From PFA member:
5 HOURS BEFORE:
The airplane was running rough before the October 07 annual and was fine when we picked it up and for the first 5 hours. The AP said the mags needed a slight adjustment and did so per Lycoming specs. The engine has about 120 hours since major overhaul at which time the magnetos were replaced. We are training in the plane and it does get a number of T&Os, missed approaches, and holding patterns performed weekly and so it does get some extra power and rich running. At the last oil change during annual I sent in an oil sample and filter piece for analysis. Nothing abnormal came up as well as the four tests sent in prior.
PREFLIGHT TRIP FROM KBUU – KIMT:
All run-up was normal check was normal and running fine. The one exception, #2 cylinder exhaust temp not reading, but cylinder temp working.
THE TRIP:
Everything was beautiful 5,500 feet and smooth VFR conditions. North of KIMT they were PIREPS of icing at 5,000 ft but was clear where I was with 50 miles of visibility. About 25 miles off of GRB VOR the engine began to run rough and get progressively worse. It was sputtering, some backfire, and losing power. I was at 5,500 feet, VFR, in clear air. I thought perhaps carb ice as the 260 seems to get some ice in the winter. Note, there was no drop in manifold pressure prior to engine sputtering which would confirm that situation. No remedy with carb heat pulled which I left out for 1 minute to make sure while I pulled my emergency checklist. At this time I had lost 500 feet of altitude and progressed to emergency check list items and at this time switched to the left magneto set. The engine smoothed out and power restored. I reduced power to 21-22 inches and flew into KIMT ( a bad decision in retrospect as the headwind would have pushed me back to KGRB quicker than going forward ). A ground run-up after touch done again indicated missing and backfiring on both, and engine shutdown on right magneto, but stabilized on left side. I also noticed that the engine appeared to run hotter on the exhaust gas temperatures for the fuel flow it was using. Normally the 260 will run about 14.2 gallons per hour leaned at 5,000 feet with hottest exhaust gas at 1275 1295 rich of peak 1,350. I try to keep the total of EGT and CHT below combined of 1,700. This trip up it was running at 17.0 gallons per hour and the #3 cylinder was at 1,300 EGT. The #2 is usually the hottest cylinder but not working. I could not do better than this and it seemed odd but all seemed to be running okay oil temp and pressure all on the low ranges of the green arcs.
CONCLUSION: Right magneto failure to hold its adjustment and subsequently was firing out of time, interfered with the left magneto on both, and could not sustain engine firing alone. It was replaced. The decision to continue on with one mag was not good, especially in Northern Wisconsin with few open areas and airstrips available. Since I did not know why the failure occurred I had no real assurance that the left would not be shortly behind. The correct decision would have been to do a 180 and return to KGRB (30 nautical miles downwind, groundspeed of 155 kts, ETA 12 minutes) instead of proceeding to KIMT ( 50 nautical miles upwind , groundspeed of 95 kts, ETA 30 minutes) with a notice of a rough running engine. At my altitude of 5,500, 95 kt best glide speed, 700 800 ft per minute altitude loss with engine out I had about 5 minutes to the ground. 12 minutes to KGRB was a much better decision than 30 minutes to KIMT. And more open fields for emergency purposes back to KGRB. The engine sputtering out is a real character building experience. The good news is that all the ongoing practice of emergency procedures paid big dividends in that I switched to the left magneto. But the decision to push on was poor. I just thought this story might help another pilot in the same mode some day.
