLATES P/N 641909 FOUND IN TACT IN OIL SUMP AND SHOW SIGNS OF PROPER INSTALLATION AND BOLT HEAD LOCKING CONDITION AS REQUIRED.  
ENGINE IS COMPLETELY DESTROYED AS THE GEAR FACE WAS PUSHED AFT, WEARING INTO THE CRANKCASE.  THE CRANKSHAFT WAS DAMAGED AS WELL AS THE ACCESSORY DRIVE TRAIN COMPONENTS IN THAT AREA.UNABLE TO DETERMINE WHERE THE FAILURE BEGAN, HOWEVER IT ALWAYS INVOLVES AN ASSEMBLY INVOLVING DRIVE COUPLINGS INSTALLED ON PLANE POWER ALTERNATORS.   IN ALL INSTANCES, THERE'S BEEN A SUBSEQUENT FAILURE OF THE CRANKSHAFT GEAR RETAINING BOLTS.  WE HAVE NOT SEEN THIS TYPE OF FAILURE WITH ANOTHER ALTERNATOR MANUFACTURER OR MODEL.  IT SEEMS TO BE A COMMON OCCURRENCE WITH CRANKSHAFT DRIVEN HARTZELL PLANE POWER ALTERNATORS.  THIS IS ONE OF THREE INSTANCES OF CATASTOPHIC ENGINE FAILURES WITH A PLANE POWER ALTERNATOR INSTALLATION THIS ENTITY HAS SEEN.  CONVERSATIONS WITH OTHER ENTITIES INCLUDE CATASTROPHIC ENGINE FAILURES WITH PLANE POWER ENGINE DRIVEN ALTERNATORS INSTALLED.  
THE DRIVE COUPLING WAS PROPERLY INSTALLED AND THE CRANKSHAFT GEAR FACES AND TEETH WERE INSPECTED IN ACCORDANCE WITH CMI MANUAL M-0.

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