Narrative:

Approximately 20 NM northwest of love field, tx I leveled off at 6000 MSL, my assigned altitude on an IFR flight plan from dal to okc. While setting cruise power (full throttle, 23 map and 2400 RPM) I noticed abnormal vibration in the engine. Over the next 10-15 minutes I monitored the engine operation, thinking that possibly the vibration would soon clear. It did not. In fact after about 15 minutes there was a sudden increase in vibration for about 5 seconds. At this point I started a return to dal. Several times during the return ftw center and regional approach asked if I wished to declare an emergency. Each time I declined due to considering this only a precautionary landing for unknown engine problems. During the descent in the dfw dfw TCA with reduced power the engine began to run smooth again. At one point I asked approach control if they would approve an ILS 13R at dal. They declined because of heavy traffic landing T dal on both 31L and 314. Since an approach to 31L or 31R would require flight over a large portion of dallas in IMC and I was still unsure of the reliability of the engine, I elected to not fly one of those apches even though my company's maintenance facility is located at dal. At this point I agreed to declare this operation an emergency in the hope that approach would provide ILS 13R to me. They did. So I elected to land at dfw which was VFR conditions and would not require undue flight over populated areas. After the above flight maintenance guessed that the problem had been water in fuel. This flight had been preflted at tul and water had been drained from the fuel system until no water remained. This flight flew from tul to okc the same day before the subject flight with no trace of problems. Again, the same day this flight flew from okc to dal with no trace of problems. The takeoff and climb from dal showed no problems.

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Original NASA ASRS Text

Title: EMERGENCY LNDG.

Narrative: APPROX 20 NM NW OF LOVE FIELD, TX I LEVELED OFF AT 6000 MSL, MY ASSIGNED ALT ON AN IFR FLT PLAN FROM DAL TO OKC. WHILE SETTING CRUISE POWER (FULL THROTTLE, 23 MAP AND 2400 RPM) I NOTICED ABNORMAL VIBRATION IN THE ENGINE. OVER THE NEXT 10-15 MINUTES I MONITORED THE ENGINE OPERATION, THINKING THAT POSSIBLY THE VIBRATION WOULD SOON CLEAR. IT DID NOT. IN FACT AFTER ABOUT 15 MINUTES THERE WAS A SUDDEN INCREASE IN VIBRATION FOR ABOUT 5 SECONDS. AT THIS POINT I STARTED A RETURN TO DAL. SEVERAL TIMES DURING THE RETURN FTW CENTER AND REGIONAL APCH ASKED IF I WISHED TO DECLARE AN EMER. EACH TIME I DECLINED DUE TO CONSIDERING THIS ONLY A PRECAUTIONARY LNDG FOR UNKNOWN ENGINE PROBLEMS. DURING THE DSCNT IN THE DFW DFW TCA WITH REDUCED POWER THE ENGINE BEGAN TO RUN SMOOTH AGAIN. AT ONE POINT I ASKED APCH CTL IF THEY WOULD APPROVE AN ILS 13R AT DAL. THEY DECLINED BECAUSE OF HEAVY TFC LNDG T DAL ON BOTH 31L AND 314. SINCE AN APCH TO 31L OR 31R WOULD REQUIRE FLT OVER A LARGE PORTION OF DALLAS IN IMC AND I WAS STILL UNSURE OF THE RELIABILITY OF THE ENGINE, I ELECTED TO NOT FLY ONE OF THOSE APCHES EVEN THOUGH MY COMPANY'S MAINT FAC IS LOCATED AT DAL. AT THIS POINT I AGREED TO DECLARE THIS OPERATION AN EMER IN THE HOPE THAT APCH WOULD PROVIDE ILS 13R TO ME. THEY DID. SO I ELECTED TO LAND AT DFW WHICH WAS VFR CONDITIONS AND WOULD NOT REQUIRE UNDUE FLT OVER POPULATED AREAS. AFTER THE ABOVE FLT MAINT GUESSED THAT THE PROBLEM HAD BEEN WATER IN FUEL. THIS FLT HAD BEEN PREFLTED AT TUL AND WATER HAD BEEN DRAINED FROM THE FUEL SYSTEM UNTIL NO WATER REMAINED. THIS FLT FLEW FROM TUL TO OKC THE SAME DAY BEFORE THE SUBJECT FLT WITH NO TRACE OF PROBLEMS. AGAIN, THE SAME DAY THIS FLT FLEW FROM OKC TO DAL WITH NO TRACE OF PROBLEMS. THE TKOF AND CLIMB FROM DAL SHOWED NO PROBLEMS.

Data retrieved from NASA's ASRS site as of August 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.