The REAL Cause of Many Unexplained Accidents?

A topic we tend to dismiss in hot weather is carburetor icing. Humid air is plentiful in the summer, and temperatures inside the carburetor can drop 30 to 40 degrees. Summer or early fall are not the times to forget "carb ice." No matter how many hours we have logged, that "carb ice" gremlin can sneak up and catch us by surprise. Engine runup on the ground is by far a better place to discover it than during flight. But many times it is during flight that carb ice rears its ugly head. And when it happens it may have progressed to the point where the only way is down; that is, an immediate landing with little or no available engine power. But first, let's review this ever-present problem, look at means of detection, and share some timely methods for staying ahead of engine icing problems. 

What is Carb Ice and What Causes it?

There is always some degree of moisture (humidity) in the air that flows into and through an aircraft engine for every unit of fuel burned. A carburetor provides the explosive air/fuel mixture to each cylinder in the engine, where your power is generated.

As air is drawn into the small throat of a carburetor, the venturi effect accelerates the air and cools it. It cools even further when mixed with vaporized fuel. When this moist air reaches the freezing level of 32°F, the ice particles that begin to form deposit themselves on the throttle plate. The carburetor can then become choked up by this ice to the point that the engine receives less air than is required for full power. The once-explosive air/fuel mixture becomes so rich from excess fuel that the engine ceases to fire.

What Conditions are Conducive to Carb Icing?
Basically: Whenever the cooling effect of the air flowing through the carburetor is sufficient to bring the temperature of the carburetor throat down to 32¼F or colder AND there is sufficient moisture in the air.


If the outside air temperature (OAT) is between about 20¼F and 30¼F with visible moisture or high humidity If the relative humidity of the outside air is high, even in a cloudless sky, with an OAT as low at 15¼F and as high as 100¼F In the spring and fall, especially just after a rain In other words, carbureted engines are susceptible to icing almost anytime.

Are All Internal Combustion Engines Prone to Carb Ice?

No. In pressure carburetors, fuel is admitted at a point past the throttle plate and downstream from the air inlet throat. It is well on its way to the hot engine before ice can form. In fuel- injected engines, the air/fuel mixture sprayed into the engine cylinders is metered into the hot cylinder at temperatures near that of a volcano. So, there is no potential for carb ice on either of these types.

Our concern is the mixture of evaporating fuel and moist air before it gets near the heat of the running engine, and this only occurs in the float-type carburetors found on most light aircraft.

There are two opportunities to detect the subtle indication of developing carb ice. The subtlety is a gradual, small drop in RPM on a fixed-pitch prop aircraft, even though the pilot did not retard the throttle. On a constant-speed prop aircraft, carb ice is manifested by a gradual, small drop in manifold pressure (MP) while in flight.

If detected early and dealt with correctly you can easily prevent an untimely engine stoppage.

During Pre-flight Engine Run-up

On the ground during engine run-up, ice is easy to identify positively and remove. On a Cessna, for example, at 1,700 rpm the carburetor heat control is pulled out fully to the hottest position. Because air entering the carburetor after application of carb heat is warm (from the engine compartment) and less dense, you will notice an rpm decrease of 100 to 300 rpm, and the rpm should remain low until the carburetor heat control is pushed all the way back in. However, if the 100 to 200 rpm decrease is noted but slowly begins to increase so that when the carb heat control is pushed back in the rpm reads more than the original 1,700 rpm, you had carb ice.

If it happened on the ground, it can happen again during takeoff. While lined up on the centerline, just before takeoff, I heartily recommend another carburetor heat check.

In Flight At Constant Cruise Throttle Setting

An often asked student question during their ground school training is, "If the engine fails in flight because of carb ice, why not just apply carburetor heat to melt the ice?" That is a reasonable question, but let's look at why that action is more than likely too late to help.

Carburetor heat is obtained essentially from within the engine compartment (rather than directly from the intake air filter on the front of the aircraft). If the engine has cooled sufficiently because of an excessively rich air/fuel mixture, there may not be sufficient hot air in the engine compartment to melt the accumulation of carb ice even with the carb heat control to full "hot." That is the point when the engine will cease developing enough power to keep your airborne.

It is of the utmost importance to keep a sharp eye on engine performance at all times. At the slightest hint of deteriorating power (from decreasing rpm or MP) use carburetor heat for at east eight to 10 seconds or for however long the aircraft's manual recommends.

The Explanation

Many accidents and off-airport landings may have occurred because unsuspecting pilots-having noted decreasing engine power during cruise--pulled on carburetor heat. Their engine immediately began running very roughly and/or backfiring. In response to such noise, they removed carburetor heat and continued on their way. Continued, that is, until the engine continued to slow and eventually quit.

What happened?

When the pilot applied carburetor heat at the first sign of decreasing power, the warm air from the engine compartment did just what it was supposed to do. It melted the ice from the carburetor throat and throttle plate. Where did the melted ice (now water) go? Right into the engine that wants to burn gasoline, not water; hence, a momentarily rough and perhaps backfiring engine.

If carburetor heat had been kept on long, the hot engine and warm carburetor heat air would have kept the carburetor ice-free.

How to Fly Safely in Carb Ice Conditions

When icing conditions exist, apply carburetor heat often at your cruise throttle setting.

Never use partial carburetor heat unless the aircraft is equipped with a carburetor temperature gauge, ice light, or similar instrumentation. Partial heat can possibly make matters worse, unless you know exact carburetor temperatures. Remember, full heat or nothing.

When below the green band on the rpm or MP gauge, apply carburetor heat. It is good practice to use it on any descent where you have reduced power. Use that carburetor heat control on any aircraft at reduced power settings and especially for landing.

In conditions conducive to icing or if ice was noted during engine run-up, perform another carburetor heat check immediately before takeoff. DO NOT leave carburetor heat on for the actual takeoff. Check your POH or aircraft manual for general guidance.

To be totally knowledgeable, I recommend that everyone read FAA Advisory Circular 20-113 concerning precautions and procedures for preventing induction and fuel system icing.

Final Reminder

Carburetor heat will keep carb ice from forming if applied early enough, often enough, and long enough.

AC 20-113, Pilot Precautions and Procedures to be taken in Preventing Aircraft Reciprocating Engine Induction System and Fuel System Icing Problems, is free from U.S. Department of Transportation, Subsequent Distribution Office, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785.

by Pete Humphrey

Mr. Humphrey is from Hagerstown, MD and is an Aviation Safety Counselor for the FAA's Baltimore Flight Standards District Office. This article originally appeared in the Baltimore FSDO's Aviation Safety Program newsletter, Mid-Atlantic Flight Safety Monitor.
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