Critical Understanding #10 – Carb Ice


About 10 years ago, a builder completed a magnificent Zenith 601XL and readied it for it’s first flight. He chose a man who presented himself as an experienced pilot to do the first flight. Although every manual I have sold in the last 15 years contains the Carb Ice story below, the pilot didn’t believe it. I know this because he said it directly after he wrecked the plane on the first flight. He flew away from the airport, and elected to fly at low altitude and low power for some reason, never using carb heat. The probable cause of resulting forced landing was ruled carb ice. I am quite sure almost all the people who watched the events blamed the Corvair, in spite of the fact the carb was identical to the one used on an O-200, and had the plane been equipped with a Continental, it would have had the exact same forced landing, because Physics doesn’t make exceptions for idiots who debate its existence.


Carb Ice is a topic that everyone who flies light planes should understand, but unfortunately, the percentage of people who have taken the time to learn this critical part of operating light aircraft is dropping. This is partially tied to the demise of the traditional, career flight instructor, and partially due to an ever increasing percentage of people who approach all learning opportunities with the question “Will this be on the test?”


Funny thing; the subject of carb ice isn’t prominently featured in FAA tests, but it is a very real part of the tests run by flying’s oldest law firm P,C & G.*  and before you fail one of their tests, I’d like to point out they have never had a case overturned on appeal.


Take some time to read and understand this story: It was written by Grace in 2001, and it is the most reprinted story we have ever put out. It has appeared in magazines all over the world, but idiots have a certain kind of coating, that you can pour information on them, but evidently it beads right up and rolls off without sinking in. Don’t be one of those people.


Armed with the information above, now go back and review the information below in CU #9.  As a general rule, anytime your engine is operating below 75% power, you should use carb heat. Critical Understanding #9 -Percent of Power and fuel flow.


There will be people who tell you some aircraft are immune to carb ice. This is a very foolish myth. Fuel injected aircraft have alternative air doors for a reason. Training with Rotax 912s have provided a generation of pilots who don’t understand carb heat, because Rotaxes have full time carb heat. Ellison carbs are often said to be immune, but “Carb Heat Required” is cast right into the body of the carb. There are variations in how susceptible some installations are, but this is nothing you would bet your life on. As a Corvair guy, you will know how to operate any aircraft, and not be restricted to operating a control-less Rotax 912 after driving a Prius with an automatic transmission and automatic braking to the airport. The reward for understanding is being able to operate all types of planes and engines, real machines, not just appliances which have allegedly been ‘idiot proofed.’


I don’t care what temperature it is outside, or if the plane has a Lycoming Continental or a Corvair, when I pull the power back, I put the carb heat on. I am sure that 90% of the time, the conditions in Florida preclude needing carb heat, but I use it anyway. I am sure that I could drive through red lights 90% of the time and not have an accident, but I don’t because there is a certain penalty associated with being wrong, and there is no reward for guessing when you might not need Carb heat.  If a conversation starts up about carb ice, and the first thing out of someone’s mouth is that you don’t really need it, nod politely, and give that person a wide berth. I could fill an entire evening with stories of collateral damage done to people by idiots in aviation. A wide berth means you never get in a plane with them, you don’t fly in the pattern when they are out, and you don’t listen to their opinions nor advice. If that sounds harsh, come find me after hours at Oshkosh and I will share how Phil Schacht, the aviator who was an irreplaceable element is Grace’s development as a pilot, was killed by an idiot. Come with an empty stomach, because the story includes him burning to death in his plane, while the idiot responsible escaped to flee our country with his worthless life.



What carburetor ice looks like; get a glance at the intake manifold tube above the carburetor in this photo of Jim Barbour’s running at Barnwell 2011. Despite the engine being quite warm, the solid white that you see is pure ice that is frozen on the outside of the test stand’s intake manifold. You don’t need x-ray vision to understand that there is matching ice on the inside of the manifold also. Although it was cold, the main effect of icing is caused by the evaporation of the fuel coming out of the carburetor. Look at the sunlight and shadows and understand it was a clear blue day out, so all people who say it has to be overcast to ice are idiots.  The fact this is a Corvair has no bearing, this is a Continental O-200 carb, and it would look exactly the same running on an O-200.




flying’s oldest law firm P,C & G.*  = The laws of Physics Chemistry and Gravity. Read: Risk Management – Human factors ” The evidence that fools present for the existence of luck is vague and anecdotal at best.  Hard, proven and factual evidence for the existence
of Physics, Gravity and Chemistry can be found at any crash site.”


