Carb Ice

[Larry Biever]

Hi,

I was wondering if anyone has had carb icing in a 180? Last weekend I was flying ifr at 15,000 and had issues with carb ice, but I'm not sure what I did wrong that it became an issue that I couldn't resolve with carb heat. To make a long story short I didn't expect to be in the clouds very long and had troubles keeping the engine running smoothly. I was over the mountains so descending below 14,000 was not a option. OAT started at +10 and was -18C when level at 15,000. I was in clouds - absolutely no airframe icing. I entered the clouds at -5C no carb heat. After about 5 min in the first cloud I did fly thru snow pellets for a few minutes. Everything was fine for about 10 min then got a burble in the engine. Mixture was leaned to rich of peak prior to cloud entry. As soon as I got the burble I added full carb heat - did not touch the mixture. Got the normal drop when heat is added and I left the heat on for about 2 min, then went back to full cold. Everything was good for about 10 min then the engine did it again. This time I went to full heat and left it on and took action to get out of the clouds. I climbed up to 17,000 and was still in the soup. After about 5 min, with the carb heat on the engine once again started running rough. I had to lean the mixture to get it to smoothen. I was diverting to the closest airport, which was 60 nm away so spent 30 min, in and out of the soup adjusting the mixture to keep the engine smooth. I completed a circling decent over the airport vfr, and prepared for the engine to quit at any moment. On final, once over the runway, about 40 feet above the runway, I adjusted the throttle slightly and the engine completely stopped. It was a harder landing than I would have liked, but it was safe. The engine was windmilling so I was able to restart after coasting down the runway. The engine now had a knock in it. After parking I noticed water was dripping from the belly just behind the engine. I did a run up after 15 min and it was perfect. A mechanic went thru the engine the next day for a couple of hours and nothing could be found - when in my mind confirmed carb ice. No water in the fuel sump was found for either tank.

So - what I'm trying to figure out is why carb heat did not look after the carb ice. I've heard different ideas ranging from I added heat too late and it couldn't catch up, to I should have gone full rich as soon as I added heat and leaving the mixture leaned counter acted the heat. I've also heard I should have added heat as soon as I entered the first cloud when the temp was below freezing. I won't allow myself to get into this situation again, but I'm still hoping to find out what I should have done differently from an engine management perspective to prevent the carb ice. I have a carb temp sensor system on order as I type.

Thanks
Larry

[Mark Anderson]

Larry,

Any chance this may have not been carb ice and could have been your magnetos breaking down at altitude? I had this happen once and will make the engine run extremely rough. When the seals start leaking oil that will cause the cylinders to fire at the wrong time. If this would have been the cause it should have gotten much better with decrease in altitude. I have had carb ice quite a few times in my 250, however never had a case where it would not clear. Would like to know the cause if you figure it out.

Thanks,

Mark

[Wayne Braden]

I also had similar incident in Michigan, IFR, in clouds 8K, no airframe ice, engine ran rough with carb heat and lean mixture, lowered my altitude to 6K and situation improved. I had new mags, plugs, and ignition harness, at annual. I took off from TVC with the temp -12F, I believe some other air restriction to the carb was at fault! I have spent enough time trying to troubleshoot this situation, have had little issue with it since. I think someone has info on this problem, next is a carb overhaul!
Wayne

[Larry Biever]

Hi Mark,

I'm not sure if it could be a mag problem. The plane just came out of an extensive annual about a week before so the ignition system was checked. The engine completely quit when I was 40 feet above the runway, as soon as I reduced power, so I wouldn't land too long. The ambient temp had warmed up from -18C at altitude to +3C at the airport. I managed to restart the engine on rollout and it had a knock similar to a stuck valve. But after the plane sat for about 15min, the run up was fine.

I've done a lot of reading and one thing that I'm wondering about is, at 15 to 17000 feet the engine is only about 50 ish % hp and when the ambient temp was -18C, is it possible I didn't have enough heat to warm the air enough to get out of carb freezing range? I don't have a carb heat temp, so maybe the air didn't get hot enough to melt the carb ice? I've talked to a lot of people trying to figure out what happened so I don't ever do it again, and also to share the info with fellow pilots.

Thanks,
Larry

[Larry Biever]

I was just thinking of another question. Is flying into clouds below freezing with a carbureted engine somewhat routine or is it something that is rare and unusual?

[Mark Anderson]

Larry,

I wonder if you could of had a small amount of water in your fuel that froze and blocked the line? Did you try swapping tanks?

Would not call it unusual; given the choice obviously would try to avoid it; wouldn't want to stay in it for extended amount of time; wouldn't cancel a flight because of it; would rather have fuel injected if given a choice.

Never hurts to be early or preemptive with the carb heat.

Mark

[Larry Biever]

I turned the electric pump on and tried both tanks. My fuel pressure stayed constant so I don't think I had fuel line icing.

I've been replaying the flight over and over in my head. I remember when descending into the airport I had a small amount of backfiring. Not big ones, but small ones that were just noticeable. Is this from too little air and incomplete combustion?

