Narrative:

On [date] I filed and departed ZZZ with another pilot; who is also a cfii; and I was acting as PIC for the flight. The plan was to fly to ZZZ1 at FL210. Proper pre-flight analysis/checklist was completed and the flight plan was opened with local ATC. During the climb to FL210 there were no indications of any abnormality with the aircraft. Performance was normal; climb rates were strong; and the plane was flying well with engine parameters normal; and other matters related to the flight also normal. As we climbed; we took our O2 concentration levels using a pulse oximeter to establish baseline at lower altitudes and continued to assess through the climb. We had set up our O2 masks and put them on one at a time with each pilot watching the plane and controls and engine while the other secured the mask properly and connected it to the voice section of the audio inputs.once we established cruise flight; engine parameters and other flight conditions were again monitored. Proper checklists were used at this point for cruise and other checklists were also noted and completed during the other phases of flight. The pilot flying with me and myself noticed there as a strong left quartering tailwind/crosswind of 48-52 kts from 16;000 feet up to and including FL210 when we were on course. We began to notice a minor knock or what I would describe as a hitch that felt very minor and inconsistent in the nose of the aircraft; which I thought might be the nose fairing moving from side to side with the strong side wind load. As we continued to fly; the knock continued intermittently but not sufficient to consider this a mechanical problem or any reason to discontinue the flight.then we began to notice our cht temperatures began to rise mildly at first and then progressively quicker. As they started to move above 380 degrees I called ATC approach/center and let them know I needed to descend to attempt to lower the temperatures on the engine and began to troubleshoot the problem. We were already on high boost with the fuel pump and noted the fuel flow was adequate and normal. As we descended the knock began to increase mildly at first and then more strongly but not violent enough to believe we had an emergency yet. The engine seemed to still be producing adequate power and ATC asked if I'd like to request priority handling; but I did not feel it was necessary at this time.I was given a block altitude from FL190 to 10;000. I informed them I would let them know if I needed to priority handling shortly. As we descended we followed the checklist for high cht temps and reduced power and continued to increase airspeed to cool the engine. Temps began to settle and come down and the knocking sensation reduced but would increase once again when attempting to increase the throttle above 45-50% power. During the descent; the cht temps and the engine condition continued to fluctuate with temps going up; once elevating to 436; and I would make power adjustments and fuel mixture adjustments to bring them under control while continuing to hand fly the airplane being sure to maintain control and develop a plan. Working with my co-pilot who was flying with me; we determined it would be best to divert and we considered ZZZ2 as an option since it was a class C airport; likely with more services and maintenance support. However; as we crossed ZZZ3; the shaking and temperatures began to rapidly deteriorate and I knew it would be necessary to make a diversion and at this time; I advised ATC and asked for priority to land at ZZZ3. I was informed however the airport was closed and looking off my left-wing; I could tell by looking down on the dark airport the information was correct and landing there would not be possible or at the very least; be a less desirable option if we could continue to a more favorable option. Then we identified ZZZ4 as a possible option prior to ZZZ5 and a class D airspace that would likely have services and maintenance so we informedatc we would fly toward ZZZ4 and then once arriving there decide if it made sense to continue on to ZZZ5.during the short time span from ZZZ3 to ZZZ4; the plane's engine began to calm as I maintained a low power setting below 40% and a stable airspeed of 100 kts and held altitude at 10;800 feet. My thinking was I would retain as much altitude as possible and also choose a forward airspeed that got us there faster and also was above vg and safely high enough above clean stall speed of 70. As we moved closer to ZZZ4; it became apparent that we could make this airport and continuing on to ZZZ5 would not be advised flying over populated areas and other hazards we may not be able to see if we were to suddenly have the engine fail completely. ATC continued to give us priority and I asked if they would like me to switch over to the tower for ZZZ4 and they said I could stay with them. I began a controlled circling descent; still at 10;000 - 10;800 feet; over the airport; keeping the 600 foot elevation airport in view off my left wing and appreciated the lights being lit to their brightest intensity. I was provided winds and weather by ATC and knew I could land safely on either runway xxr or xxl and or the opposite direction if this became necessary. However; I was able to line up and chose the longer runway and the runway that favored the wind. Once certain I would be able to safely land on the runway; I let ZZZ5 approach know I could move to tower and was cleared to land. I rolled out on my last downwind turn 800 - 1000 feet above pattern altitude purposely giving myself still additional altitude in the event of a total loss of power at this time and also know that I wasn't likely to generate adequate power to execute a failed landing on a go-around. As I descended; I lined up the runway; completed all final landing checklists and landed on the centerline of runway xxr and then was taxied back to the FBO and greeted by fire and ambulance safety members.the next day a local maintenance facility identified the problem as a failed left magneto that likely failed while in cruise causing a fuel/combustion problem from advance timing and elevating engine temperatures dramatically.the failed magneto was partially delivering spark that was ill-timed and causing the temps to rise. At the time of the mechanical issues; it was not readily apparent this was a magneto problem and therefore was not something myself or the experienced co-pilot with me considered as a consideration.since that time in discussions with several experienced pilots; magneto problems often are not even recognized when they occur in flight and several have confirmed they too would have not probably recognized this connection. Fortunately; no engine damage occurred and the plane was landed safely and this situation ended up becoming a mostly non-event and fairly routine.

Related reports:

Google
 

Original NASA ASRS Text

Title: SR22 pilot reported a rough running engine caused by a magneto failure resulted in a diversion.

