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Carburetor Ice: A Nasty Surprise For All Types Of Weather

Carb-Ice Header FlyingLotus1983 / Flickr

NTSB safety alert: from 2000 to 2011, carburetor icing was a cause or factor in about 250 accidents - on average, causing or contributing to two fatal accidents per year.

The FAA found that between 1998 and 2007, there were 212 accidents attributed to carb icing - with 13 fatalities.

Surprised? Before you read on, think about the temperature outside right now. If you were flying today, is carb ice possible during cruise or descent? When you're done reading, tell me where you fall in the comments section. (Turbine pilots get a free pass on this one...)

The Conditions May Surprise You

We know carb ice happens - but when, and at what humidities and temperatures?

It makes sense that carb ice can happen at high humidity - lots of moisture means lots of icing potential. But, according to the NTSB, it's possible to pick up carb ice with relative humidity as low as 35% at glide power. Not a problem you want when turning base to final.

Icing Potential Chart

And, the FAA's chart above shows that carb icing is possible from 10F to over 100F (-12C to 38C), with serious icing possible from 20F to over 90F. (-7C to 32C) (We've remastered the chart to make it look better, but the raw version's available here.)

Preventing Ice is Much Better Than Fixing It

Preventing ice is your best bet to avoid becoming an NTSB statistic. Use your carburetor heat whenever icing is probable, following your airplane flight manual or operating handbook's instructions. You can even pick up ice when taxiing or idling on the ground. If ice is likely, run the carb heat for a few minutes prior to takeoff to make sure that the carburetor is clear.

Picking Up Ice? Here's How to Detect It

Your first indication of carburetor icing is usually a drop in RPM or manifold pressure. If you don't correct, you'll notice engine roughness after a while. If you're still flying around with your head in the clouds, you'll soon be gliding.

Fixing It Sounds Even Worse

Remember the first time you pulled back the mixture to lean the engine in-flight? You hands were sweating - waiting intently for that rough sound, concerned that you'd kill the engine? Now imagine hearing that rough sound when you're trying to fix an engine problem - stressed and under pressure.

Carb heat works by directing warm air from around the engine into your carburetor. It melts the ice - which is then sucked up into the cylinders. Water isn't avgas, and your engine will cough and rumble as it digests it. But stay calm - that's a great sound because you're clearing the ice. Search the NTSB records and you'll find lots of cases where pilots turned the carb heat off because they thought they were making the situation worse - only to become a glider after a few minutes.

What Causes It?

The FAA gives a great description in AC 20-113, describing the three types of induction icing:

Impact Ice - Moisture Impacts and Freezes

When you have moisture laden air at temperatures below freezing, ice can form instantly as the moisture contacts anything solid. Imagine your air scoops, filters and carburetor components rapidly picking up ice as moisture packs on.

This type of icing is especially prevalent when you have snow, sleet, rain or other visible moisture. It can come on fast and build up fast, so quick action is a must and prevention is even better. Use carb heat, or alternate air for fuel-injected engines, anytime you're in these conditions. Don't use partial heat - you can actually make things worse by raising temperatures in sub-32F (0C) weather into the danger range. Full heat keeps you warm enough to stay ice free.

Throttle Ice - Throttle Back, Ice Up

Throttle ice occurs around a partially closed throttle, especially when your engine's at a cruise power setting or near idle. The cooling caused by the throttle's Venturi effect lowers the air's temperature, condensing the water vapor in the air. This water freezes on the throttle components, restricting airflow. Eventually, you'll lose RPM or manifold pressure, followed later by a rough running engine and eventually power loss.

The best, or worst, part is that you don't need visible moisture in the air - so you're probably not thinking about ice when you pick it up. Again, fix it with alternate air or carburetor heat - and expect roughness as it clears.

Throttle Ice

Fuel Vaporization Ice - Vaporizing Fuel And Building Ice

The third type of induction ice, fuel vaporization ice, results from the cooling effect when your fuel mixes with air and vaporizes in the carburetor. This drop in temperature causes the same result as throttle ice - condensing moisture out of the air and freezing it around your carburetor Venturi and butterfly valve. It's not a problem in fuel-injected engines, because those engines mix the fuel and air after the throttle valve.

As with throttle ice, the symptoms are the same: power loss (RPM or manifold pressure), followed by a rough running engine, followed by glider mode. The fix is the same, as well - turn on carb heat and let it rumble.

Carb Heat - Getting Over The Fear

One of the worst parts about induction icing is that the fix can sound so terrifying - making a rough running engine sound worse is nobody's idea of a fun time. However, once the ice has cleared, the engine will run smoothly.

On your next flight with a carbureted engine, take the time to turn on carb heat during cruise and note the sound. If you don't have ice, the engine should lose a little power, but run smoothly. The carb heat air is warmer than the free air, essentially enriching your mixture slightly.

What's Your Condition

Ok - so now that you know, is ice probable or possible today where you are? Let us know in the comments. And, if you liked this article, please share it! If you learned something, I'm sure readers on Twitter, Facebook or Reddit would, too!

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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