Narrative:

My passenger; who is also a private pilot; and I; had planned to ferry this airplane back for its new owner. We agreed to share operating costs as this was a great hour building opportunity. I as a pilot had flown numerous times across huge portions of the country and took up the opportunity as more experience to be added to my logbook. The winds were calm; visibility was greater than 10; ceiling was reported as greater than 12;000; temperature 66; dew point 64; 94% humidity; and an altimeter of 30.20. I performed our run-up per the poh. All systems were functioning to spec and within operating limits. We took off intending to go to [an enroute airport] as the first leg of the trip. The climb out and level off all went smoothly. We leveled off at 8;500 feet MSL and were on flight following. About 5 minutes into the cruise; while scanning the gauges; we noted that the egt had been slowly dropping; and as it was doing so; the engine began to run roughly. I noted that this was a definite sign of carb ice; so we pulled out the checklist for carb ice; and pulled on full carb heat and applied full throttle. This made things worse. The engine ran extremely roughly to the point that the entire airplane was shaking. Keeping the throttle at this setting for any longer than 5-10 seconds would make the RPM drop from 2200 down to below idle RPM; so I had to pull out the throttle to keep the engine running smoothly. The engine only ran smoothly [at] 1500 RPM and below; so at this point I made a transmission on flight following saying that we weren't going to declare anything but that we are going to deviate to [a nearby airport] due to engine roughness. As we got lower; I continued to use the carb heat technique in the checklist and it failed to make any difference until we were about 2;000 feet above the [field] elevation. As we were approaching 2;000 AGL; the engine with full throttle and carb heat did lose power at full throttle quickly; but the longer we kept it there; the roughness did begin to subside; so I chose to stay in a circuit 2;000 feet above [the airport] until the roughness went away and the ice was fully melted. It took about 5 minutes for it all to clear out; but by the end of the 5 minutes; the engine ran smoothly at full throttle and with carb heat on. I checked to see if it would also run smoothly with carb heat off and it did. At this point I decided to land to wait about 30 minutes to ensure that the ice had fully melted. After waiting; we started up again intending to continue. I performed our normal run-up and everything; including carb heat; functioned the way it should. The only difference in this run up was that I also elected to perform a full throttle static RPM run-up for 30 seconds to ensure there was no roughness at full power. The engine ran smoothly and to spec so I decided that it would be safe to continue. We took off and I was a bit more cautious about the possibility of carb icing. On departure; the winds were calm; visibility was 10 miles; sky condition was 12;000 broken; temperature 64; dew point 63; humidity 93%; and altimeter 30.25. Every few minutes; I would pull the carb heat on to ensure that there was a normal drop in RPM; rather than an excessive one that would indicate to use that carb ice was forming again. We made it to our cruising altitude of 6;500 feet and planned to cross the mountains by way of overflying [an enroute airport] and then following [a] road. I have always followed roadways over mountain ranges just to be safe in case of engine problems. We were on flight following with center throughout this leg as well. Everything was running smoothly in cruise and carb heat checks were to standards until reaching about 12 miles to the west of [the enroute] airport. At this point; the same issue occurred again in the same way; but this time much worse and much more rapidly. The egt gauge's needle dropped to the bottom of the peg; and the engine began to run rough again; this time even more roughly than before. I again pulled on carb heat and applied full throttle; only to [experience the] airplane shaking violently and the engine losing power rapidly at this power setting. The engine again only ran smoothly at 1500 RPM or below; and at this point the closest we could get to level flight was a 500 foot per minute descent. I then made a call on center alerting them that we had some engine problems and were addressing them. They told us that the nearest airport was a small private airport 5 miles to the southwest of us nestled in the hills. By this point in time; I planned on attempting to make it to [that airport] as the emergency landing site. I continued to attempt to melt the carburetor ice as we had done on the last leg; but it didn't work this time. The engine only ran rougher and at this point I made the decision that we need to land; and I knew that no further attempts to fix the problem would permit us to maintain level flight. We were approaching about 5;500 feet MSL now and time was running out to reach [the airport]. I continued to try to get the carb ice to go away and had pointed the nose towards [the airport]; but it wouldn't work. At this point I decided that any further attempts to try and get the icing away would not help due to our altitude AGL. I then pulled the power to idle and pitched for best glide. At best glide; our ground speed; altitude; and distance from [the airport] wouldn't permit us to reach it; especially with all the hills in the way that would make it an even more dangerous idea. Once I realized this; I [called] center and told them that I would not make [the airport] and that I would be landing on the road below us. I then made the descent to set up for the road. There was a straight stretch; then a slight bend in the road towards the right; then a long straightaway without power lines running parallel to it. The long stretch looked like our best bet. There were a few cars running in the direction I was headed; and I did note one headed towards us; but at this point I was committed and there were no better options for locations to land. I picked the touchdown spot; dropped the flaps; and turned final to the spot. I entered a forward slip so as to reach the spot properly as I was a little high. We overflew a set of power lines running over the road; and overflew the semi-truck and pickup truck that we had seen on long final. At the start of the bend in the road; my passenger pointed out a power line that was going across the road about 30 feet above the height of the road that we hadn't seen from the air before. We pushed the nose down rapidly to avoid it; and once under it; I had started in the slight bend in the road. I followed it to the right; stayed about 10 feet AGL to avoid a second set of power lines that crossed over the road and also had to avoid a truck that passed by our left wing as I was in the turn back towards the straightaway. I finally leveled the wings upon reaching the long straightaway in the road and landed the airplane. Thankfully there were no cars ahead of us; only behind us; and we then shut the airplane down and shut fuel off and got out. There was no damage to the airplane; my passenger and I; or anyone or anything outside of the airplane as well.

