Carb ice brings down Cessna

Aircraft: Cessna 150. Injuries: 1 Serious. Location: Fitchburg, Mass. Aircraft damage: Substantial.

What reportedly happened: On the day of the accident, the student pilot, accompanied by a CFI, conducted a touch-and-go landing. While on the downwind leg of the traffic pattern for the second landing the engine began to run roughly.

The student applied carburetor heat and kept it on until touchdown. After landing the student disengaged carburetor heat.

During climb out after the second landing, the engine began running roughly again. The CFI took control of the airplane, lowered the angle-of-attack, and reapplied carburetor heat.

According to the CFI, the engine subsequently ran more roughly, so he immediately placed the carburetor heat to “off” but the engine still ran roughly.

The CFI decided that there was not sufficient remaining runway to land and he was not sure how much power the engine was producing or if he could maintain level flight. The CFI decided against attempting to turn back to the runway, so he continued straight ahead. The CFI then saw a building ahead and realized that the airplane was too low to fly over it, so he banked to the right to avoid it. The airplane mushed into trees.

A post-accident review of weather data and a carburetor icing probability chart revealed that, at the time of the accident, the ambient temperature and dew point favored serious icing.

Industry and FAA guidance advised pilots to be aware of the warning signs of carburetor ice, including loss of rpm and rough running engine and advised that the pilot should respond to these warning signs by immediately applying full carburetor heat, and that the engine may initially run roughly for a short time while the ice melts. Therefore, the flight instructor should not have turned off the carburetor heat when the engine was still running roughly after he turned it on.

Probable cause: A partial loss of engine power due to the formation of carburetor ice and the flight instructor’s improper application of carburetor heat.

NTSB Identification: ERA12FA143

This January 2012 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.


  1. Jeff says

    Not sure where some get their information, but the rough running when you pull carb heat with carb icing is the melting water going into the engine, not an overly rich condition. Lean it out and you may stop the engine.
    Why would you put the carb heat back in 10 – 15 feet from the runway? At that point you may increase the engine power and cause a float. Leave it out until landing.

    • Henrik Værø says

      Carb ice restricts the amount air going into the engine, so the mixture is already enrichened by carb ice before carb heat is applied. Adding carb heat reduces the weight of the air going into the engine, further enriching the mixture. (Because of this, carb heat always enrichens mixture, ice or not.) The added effect of ice plus carb heat will in some cases make the engine lose power or even threaten to quit due to over-richness. That is why leaning helps in these cases. And yes, the melting ice running through the engine does not make it easier for the engine, either.

  2. Henrik Værø says

    Rough running after carb hear application is due to over-rich mixture. Leaning helps and can actually prevent the engine from losing all power. Just remember to enrichen the mixture again as the ice goes away.

  3. John says

    Reading this story reminded me of a flight in a Swift with a 90 hp Continental in solid IFR (yes, a Swift can be flown IFR) about 25 years ago. It was December I was flying at 6000 feet in solid cloud and had been in that situation for over an hour. ATC was giving me vectors on a ILS approach to Brunswick Georgia and the power reductions while on decent caused the engine to run rough. I applied carb heat and it seemed to get worse so I removed carb heat and it stayed about the same then the engine almost seemed to be coming apart. I reapplied carb heat and left it there this time and what seemed like hours, probably 30 seconds, it smoothed out just as I broke out at about 500 feet with the runway in front of me. Prior to this event I had flown a lot of little Continentals on clear days VFR never even using carb heat. I haven’t the last 25 years. Oh and once I pull carb heat I do not push it in until I am 10-15 feet from the runway on very short final.

    • says

      John –

      That rough running sound is one of the worst parts of induction ice – the fix can sound worse than the problem! It’s tough to teach pilots that with carb heat, if it sounds worse – it’s getting better.

      Nice comment.


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