Narrative:

On IFR flight from M32 to tlh, the ride was intermittent IMC and light turbulence. After cruising at 7000, mostly out of clouds but with an unusual headwind, I asked for and received 9000 cruise altitude. I left ram air (alternate air bypass for the small aircraft) on, even though intermittent rain was occurring about 20 mins later, I requested 11000 due to continued light turbulence and rain. The ride was no better at 11000 and outside air temperature was 0 degree C. I requested and received 9000 trying to get out of the IMC. As I leveled off from the descent at 9000, adding cruise power, the engine shuddered and lost power. Manifold pressure indicated approximately 20 inches no matter where throttle was set. I set up glide, contacted atlanta center to indicate loss of power, and began to troubleshoot the problem. Mgm was visible at approximately 9 mi the northeast -- took up heading in direction of mgm. Fuel tanks were switched, boost pump turned on, and throttle was adjusted in/out (though not to idle). I arrived over mgm at approximately 5000 ft still troubleshooting. I reduced throttle setting to idle and noticed ram air light on -- the ram air actuating lever is hidden from view by the throttle quadrant in the small aircraft. I immediately disengaged the ram air and the engine immediately smoothed out. Throttle movement now produced the desired effect. I made 2 360 degree turns, landing at the end of the second 360. Rollout and taxi was normal. The center and approach control and the tower were quick in their reaction and are to be commended in the handling of the situation. As precautionary check, I had maintenance shop inspect the engine do a compression check, and run-up. Everything was normal -- I proceeded to my original des. Iþm satisfied that rain at 11000 ft was freezing at the butterfly valve of the ram air door and blocked combustion air to the engine. A recent service bulletin on the ram air mechanism had been completed, but I believe that had no effect on this situation. Callback conversation with reporter revealed the following information: reporter states this aircraft has ram air control to bypass air filter for induction air. This bypass adds 5 to 7% power. It should only be used when aircraft is above any possible pollutants in the air. Reporter had just plain forgotten ram air was on, with the stated results. He feels this will never happen again as it is emblazoned on his memory.

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Original NASA ASRS Text

Title: SMA HAS ICING PROBLEM AND RESULTING PWR LOSS. HEADS FOR NEARBY ARPT WHILE TROUBLESHOOTING. DISCOVERS RAM AIR BYPASS IS ON.

Narrative: ON IFR FLT FROM M32 TO TLH, THE RIDE WAS INTERMITTENT IMC AND LIGHT TURB. AFTER CRUISING AT 7000, MOSTLY OUT OF CLOUDS BUT WITH AN UNUSUAL HEADWIND, I ASKED FOR AND RECEIVED 9000 CRUISE ALT. I LEFT RAM AIR (ALTERNATE AIR BYPASS FOR THE SMA) ON, EVEN THOUGH INTERMITTENT RAIN WAS OCCURRING ABOUT 20 MINS LATER, I REQUESTED 11000 DUE TO CONTINUED LIGHT TURB AND RAIN. THE RIDE WAS NO BETTER AT 11000 AND OUTSIDE AIR TEMP WAS 0 DEG C. I REQUESTED AND RECEIVED 9000 TRYING TO GET OUT OF THE IMC. AS I LEVELED OFF FROM THE DSCNT AT 9000, ADDING CRUISE PWR, THE ENG SHUDDERED AND LOST PWR. MANIFOLD PRESSURE INDICATED APPROX 20 INCHES NO MATTER WHERE THROTTLE WAS SET. I SET UP GLIDE, CONTACTED ATLANTA CTR TO INDICATE LOSS OF PWR, AND BEGAN TO TROUBLESHOOT THE PROBLEM. MGM WAS VISIBLE AT APPROX 9 MI THE NE -- TOOK UP HDG IN DIRECTION OF MGM. FUEL TANKS WERE SWITCHED, BOOST PUMP TURNED ON, AND THROTTLE WAS ADJUSTED IN/OUT (THOUGH NOT TO IDLE). I ARRIVED OVER MGM AT APPROX 5000 FT STILL TROUBLESHOOTING. I REDUCED THROTTLE SETTING TO IDLE AND NOTICED RAM AIR LIGHT ON -- THE RAM AIR ACTUATING LEVER IS HIDDEN FROM VIEW BY THE THROTTLE QUADRANT IN THE SMA. I IMMEDIATELY DISENGAGED THE RAM AIR AND THE ENG IMMEDIATELY SMOOTHED OUT. THROTTLE MOVEMENT NOW PRODUCED THE DESIRED EFFECT. I MADE 2 360 DEG TURNS, LNDG AT THE END OF THE SECOND 360. ROLLOUT AND TAXI WAS NORMAL. THE CTR AND APCH CTL AND THE TWR WERE QUICK IN THEIR REACTION AND ARE TO BE COMMENDED IN THE HANDLING OF THE SITUATION. AS PRECAUTIONARY CHK, I HAD MAINT SHOP INSPECT THE ENG DO A COMPRESSION CHK, AND RUN-UP. EVERYTHING WAS NORMAL -- I PROCEEDED TO MY ORIGINAL DES. IþM SATISFIED THAT RAIN AT 11000 FT WAS FREEZING AT THE BUTTERFLY VALVE OF THE RAM AIR DOOR AND BLOCKED COMBUSTION AIR TO THE ENG. A RECENT SVC BULLETIN ON THE RAM AIR MECHANISM HAD BEEN COMPLETED, BUT I BELIEVE THAT HAD NO EFFECT ON THIS SITUATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THIS ACFT HAS RAM AIR CTL TO BYPASS AIR FILTER FOR INDUCTION AIR. THIS BYPASS ADDS 5 TO 7% PWR. IT SHOULD ONLY BE USED WHEN ACFT IS ABOVE ANY POSSIBLE POLLUTANTS IN THE AIR. RPTR HAD JUST PLAIN FORGOTTEN RAM AIR WAS ON, WITH THE STATED RESULTS. HE FEELS THIS WILL NEVER HAPPEN AGAIN AS IT IS EMBLAZONED ON HIS MEMORY.

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