Narrative:

On jul/wed/89, I was the PIC of a flight from auburn, ca to oakland, ca. I had one passenger on board. We departed auburn at approximately XA00 pm. At about the time we reached our cruising altitude of 6500', the aircraft experienced an unusual vibration, and the engine lost some power. My attempts to diagnose the problem, through changes in power settings, gas tanks, magnetos, etc, were unsuccessful. I reduced power to about 50 percent. All engine temperatures and pressures remained normal. The power loss was reflected in a 15 KT decrease in airspeed from normal cruise at that power setting. I decided to divert to concord, and so advised sacramento approach. However, at about travis AFB, the engine smoothed out somewhat, and we regained about 10 KTS in airspeed. At that time I decided that we would continue to oakland, provided that ATC would permit us to enter the TCA and maintain enough altitude to accomplish an emergency landing at no risk to public safety. I advised travis of my intention to continue to oakland. As a precautionary measure, however, at my first contact with bay approach I stated that we were experiencing engine roughness, but all gauges were 'in the green'. I asked to enter the TCA at 5500' and remain at that altitude until in the immediate vicinity of the airport. The request was granted. I declined emergency equipment at the airport. We landed without incident. I am very familiar with the san francisco bay area. At all times during the approach to oakland I had in sight unpopulated areas suitable for an emergency landing in the event of total power loss. Further, at all times I maintained sufficient altitude to accomplish a landing in those areas. On the basis of normal engine instrument readings, and a partial lessening of vibration and a partial recovery of power at travis, I believed that a complete engine failure was not imminent. For the above reasons, I judged that an approach to oakland was a safe and prudent decision.

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Original NASA ASRS Text

Title: PLT OF SMA PROCEEDS TO DESTINATION IN SPITE OF ROUGH RUNNING ENGINE.

Narrative: ON JUL/WED/89, I WAS THE PIC OF A FLT FROM AUBURN, CA TO OAKLAND, CA. I HAD ONE PAX ON BOARD. WE DEPARTED AUBURN AT APPROX XA00 PM. AT ABOUT THE TIME WE REACHED OUR CRUISING ALT OF 6500', THE ACFT EXPERIENCED AN UNUSUAL VIBRATION, AND THE ENGINE LOST SOME POWER. MY ATTEMPTS TO DIAGNOSE THE PROBLEM, THROUGH CHANGES IN POWER SETTINGS, GAS TANKS, MAGNETOS, ETC, WERE UNSUCCESSFUL. I REDUCED POWER TO ABOUT 50 PERCENT. ALL ENGINE TEMPS AND PRESSURES REMAINED NORMAL. THE POWER LOSS WAS REFLECTED IN A 15 KT DECREASE IN AIRSPEED FROM NORMAL CRUISE AT THAT POWER SETTING. I DECIDED TO DIVERT TO CONCORD, AND SO ADVISED SACRAMENTO APCH. HOWEVER, AT ABOUT TRAVIS AFB, THE ENGINE SMOOTHED OUT SOMEWHAT, AND WE REGAINED ABOUT 10 KTS IN AIRSPEED. AT THAT TIME I DECIDED THAT WE WOULD CONTINUE TO OAKLAND, PROVIDED THAT ATC WOULD PERMIT US TO ENTER THE TCA AND MAINTAIN ENOUGH ALT TO ACCOMPLISH AN EMER LNDG AT NO RISK TO PUBLIC SAFETY. I ADVISED TRAVIS OF MY INTENTION TO CONTINUE TO OAKLAND. AS A PRECAUTIONARY MEASURE, HOWEVER, AT MY FIRST CONTACT WITH BAY APCH I STATED THAT WE WERE EXPERIENCING ENGINE ROUGHNESS, BUT ALL GAUGES WERE 'IN THE GREEN'. I ASKED TO ENTER THE TCA AT 5500' AND REMAIN AT THAT ALT UNTIL IN THE IMMEDIATE VICINITY OF THE ARPT. THE REQUEST WAS GRANTED. I DECLINED EMER EQUIPMENT AT THE ARPT. WE LANDED WITHOUT INCIDENT. I AM VERY FAMILIAR WITH THE SAN FRANCISCO BAY AREA. AT ALL TIMES DURING THE APCH TO OAKLAND I HAD IN SIGHT UNPOPULATED AREAS SUITABLE FOR AN EMER LNDG IN THE EVENT OF TOTAL POWER LOSS. FURTHER, AT ALL TIMES I MAINTAINED SUFFICIENT ALT TO ACCOMPLISH A LNDG IN THOSE AREAS. ON THE BASIS OF NORMAL ENGINE INSTRUMENT READINGS, AND A PARTIAL LESSENING OF VIBRATION AND A PARTIAL RECOVERY OF POWER AT TRAVIS, I BELIEVED THAT A COMPLETE ENGINE FAILURE WAS NOT IMMINENT. FOR THE ABOVE REASONS, I JUDGED THAT AN APCH TO OAKLAND WAS A SAFE AND PRUDENT DECISION.

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.