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Original NASA ASRS Text

Title: C172 pilot reported severe carburetor icing that ended in an uneventful forced landing on a road.

Narrative: My passenger; who is also a private pilot; and I; had planned to ferry this airplane back for its new owner. We agreed to share operating costs as this was a great hour building opportunity. I as a pilot had flown numerous times across huge portions of the country and took up the opportunity as more experience to be added to my logbook. The winds were calm; visibility was greater than 10; ceiling was reported as greater than 12;000; temperature 66; dew point 64; 94% humidity; and an altimeter of 30.20. I performed our run-up per the POH. All systems were functioning to spec and within operating limits. We took off intending to go to [an enroute airport] as the first leg of the trip. The climb out and level off all went smoothly. We leveled off at 8;500 feet MSL and were on flight following. About 5 minutes into the cruise; while scanning the gauges; we noted that the EGT had been slowly dropping; and as it was doing so; the engine began to run roughly. I noted that this was a definite sign of carb ice; so we pulled out the checklist for carb ice; and pulled on full carb heat and applied full throttle. This made things worse. The engine ran extremely roughly to the point that the entire airplane was shaking. Keeping the throttle at this setting for any longer than 5-10 seconds would make the RPM drop from 2200 down to below idle RPM; so I had to pull out the throttle to keep the engine running smoothly. The engine only ran smoothly [at] 1500 RPM and below; so at this point I made a transmission on flight following saying that we weren't going to declare anything but that we are going to deviate to [a nearby airport] due to engine roughness. As we got lower; I continued to use the carb heat technique in the checklist and it failed to make any difference until we were about 2;000 feet above the [field] elevation. As we were approaching 2;000 AGL; the engine with full throttle and carb heat did lose power at full throttle quickly; but the longer we kept it there; the roughness did begin to subside; so I chose to stay in a circuit 2;000 feet above [the airport] until the roughness went away and the ice was fully melted. It took about 5 minutes for it all to clear out; but by the end of the 5 minutes; the engine ran smoothly at full throttle and with carb heat on. I checked to see if it would also run smoothly with carb heat off and it did. At this point I decided to land to wait about 30 minutes to ensure that the ice had fully melted. After waiting; we started up again intending to continue. I performed our normal run-up and everything; including carb heat; functioned the way it should. The only difference in this run up was that I also elected to perform a full throttle static RPM run-up for 30 seconds to ensure there was no roughness at full power. The engine ran smoothly and to spec so I decided that it would be safe to continue. We took off and I was a bit more cautious about the possibility of carb icing. On departure; the winds were calm; visibility was 10 miles; sky condition was 12;000 broken; temperature 64; dew point 63; humidity 93%; and altimeter 30.25. Every few minutes; I would pull the carb heat on to ensure that there was a normal drop in RPM; rather than an excessive one that would indicate to use that carb ice was forming again. We made it to our cruising altitude of 6;500 feet and planned to cross the mountains by way of overflying [an enroute airport] and then following [a] ROAD. I have always followed roadways over mountain ranges just to be safe in case of engine problems. We were on flight following with