Note Book Section:


Make line 10.1 in your Hand book a hand written entry, stating  “As a general rule, anytime your engine is operating below 75% power, you should use carb heat” and “Cab heat is to be used as Anti-ice, not De-ice.”


Make line 10.2 in your Hand book a note showing the RPM drop from applying carb heat at idle. Note both RPMs and the OAT. The minimum acceptable RPM drop when the engine is warmed up, is 125 RPM.


Make line 10.3 in your Hand book a list of procedures when the pilot will use carb heat, other than power reduction. They would typically include, but not be limited to,  inadvertently flying into rain,  Engine running rough in flight, etc.  


Make line 10.4 in your Hand Book a notation on the effect of automotive fuels being more prone to ice than aircraft fuels.


Make line 10.5 in your Hand Book a series of sample conditions where the rpm and MAP would indicate less than 75% power and therefore require the use of carb heat.


About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 30 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at and in more than 50 magazine articles.

11 Responses to Critical Understanding #10 – Carb Ice

  1. Dennis McGuire says:

    I built a Volksplane back in the 70’s
    and it had a 1600 VW engine with
    a Posa carb. Started out with a heat box but due to the Posa carb leaked
    when the the fuel was turned on.
    Well it leaked fuel into the carb heat box
    and when I started her she backed fired and caught fire. No real damage done but I took the heat box off and never put it back on. I flew her many hours with no carb heat.

    • Dennis, none of Steve Wittmans planes had carb heat, because he always flew at 100% power. However, here we are focused on today’s typical operations. I don’t know anyone, nor do I want anyone using a Posa on a Corvair, and I want the discussion focused on what works, not what can be gotten away with. 8% of our builders are in Canada, and heat is required there.

  2. Ron. Lendon says:

    The best advice I received about carb heat was from you WW. I do many local flights at reduced throttle, 65%. Always use carb heat.

    Most of my cross country flights are at 75-80% power and for landing, as soon as I see or should see the windsock, it’s full carb heat to landing.

  3. Dan Branstrom says:

    Because I live in the desert, where the humidity is very low, some pilots here often get nonchalant about carb heat. That’s a big mistake, especially if they fly to areas with much more humidity, and the rare days when the humidity is high, if, for no other reason than it’s a good habit.

    Murphy’s law (If something can go wrong, it will) is always in operation, and it’s a simple preventative for carb ice. My corollary to Murphy’s law? It will go wrong at the worst possible time.

  4. Craig Owen says:

    Iowa Bearhawk : carb ice it will kill you . I conected the throlle to the carb heat lever they move as one.
    Thanks Craig Owen

  5. Mike Crowley says:

    What do you think about the Aeroinjector supposedly not needing carb heat do to the lack of Venturi or throttle plate? I noticed on your reference pages you initially had one on your 701 test aircraft. Personally I’m planning on sticking with the proven MA-SPA3 combination, but I’m interested in your thoughts with the slide carbs.

  6. David Jones says:

    Once again, you are right on. As an air traffic controller, I witnessed the apparent decline in student pilot training. Carb heat had been pounded into my by a USAF tanker instructor. Your review here may save lives my friend. Thank you

  7. toomanyps says:

    Ice is good in soda and water on a hot day. Not so good in carburetors. I’ll be using a carb-heat box.
    Looking forward to seeing everyone in Barnwell next month. Hope you’re all having a great day.

    Lou Casella

  8. Phil Davis says:

    Articles like this are the reason I go to your website daily! Since I fly my Corvair-powered Sonex most everyday, cloudy and sunny, I use carb heat religiously! I like to know WHY I use it. Thanks Professor Wynne

    • Phil, for a long time after the NASCAR spoof Talladega Nights came out, Grace thought I should use the title ‘Professor Dickweed’ Like Ricky Bobby’s dad . That is as close as I have been to the title Professor.

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