carburettor icing and the defensive use of carburettor heat

Richard Keech's advice to readers of Vintage Piper Aircraft Club News is relevant to many of us.
reproduced from GASCO

A Piper J3 Cub. You get virtually no heating of the induction air with this cowling arrangement

Earlier this summer one of the main UK general aviation flight safety magazines (Flight Safety) published a leading article that was very critical of the design of aviation carburettor induction systems. Specifically, the article highlighted that these systems, which are in such common use and so critical to flight safety, are prone to suffering from icing, icing that statistically causes 10 occurrences, including 7 accidents a year. Now, since all our Vintage Pipers are equipped with carburettor induction systems we clearly have a problem ... or do we?

Well, before I proffer my own thoughts on the subject you'll be relieved to hear that I'm not proposing to delve into theory of carburettors and carb icing, all that should be, hopefully, second nature to any licensed pilot who flies in the moist skies of the U.K. In any case the CAA have provided all the necessary information in AIC 145/1997 (Pink 161) and that is freely available from Given that our type of flying is almost exclusively VFR and clear of airframe icing conditions a review of the CAA information suggests that carburettor icing is our principal problem and that fuel icing and intake/impact icing are not really going to be a factor for our Vintage Pipers. And to be sure, carb. icing is most certainly a problem if we don't take the  necessary steps to prevent its formation. The potential for icing was brought home to me after landing in Lyle Wheeler's J3 Cub following a 90 minute flight in the heat of Florida.

As you can imagine that little Continental was HOT, but its carburettor was dripping with condensation; it looked like a can of Pepsi that had come straight from the fridge. As we all know, our carburettors are usually operating in potential icing conditions in the U.K. especially when the engine is giving reduced power such as when taxiing or descending and, equally, we also know that the solution to our problem is to apply carb. heat. OK, but the question is when to apply that heat? I say this because most of us were trained to fly on slightly more modern aircraft than our Pipers and those aircrafts' flight manuals give conflicting advice as to when to use the carb. heat. For example, one well known type that I once instructed upon had a flight manual that advised pulling the carb. heat for a few seconds on the downwind leg and then returning it to cold whereas another type's manual advised applying the carb. heat on the downwind leg and leaving it on until after landing. My Colt's owner's handbook advises that the carb. heat should be "off unless carburettor icing conditions prevail". Brilliant! So when is it best to apply the carb heat? I will now humbly proffer my own thoughts on this particular topic.

Before I spell out my personal recommendations regarding the appropriate use of carb. heat I just want to look at the advantages and disadvantages of putting hot air from the exhaust shroud down the carburettor.

Timely application of carb. heat prevents carburettor icing and can melt ice that has already formed


1.  Use of carb. heat reduces engine power and efficiency.

2.  During the application of carb. heat the inducted air is unfiltered.

3.  Application of carb. heat under conditions of high engine power can cause detonation.

Clearly, these disadvantages preclude the permanent use of carb. heat, but equally, there are times when lack of use is definitely a hazard to flight safety so it's easy to see why there are so many problems. However, the positive side of this is that during those occasions when the use of carb. heat is not appropriate the engine is not likely to suffer from carburettor icing. For example, during takeoff and climb when the throttle butterfly is fully open and the engine is developing full power the use of carb. heat is not recommended due to the disadvantages 1 and 3; in fact detonation can cause long term damage to the engine.

Happily however, the formation of carburettor ice is most unlikely in this condition. To those who seem to have experienced carburettor icing in the climb I would say that it is likely that the icing formed when the aircraft was at low power on the ground prior to takeoff. Conversely, during those low power flight regimes when carb. icing is most likely, the extended use of carb. heat will cause no damage to the engine except in very dusty conditions when the lack of air filtering could be a factor. However, on balance, I feel that most would prefer to accept this fact rather than risk possible engine stoppage. Therefore, taking all this into account my personal recommendations concerning the use of carb. heat during the usual flight conditions are as follows:

Pre Takeoff: Prior to checking the carb. heat during the engine run-up note the stabilised engine rpm. Apply full carb. heat for about 10 seconds and check that the rpm drops by around 100. After returning the control to "cold", note the rpm; if it has increased from the previously noted reading then carb. ice was present. The procedure must then be repeated until no rpm increase is noted following the check i.e. all the ice has been melted. If prolonged holding is required following the run-up then repeat the check.

Takeoff and Climb:
The carb. heat should be left in the "cold" heat to cold on short finals to cater for a possible go-around. In any event set the carb. heat to cold when on the ground so that the engine receives filtered air since this is the environment that is most likely to be dusty.

And there we are, my own recommendations for the defensive use of carburettor heat. Defensive because I feel that is safest to assume that conditions in the U.K. are always conducive to the formation of carburettor ice and I can testify that these procedures work; I have never been troubled by carburettor icing. Finally, a historical thought; Charles Lindbergh trusted his life to the Wright radial engine that, as we all know, ran faultlessly during his epic flight between New York and Paris. It wasn't always that reliable. Prior to the famous 1927 transatlantic trip Lindbergh was plagued with engine problems during his positioning flight from California to the east coast due to, you've guessed it, carburettor icing. Needless to say, before the Paris flight he had a carb. heat system fitted to the "Spirit of St Louis". I hope that he remembered to apply and pay for a CAA major modification authorisation!
Safe Flying

Richard Keech