carburettor icing and the defensive use of
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
www.ais.org.uk. 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
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
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!