Narrative:

I was completing my run up and my right magneto was showing less than a 150 rpm drop; I leaned the mixture and burned off the carbon; returned to the proper rpm setting check the mag once more; and it indicated fine. I then departed and headed west toward my destination. I was on an IFR flight plan; my planned altitude was 12000 feet. I made it to 9100 feet and I experienced a 'rough running' engine. My climb setting for power is 32' mp (manifold pressure) and it dropped to below 25'mp in a blink of an eye. I kicked on the emergency fuel pump; disconnected the autopilot and notified ATC that I was experiencing engine troubles. They asked what I wanted to do and where I wanted to go; I replied the closest airport possible. ATC suggested [a nearby airport] which I programmed in the GPS and headed in that direction. While on the way I tried to advance the throttle which seemed to make matters worse and also tried adjusting the mixture which also seemed to aggravate the situation; so I returned the throttle and mixture to the original spots and continued. ATC advised me that [the airport I was diverting to] had no services and most likely had no one around to help me if and when I landed so ATC suggested I fly to [another airport]; which had services and a control tower. I agreed and programmed the GPS. I had a steady controlled descent of around 800 ft per minute. I figured at that rate and distance if my engine held on I could make [the airport] with no problems. I lined up with [the] runway and as soon as my mains touched the ground my engine quit; without me reducing any power whatsoever. I waited to the last minute to drop my gear and flaps just in case my engine quit on short final I could get a little more glide with a clean configuration. So I'm stuck on [the] runway and I tried to restart [but] it would not start at all. I had to be towed to a maintenance facility. The next day the mechanic called me and told me that both magnetos were bad and I was very lucky to have made it that far. What I think that needs to happen to prevent this from happening again is perform tests and inspections after say 500 hrs of use.

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Original NASA ASRS Text

Title: PA46 pilot reported diverting to a nearby alternate after experiencing a rough running engine.

Narrative: I was completing my run up and my right magneto was showing less than a 150 rpm drop; I leaned the mixture and burned off the carbon; returned to the proper rpm setting check the mag once more; and it indicated fine. I then departed and headed west toward my destination. I was on an IFR flight plan; my planned altitude was 12000 feet. I made it to 9100 feet and I experienced a 'rough running' engine. My climb setting for power is 32' mp (Manifold Pressure) and it dropped to below 25'mp in a blink of an eye. I kicked on the emergency fuel pump; disconnected the autopilot and notified ATC that I was experiencing engine troubles. They asked what I wanted to do and where I wanted to go; I replied the closest airport possible. ATC suggested [a nearby airport] which I programmed in the GPS and headed in that direction. While on the way I tried to advance the throttle which seemed to make matters worse and also tried adjusting the mixture which also seemed to aggravate the situation; so I returned the throttle and mixture to the original spots and continued. ATC advised me that [the airport I was diverting to] had no services and most likely had no one around to help me if and when I landed so ATC suggested I fly to [another airport]; which had services and a control tower. I agreed and programmed the GPS. I had a steady controlled descent of around 800 ft per minute. I figured at that rate and distance if my engine held on I could make [the airport] with no problems. I lined up with [the] runway and as soon as my mains touched the ground my engine quit; without me reducing any power whatsoever. I waited to the last minute to drop my gear and flaps just in case my engine quit on short final I could get a little more glide with a clean configuration. So I'm stuck on [the] runway and I tried to restart [but] it would not start at all. I had to be towed to a maintenance facility. The next day the mechanic called me and told me that both magnetos were bad and I was very lucky to have made it that far. What I think that needs to happen to prevent this from happening again is perform tests and inspections after say 500 hrs of use.

Data retrieved from NASA's ASRS site 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.