Center throughout this leg as well. Everything was running smoothly in cruise and carb heat checks were to standards until reaching about 12 miles to the west of [the enroute] airport. At this point; the same issue occurred again in the same way; but this time much worse and much more rapidly. The EGT gauge's needle dropped to the bottom of the peg; and the engine began to run rough again; this time even more roughly than before. I again pulled on carb heat and applied full throttle; only to [experience the] airplane shaking violently and the engine losing power rapidly at this power setting. The engine again only ran smoothly at 1500 RPM or below; and at this point the closest we could get to level flight was a 500 foot per minute descent. I then made a call on Center alerting them that we had some engine problems and were addressing them. They told us that the nearest airport was a small private airport 5 miles to the southwest of us nestled in the hills. By this point in time; I planned on attempting to make it to [that airport] as the emergency landing site. I continued to attempt to melt the carburetor ice as we had done on the last leg; but it didn't work this time. The engine only ran rougher and at this point I made the decision that we need to land; and I knew that no further attempts to fix the problem would permit us to maintain level flight. We were approaching about 5;500 feet MSL now and time was running out to reach [the airport]. I continued to try to get the carb ice to go away and had pointed the nose towards [the airport]; but it wouldn't work. At this point I decided that any further attempts to try and get the icing away would not help due to our altitude AGL. I then pulled the power to idle and pitched for best glide. At best glide; our ground speed; altitude; and distance from [the airport] wouldn't permit us to reach it; especially with all the hills in the way that would make it an even more dangerous idea. Once I realized this; I [called] Center and told them that I would not make [the airport] and that I would be landing on the road below us. I then made the descent to set up for the road. There was a straight stretch; then a slight bend in the road towards the right; then a long straightaway without power lines running parallel to it. The long stretch looked like our best bet. There were a few cars running in the direction I was headed; and I did note one headed towards us; but at this point I was committed and there were no better options for locations to land. I picked the touchdown spot; dropped the flaps; and turned final to the spot. I entered a forward slip so as to reach the spot properly as I was a little high. We overflew a set of power lines running over the road; and overflew the semi-truck and pickup truck that we had seen on long final. At the start of the bend in the road; my passenger pointed out a power line that was going across the road about 30 feet above the height of the road that we hadn't seen from the air before. We pushed the nose down rapidly to avoid it; and once under it; I had started in the slight bend in the road. I followed it to the right; stayed about 10 feet AGL to avoid a second set of power lines that crossed over the road and also had to avoid a truck that passed by our left wing as I was in the turn back towards the straightaway. I finally leveled the wings upon reaching the long straightaway in the road and landed the airplane. Thankfully there were no cars ahead of us; only behind us; and we then shut the airplane down and shut fuel off and got out. There was no damage to the airplane; my passenger and I; or anyone or anything outside of the airplane as well.

Data retrieved from NASA's ASRS site 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.