Narrative:

I noticed a rough running engine on climb out. Was hoping it would smooth out at cruise power and being leaned back. Fouled plug? It continued to run rough; right engine at 15;000 ft. Cruise and I noted that the cylinder head temp on the right engine was at redline. With widespread IMC and forecast moderate icing ahead; I decided to take advantage of the area of VMC that I was in at that time. I had a visual on ZZZ so I advised ATC that I had a rough running engine and was going to spiral down into ZZZ; which is surrounded by high terrain. The engine was still running and I had plenty of altitude and VMC. ATC advised they showed no notams for ZZZ. I started an immediate descent due to the engine temp being at redline. With the mountainous terrain I very quickly lost radio contact with ATC. Another aircraft relayed to ATC that I was about to land and as soon as I landed I called FSS and reported safely on the ground and to please relay that to ATC.my concern is that I don't recall; very busy single-pilot ops in high terrain; rough engine; and unfamiliar airport at night; cancelling IFR prior to my rapid descent. The controller was helpful and onboard with what I was doing; but I suppose it could be argued that I descended through unassigned altitudes without having cancelled my IFR.

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Original NASA ASRS Text

Title: 402C pilot reported possibly not cancelling IFR while diverting due to a rough running engine.

Narrative: I noticed a rough running engine on climb out. Was hoping it would smooth out at cruise power and being leaned back. Fouled plug? It continued to run rough; right engine at 15;000 ft. cruise and I noted that the cylinder head temp on the right engine was at redline. With widespread IMC and forecast moderate icing ahead; I decided to take advantage of the area of VMC that I was in at that time. I had a visual on ZZZ so I advised ATC that I had a rough running engine and was going to spiral down into ZZZ; which is surrounded by high terrain. The engine was still running and I had plenty of altitude and VMC. ATC advised they showed no NOTAMs for ZZZ. I started an immediate descent due to the engine temp being at redline. With the mountainous terrain I very quickly lost radio contact with ATC. Another aircraft relayed to ATC that I was about to land and as soon as I landed I called FSS and reported safely on the ground and to please relay that to ATC.My concern is that I don't recall; very busy single-pilot ops in high terrain; rough engine; and unfamiliar airport at night; cancelling IFR prior to my rapid descent. The controller was helpful and onboard with what I was doing; but I suppose it could be argued that I descended through unassigned altitudes without having cancelled my IFR.

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.