[N3322G]

Larry,

Decades back we had a similar set of symptoms - turns out some fuel system components had aged and were throwing off debris. Replaced some lines/hoses, cleaned everything - sump screens, injectors etc. So while we have injectors, the symptoms - especially the backfiring - sounded similar so I thought I'd pass along our experience.

[Larry Biever]

Thanks Pat. I'll check my carb sump.

[Tim Weigle]

A couple notes to think about. Not knowing which exhaust system your engine has stock or update dual. I would check the heat muff closely for any air leaks around the shield and the condition of the hoses going to the carb air box. I would assume at your altitude and the cold OAT your cabin heat was on full which reduces the air pressure inside the heat plenum. It may have been just not enough warm air getting to the carb. The engine will run much cooler at those altitudes which reduces temperature through the entire system. In my 250 I can recall one instance where I was a little lax getting the carb heat on in perfect icing conditions and it took a while along with clear air to clean it up. I have a carb air temp gauge but I generally watch manifold pressure closely as it usually warns you by dropping first when ice is present, and keep in mind the prop governor will compensate to a point keeping RPM steady when engine power drops.

The last thing to check is the fine screen in the carb. Make sure there is no debris present.

Tim

[Zach Grant L1011jock]

Larry,
I am assuming you had the throttle full open during this entire episode at altitude down to when you pulled the power? There are couple of things here. First, if you were not full throttle, the butterfly valve is not going to be in its most open position, and you will have more impact surface area for ice to accumulate, and less space before you close off the airflow to the engine. The second is that you open the economizer circuit in the carb, venting cold air directly to the main jet. IF you had suspended water/ice in the fuel, you could have iced up the main jet as well as having ice in the venturi. If the engine was not creating much heat, you would not have much carb heat to work with, and at those temps you would need some pretty good heat to overcome a cold soak.

I fly in the clouds all the time with carburetors, and especially in my Comanche 180, full throttle, full carb heat has solved any issues I have had. I have never gotten carb ice in the 180 at wide open throttle. I have seen it many times at reduced throttle settings, and I have had induction ice from freezing rain/heavy snow that has clogged the air filter and I have had to use the carb heat like alternate air to keep the engine running. As has been pointed out here, simply a rough running engine is not usually a signal of carb ice. The most reliable signal of carb ice is the commanded reduction of manifold pressure, and then a recovery of same when carb heat is applied.

Being that you were at a high altitude/low temperature condition, you may very well have other issues. The backfiring leads me to think you had a stuck/obstructed valve, or a mag/plug issue. either of these can cause you to get afterfires in the exhaust as unburned fuel is expelled from the engine. Unless you have a graphic engine monitor, it will be very difficult for you to figure out. In fact, you may actually be chasing two issues, and the engine quitting on final and the roughness at altitude may not be related.

-Zach

[Larry Biever]

Thanks Tim and Zach. I have factory exhaust and was likely running full cabin heat. I don't recall pulling back from full throttle during or after climb out and cruise. The engine running rough was periodic only lasting 5 to 10 seconds while at cruise, happening about 6 times. I did reduce throttle when starting my circling descent while beside the airport. I had to come down 9000 feet so had no choice. I was clear of cloud at this time. I don't recall if I used carb heat at this time. I agree I might have 2 different issues. When I got my engine restarted by adjusting the mixture on rollout, there was a very noticeable knock that I could feel in my foot pedals. This knock disappeared after the plane sat 15 min and I restarted. Could this have been from ingesting the water from the carb or is there more I should look for? The mechanic pulled all 4 bottom plugs and they we're clean and properly gapped. We did not pull the top plugs. I am seriously looking at at EI EBG16 with carb temp so I can better understand what is going on.

[Michael Bryant]

Larry,

I note that you indicate you did not touch the mixture after applying carburetor heat. Carb Heat does enrichen the mixture and you may have needed to adjust the mixture.

A side note. During the first annual after purchasing our 180, it was noted there was some light leaking through a crack in the carb heat box. On removal, it was discovered that the carb heat valve was installed backwards and with nuts/bolts. It would not fully close nor fully open, we had partial carb heat/unfiltered air full time. We also could never obtain full carb heat if it was needed.

We had Webco repair the carb heat box. On return, the carb heat valve was properly riveted in place and fully closed in both directions as it was supposed to. A look at your carb heat box could be in order.

Michael

[Larry Biever]

Thanks Michael. I did lean after turning the carb heat on, just forgot to mention it. Interestingly enough I've had mixed response about this. Some agree you need to lean since you've enrichened by adding hot air and some say you should go full rich because leaning also provides more cooling by restricting the flow. Their thought is that when going carb heat on, you also want full power and full rich. I don't think our POH discusses this though?

After talking to my mechanic a bit more, we discussed that the engine just passed 400 hrs since overhaul so it's time for the SB that checks the valve clearance, wear and carbon deposits. That should confirm or rule out a sticky valve I would think.

[James Turner]

Hi Larry,

For what it is worth if I apply carb heat at any altitude (above say 9000') with full rich mixture my engine will run rough. I always lean a little through ~5000' on climb out now (to ~15gph providing CHT's remain less than 400 deg).

James.

PA24-180.