Narrative:

Routing was elh direct tym direct ggt direct fll. En route from tym to ggt, a 20 min hop, I noticed a 'miss' sound but could not identify which engine. All power and engine indication normal. Did a magneto check. All checked normal. On climb from ggt, noticed 'miss' again. All other indications were normal. I attributed this to a common occurrence where the spark will 'jump' intermittently at altitude. This is not unusual, so I did not return to ggt. As I was handed over to NAS approach, the 'miss' happened again. It was more noticeable now. I checked the mixture and boost (fuel) pumps. All engine instruments read normal. About 5 min later, there was a definite change in engine sound and the plane started a noticeable vibration. All engine indications were still normal, there was no surge or yawing. I still could not identify which engine was having problems. I elected to divert to NAS, which was only about 30 mi now and 15 degree turn right. Due to my inability to identify the problem and the vibration, I requested a 'priority' landing from NAS. They cleared me #1 and sent out the emergency trucks. Approach and landing was without incident. After contact with operations, on their request I did another run up. The left engine now showed a problem with a magneto. Aircraft was left grounded in NAS.

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Original NASA ASRS Text

Title: AN ENG VIBRATION (MALFUNCTIONING MAGNETO) CAUSED A SCHEDULED COMMUTER FLT TO DEV FROM RTE, AS WELL AS DEST.

Narrative: RTING WAS ELH DIRECT TYM DIRECT GGT DIRECT FLL. ENRTE FROM TYM TO GGT, A 20 MIN HOP, I NOTICED A 'MISS' SOUND BUT COULD NOT IDENT WHICH ENG. ALL PWR AND ENG INDICATION NORMAL. DID A MAGNETO CHK. ALL CHKED NORMAL. ON CLB FROM GGT, NOTICED 'MISS' AGAIN. ALL OTHER INDICATIONS WERE NORMAL. I ATTRIBUTED THIS TO A COMMON OCCURRENCE WHERE THE SPARK WILL 'JUMP' INTERMITTENTLY AT ALT. THIS IS NOT UNUSUAL, SO I DID NOT RETURN TO GGT. AS I WAS HANDED OVER TO NAS APCH, THE 'MISS' HAPPENED AGAIN. IT WAS MORE NOTICEABLE NOW. I CHKED THE MIXTURE AND BOOST (FUEL) PUMPS. ALL ENG INSTS READ NORMAL. ABOUT 5 MIN LATER, THERE WAS A DEFINITE CHANGE IN ENG SOUND AND THE PLANE STARTED A NOTICEABLE VIBRATION. ALL ENG INDICATIONS WERE STILL NORMAL, THERE WAS NO SURGE OR YAWING. I STILL COULD NOT IDENT WHICH ENG WAS HAVING PROBS. I ELECTED TO DIVERT TO NAS, WHICH WAS ONLY ABOUT 30 MI NOW AND 15 DEG TURN R. DUE TO MY INABILITY TO IDENT THE PROB AND THE VIBRATION, I REQUESTED A 'PRIORITY' LNDG FROM NAS. THEY CLRED ME #1 AND SENT OUT THE EMER TRUCKS. APCH AND LNDG WAS WITHOUT INCIDENT. AFTER CONTACT WITH OPS, ON THEIR REQUEST I DID ANOTHER RUN UP. THE L ENG NOW SHOWED A PROB WITH A MAGNETO. ACFT WAS LEFT GNDED IN NAS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.