Narrative:

The engine trouble started by losing about 300 RPM. As I noticed a change in power, I immediately turned on carburetor heat, which took care of the problem. After 10 seconds, with heat on, everything was still running fine, so I shut the carburetor heat off which immediately caused the engine to run rough. Carburetor ice I thought, again, so I turned on heat, again, with same results -- took care of problem. I then decided to leave heat on for one min this time to see if it was ice, and to clear it out completely if it was. After one min, I went to turn off the heat, but before I had the knob all the way in, the engine was running rough again. I then decided to leave the heat on until reaching lebanon, about 15 mi away, to determine if this was a localized problem. Up to now, carburetor heat was taking care of the problem, so I reasoned we were getting a lot of ice. After about 3 mins of operating with heat on, the engine started to run rough, as it rapidly went from 2300 RPM to 2000 RPM. I then turned to my student, who was flying to this point, and told him to point us toward montpelier while I told boston center that we had a rough running engine and were returning to montpelier. Center asked if we needed assistance, and as I did a quick magneto check to rule out a bad magneto, I told them that we would need a vector and then took control of the aircraft and finished the turn to 360. Less than a min after we were on the heading, the engine was down to 1900 RPM and was beginning to surge in power -- it would drop off to about 1500 RPM and return to 1900 RPM, however, every time it was now dropping to the low point, it would drop a little lower than the previous time and not come up quite as far. I already had the aircraft slowed down to best glide speed, and we were beginning to slowly lose altitude. After maybe 3 or 4 mins of this, the RPM had dropped off to a low of 800 RPM and then the engine quit altogether. We had been advised less than a min earlier that center lost radar contact with us 8 mi south of mpv, and the controller had asked us if we had the field in sight, to which I replied I did not. He then told us to maintain 360 on the heading to get to mpv. Boston center had already diverted another aircraft to circle over us to watch us and offer assistance. We continued on the heading without power, as our best fields were still ahead of us. Spi was now working the radios full time and I was looking for a field. I had asked spi to look for a field off his side of the aircraft, although I found northeast which was looking good to me from my vantage point, about half a mi ahead and on the right. We had been without power now for about 3 mins and were down to 2400 ft MSL. I was just about to tell spi to unlatch his door, and to turn off all the fuel, magnetos, etc. When the engine came back up to 2300 RPM. I then brought the nose up to best rate of climb attitude and we climbed on a 360 heading to 2600 ft MSL, since we only had that power for 20 seconds before the engine ran rough. I had left all items set for power, in case we would get a break. The engine was again running at about 1900 RPM at full throttle, but now, at this altitude we were able to maintain 2600 ft, at 60 KTS. Since we had many more fields remaining to the airport, now 4 mi away, I decided to continue to mpv. The pilots orbiting above us, had already advised mpv unicom that we were on our way in, and obtained an advisory for us. At about 1.5 mi from the runway, we were at 2500 ft MSL, and then I knew we would make it to mpv. We went straight in on 35 and in the flare I asked to contact other aircraft and to relay to center we had made it and were safe at mpv. After landing and before shutting down, we ran the engine up and noted no problems. After checking everything we shut it down and had the mechanics check it out and neither found anything wrong, although both speculated that there may have been some dirt in the carburetor which could have caused a problem like this. This theory was later reinforced by mechanics at another airport who reported a similar occurrence several yrs ago.

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Original NASA ASRS Text

Title: FLT INSTRUCTOR AND STUDENT PLT IN A SINGLE SMA EXPERIENCE ENG FAILURE IN CRUISE. THEY NURSE THEIR ACFT, WITH PARTIAL TO NO PWR, TO MPV ARPT, 14 MI AWAY WITH THE HELP OF ZBW CTLR AND ANOTHER SMA RELAYING COMS.

