Narrative:

We departed at approximately XA30 am on 12/mon/88 from pao. We headed out on a heading of 210 degrees towards the training practice area over the coast on a dumbarton left departure. Proceeding along this course about 1/2 mi past skyline blvd we began to experience engine roughness and a partial power loss. I elected to return to pao or at least attempt to. We made it as far as highway 280 and the roughness and power loss were getting increasingly severe. At this point I called pao tower to inform them that we might need assistance and I requested a TCA clearance for sector C of the sfo TCA. I was informed that bay TRACON had been notified. The roughness continued to worsen despite all appropriate diagnostic action on my part (including carburetor heat). At this point I decided that it was not worth the risk to try to limp the plane to pao and I made the decision to land on highway 280 near the alpine road exit (northbound). Altitude was 2800' with a controled descent of about 200-300 FPM. I circled over my intended landing site and declared an emergency with pao tower. I continued circling and set up as normal an approach as possible. My intent was to land with power if at all possible. I remained in radio contact with pao tower and other aircraft that were above me. I flew a normal left traffic pattern and landed northbound on 280 (based on a wind report from pao tower). Upon T/D I pulled the mixture and turned off the fuel. It was as normal a landing as was possible. It was, at the time, a precautionary landing and as such I proceeded in accordance with the recommended procedure for this situation. Overall this should be viewed as a precautionary landing with partial power off field. To the best of my knowledge any deviations from clrncs or far's will be covered under far 91.3, PIC authority, in an emergency. Supplemental information from acn 100616: my instrument and I were on a training flight. We began experiencing engine roughness, carburetor heat worked for a short time, then carburetor heat would no longer clear the engine.

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Original NASA ASRS Text

Title: GA SMA PARTIAL ENGINE FAILURE. PRECAUTIONARY OFF ARPT LNDG.

Narrative: WE DEPARTED AT APPROX XA30 AM ON 12/MON/88 FROM PAO. WE HEADED OUT ON A HDG OF 210 DEGS TOWARDS THE TRNING PRACTICE AREA OVER THE COAST ON A DUMBARTON LEFT DEP. PROCEEDING ALONG THIS COURSE ABOUT 1/2 MI PAST SKYLINE BLVD WE BEGAN TO EXPERIENCE ENG ROUGHNESS AND A PARTIAL PWR LOSS. I ELECTED TO RETURN TO PAO OR AT LEAST ATTEMPT TO. WE MADE IT AS FAR AS HWY 280 AND THE ROUGHNESS AND PWR LOSS WERE GETTING INCREASINGLY SEVERE. AT THIS POINT I CALLED PAO TWR TO INFORM THEM THAT WE MIGHT NEED ASSISTANCE AND I REQUESTED A TCA CLRNC FOR SECTOR C OF THE SFO TCA. I WAS INFORMED THAT BAY TRACON HAD BEEN NOTIFIED. THE ROUGHNESS CONTINUED TO WORSEN DESPITE ALL APPROPRIATE DIAGNOSTIC ACTION ON MY PART (INCLUDING CARB HEAT). AT THIS POINT I DECIDED THAT IT WAS NOT WORTH THE RISK TO TRY TO LIMP THE PLANE TO PAO AND I MADE THE DECISION TO LAND ON HWY 280 NEAR THE ALPINE ROAD EXIT (NBND). ALT WAS 2800' WITH A CTLED DSCNT OF ABOUT 200-300 FPM. I CIRCLED OVER MY INTENDED LNDG SITE AND DECLARED AN EMER WITH PAO TWR. I CONTINUED CIRCLING AND SET UP AS NORMAL AN APCH AS POSSIBLE. MY INTENT WAS TO LAND WITH PWR IF AT ALL POSSIBLE. I REMAINED IN RADIO CONTACT WITH PAO TWR AND OTHER ACFT THAT WERE ABOVE ME. I FLEW A NORMAL LEFT TFC PATTERN AND LANDED NBOUND ON 280 (BASED ON A WIND RPT FROM PAO TWR). UPON T/D I PULLED THE MIXTURE AND TURNED OFF THE FUEL. IT WAS AS NORMAL A LNDG AS WAS POSSIBLE. IT WAS, AT THE TIME, A PRECAUTIONARY LNDG AND AS SUCH I PROCEEDED IN ACCORDANCE WITH THE RECOMMENDED PROC FOR THIS SITUATION. OVERALL THIS SHOULD BE VIEWED AS A PRECAUTIONARY LNDG WITH PARTIAL PWR OFF FIELD. TO THE BEST OF MY KNOWLEDGE ANY DEVIATIONS FROM CLRNCS OR FAR'S WILL BE COVERED UNDER FAR 91.3, PIC AUTHORITY, IN AN EMER. SUPPLEMENTAL INFO FROM ACN 100616: MY INSTR AND I WERE ON A TRNING FLT. WE BEGAN EXPERIENCING ENG ROUGHNESS, CARB HEAT WORKED FOR A SHORT TIME, THEN CARB HEAT WOULD NO LONGER CLEAR THE ENG.

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.