Narrative: On [date] I filed and departed ZZZ with another pilot; who is also a CFII; and I was acting as PIC for the flight. The plan was to fly to ZZZ1 at FL210. Proper pre-flight analysis/checklist was completed and the flight plan was opened with local ATC. During the climb to FL210 there were no indications of any abnormality with the aircraft. Performance was normal; climb rates were strong; and the plane was flying well with engine parameters normal; and other matters related to the flight also normal. As we climbed; we took our O2 concentration levels using a pulse oximeter to establish baseline at lower altitudes and continued to assess through the climb. We had set up our O2 masks and put them on one at a time with each pilot watching the plane and controls and engine while the other secured the mask properly and connected it to the voice section of the audio inputs.Once we established cruise flight; engine parameters and other flight conditions were again monitored. Proper checklists were used at this point for cruise and other checklists were also noted and completed during the other phases of flight. The pilot flying with me and myself noticed there as a strong left quartering tailwind/crosswind of 48-52 kts from 16;000 feet up to and including FL210 when we were on course. We began to notice a minor knock or what I would describe as a hitch that felt very minor and inconsistent in the nose of the aircraft; which I thought might be the nose fairing moving from side to side with the strong side wind load. As we continued to fly; the knock continued intermittently but not sufficient to consider this a mechanical problem or any reason to discontinue the flight.Then we began to notice our CHT temperatures began to rise mildly at first and then progressively quicker. As they started to move above 380 degrees I called ATC approach/center and let them know I needed to descend to attempt to lower the temperatures on the engine and began to troubleshoot the problem. We were already on high boost with the fuel pump and noted the fuel flow was adequate and normal. As we descended the knock began to increase mildly at first and then more strongly but not violent enough to believe we had an emergency yet. The engine seemed to still be producing adequate power and ATC asked if I'd like to request priority handling; but I did not feel it was necessary at this time.I was given a block altitude from FL190 to 10;000. I informed them I would let them know if I needed to priority handling shortly. As we descended we followed the checklist for high CHT temps and reduced power and continued to increase airspeed to cool the engine. Temps began to settle and come down and the knocking sensation reduced but would increase once again when attempting to increase the throttle above 45-50% power. During the descent; the CHT temps and the engine condition continued to fluctuate with temps going up; once elevating to 436; and I would make power adjustments and fuel mixture adjustments to bring them under control while continuing to hand fly the airplane being sure to maintain control and develop a plan. Working with my co-pilot who was flying with me; we determined it would be best to divert and we considered ZZZ2 as an option since it was a class C airport; likely with more services and maintenance support. However; as we crossed ZZZ3; the shaking and temperatures began to rapidly deteriorate and I knew it would be necessary to make a diversion and at this time; I advised ATC and asked for priority to land at ZZZ3. I was informed however the airport was closed and looking off my left-wing; I could tell by looking down on the dark airport the information was correct and landing there would not be possible or at the very least; be a less desirable option if we could continue to a more favorable option. Then we identified ZZZ4 as a possible option prior to ZZZ5 and a class D airspace that would likely have services and maintenance so we informedATC we would fly toward ZZZ4 and then once arriving there decide if it made sense to continue on to ZZZ5.During the short time span from ZZZ3 to ZZZ4; the plane's engine began to calm as I maintained a low power setting below 40% and a stable airspeed of 100 kts and held altitude at 10;800 feet. My thinking was I would retain as much altitude as possible and also choose a forward airspeed that got us there faster and also was above VG and safely high enough above clean stall speed of 70. As we moved closer to ZZZ4; it became apparent that we could make this airport and continuing on to ZZZ5 would not be advised flying over populated areas and other hazards we may not be able to see if we were to suddenly have the engine fail completely. ATC continued to give us priority and I asked if they would like me to switch over to the Tower for ZZZ4 and they said I could stay with them. I began a controlled circling descent; still at 10;000 - 10;800 feet; over the airport; keeping the 600 foot elevation airport in view off my left wing and appreciated the lights being lit to their brightest intensity. I was provided winds and weather by ATC and knew I could land safely on either Runway XXR or XXL and or the opposite direction if this became necessary. However; I was able to line up and chose the longer runway and the runway that favored the wind. Once certain I would be able to safely land on the runway; I let ZZZ5 Approach know I could move to Tower and was cleared to land. I rolled out on my last downwind turn 800 - 1000 feet above pattern altitude purposely giving myself still additional altitude in the event of a total loss of power at this time and also know that I wasn't likely to generate adequate power to execute a failed landing on a go-around. As I descended; I lined up the runway; completed all final landing checklists and landed on the centerline of Runway XXR and then was taxied back to the FBO and greeted by fire and ambulance safety members.The next day a local maintenance facility identified the problem as a failed left magneto that likely failed while in cruise causing a fuel/combustion problem from advance timing and elevating engine temperatures dramatically.The failed magneto was partially delivering spark that was ill-timed and causing the temps to rise. At the time of the mechanical issues; it was not readily apparent this was a magneto problem and therefore was not something myself or the experienced co-pilot with me considered as a consideration.Since that time in discussions with several experienced pilots; magneto problems often are not even recognized when they occur in flight and several have confirmed they too would have not probably recognized this connection. Fortunately; no engine damage occurred and the plane was landed safely and this situation ended up becoming a mostly non-event and fairly routine.

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.