Narrative: THE ENG TROUBLE STARTED BY LOSING ABOUT 300 RPM. AS I NOTICED A CHANGE IN PWR, I IMMEDIATELY TURNED ON CARB HEAT, WHICH TOOK CARE OF THE PROBLEM. AFTER 10 SECONDS, WITH HEAT ON, EVERYTHING WAS STILL RUNNING FINE, SO I SHUT THE CARB HEAT OFF WHICH IMMEDIATELY CAUSED THE ENG TO RUN ROUGH. CARB ICE I THOUGHT, AGAIN, SO I TURNED ON HEAT, AGAIN, WITH SAME RESULTS -- TOOK CARE OF PROBLEM. I THEN DECIDED TO LEAVE HEAT ON FOR ONE MIN THIS TIME TO SEE IF IT WAS ICE, AND TO CLR IT OUT COMPLETELY IF IT WAS. AFTER ONE MIN, I WENT TO TURN OFF THE HEAT, BUT BEFORE I HAD THE KNOB ALL THE WAY IN, THE ENG WAS RUNNING ROUGH AGAIN. I THEN DECIDED TO LEAVE THE HEAT ON UNTIL REACHING LEBANON, ABOUT 15 MI AWAY, TO DETERMINE IF THIS WAS A LOCALIZED PROBLEM. UP TO NOW, CARB HEAT WAS TAKING CARE OF THE PROBLEM, SO I REASONED WE WERE GETTING A LOT OF ICE. AFTER ABOUT 3 MINS OF OPERATING WITH HEAT ON, THE ENG STARTED TO RUN ROUGH, AS IT RAPIDLY WENT FROM 2300 RPM TO 2000 RPM. I THEN TURNED TO MY STUDENT, WHO WAS FLYING TO THIS POINT, AND TOLD HIM TO POINT US TOWARD MONTPELIER WHILE I TOLD BOSTON CTR THAT WE HAD A ROUGH RUNNING ENG AND WERE RETURNING TO MONTPELIER. CTR ASKED IF WE NEEDED ASSISTANCE, AND AS I DID A QUICK MAGNETO CHK TO RULE OUT A BAD MAGNETO, I TOLD THEM THAT WE WOULD NEED A VECTOR AND THEN TOOK CTL OF THE ACFT AND FINISHED THE TURN TO 360. LESS THAN A MIN AFTER WE WERE ON THE HDG, THE ENG WAS DOWN TO 1900 RPM AND WAS BEGINNING TO SURGE IN PWR -- IT WOULD DROP OFF TO ABOUT 1500 RPM AND RETURN TO 1900 RPM, HOWEVER, EVERY TIME IT WAS NOW DROPPING TO THE LOW POINT, IT WOULD DROP A LITTLE LOWER THAN THE PREVIOUS TIME AND NOT COME UP QUITE AS FAR. I ALREADY HAD THE ACFT SLOWED DOWN TO BEST GLIDE SPD, AND WE WERE BEGINNING TO SLOWLY LOSE ALT. AFTER MAYBE 3 OR 4 MINS OF THIS, THE RPM HAD DROPPED OFF TO A LOW OF 800 RPM AND THEN THE ENG QUIT ALTOGETHER. WE HAD BEEN ADVISED LESS THAN A MIN EARLIER THAT CTR LOST RADAR CONTACT WITH US 8 MI S OF MPV, AND THE CTLR HAD ASKED US IF WE HAD THE FIELD IN SIGHT, TO WHICH I REPLIED I DID NOT. HE THEN TOLD US TO MAINTAIN 360 ON THE HDG TO GET TO MPV. BOSTON CTR HAD ALREADY DIVERTED ANOTHER ACFT TO CIRCLE OVER US TO WATCH US AND OFFER ASSISTANCE. WE CONTINUED ON THE HDG WITHOUT PWR, AS OUR BEST FIELDS WERE STILL AHEAD OF US. SPI WAS NOW WORKING THE RADIOS FULL TIME AND I WAS LOOKING FOR A FIELD. I HAD ASKED SPI TO LOOK FOR A FIELD OFF HIS SIDE OF THE ACFT, ALTHOUGH I FOUND NE WHICH WAS LOOKING GOOD TO ME FROM MY VANTAGE POINT, ABOUT HALF A MI AHEAD AND ON THE R. WE HAD BEEN WITHOUT PWR NOW FOR ABOUT 3 MINS AND WERE DOWN TO 2400 FT MSL. I WAS JUST ABOUT TO TELL SPI TO UNLATCH HIS DOOR, AND TO TURN OFF ALL THE FUEL, MAGNETOS, ETC. WHEN THE ENG CAME BACK UP TO 2300 RPM. I THEN BROUGHT THE NOSE UP TO BEST RATE OF CLB ATTITUDE AND WE CLBED ON A 360 HDG TO 2600 FT MSL, SINCE WE ONLY HAD THAT PWR FOR 20 SECONDS BEFORE THE ENG RAN ROUGH. I HAD LEFT ALL ITEMS SET FOR PWR, IN CASE WE WOULD GET A BREAK. THE ENG WAS AGAIN RUNNING AT ABOUT 1900 RPM AT FULL THROTTLE, BUT NOW, AT THIS ALT WE WERE ABLE TO MAINTAIN 2600 FT, AT 60 KTS. SINCE WE HAD MANY MORE FIELDS REMAINING TO THE ARPT, NOW 4 MI AWAY, I DECIDED TO CONTINUE TO MPV. THE PLTS ORBITING ABOVE US, HAD ALREADY ADVISED MPV UNICOM THAT WE WERE ON OUR WAY IN, AND OBTAINED AN ADVISORY FOR US. AT ABOUT 1.5 MI FROM THE RWY, WE WERE AT 2500 FT MSL, AND THEN I KNEW WE WOULD MAKE IT TO MPV. WE WENT STRAIGHT IN ON 35 AND IN THE FLARE I ASKED TO CONTACT OTHER ACFT AND TO RELAY TO CTR WE HAD MADE IT AND WERE SAFE AT MPV. AFTER LNDG AND BEFORE SHUTTING DOWN, WE RAN THE ENG UP AND NOTED NO PROBLEMS. AFTER CHKING EVERYTHING WE SHUT IT DOWN AND HAD THE MECHS CHK IT OUT AND NEITHER FOUND ANYTHING WRONG, ALTHOUGH BOTH SPECULATED THAT THERE MAY HAVE BEEN SOME DIRT IN THE CARB WHICH COULD HAVE CAUSED A PROBLEM LIKE THIS. THIS THEORY WAS LATER REINFORCED BY MECHS AT ANOTHER ARPT WHO RPTED A SIMILAR OCCURRENCE SEVERAL YRS AGO.

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.