Sucking on Ice

Airframe ice isnt the only frozen stuff to worry about; induction ice can spoil the party, too


The sputtering sound of your airplanes power plant, followed by a sudden silence, and the realization that you are now the pilot of a heavy glider, can certainly get your immediate attention.

Cold weather brings with it the potential for ice – and not just ice of the airframe variety. Induction system icing brings down more than a few unsuspecting pilots every year. Even those who valiantly give wide berth to known icing conditions can be struck by the power loss that comes from ice in the engine compartment.

Induction System Icing
Induction system icing takes several forms – including impact ice, throttle ice and fuel vaporization ice. Any or all of these can play troublemaker by restricting the ability of the engine to create the proper fuel/air mixture. That means power loss or complete engine stoppage. The notorious carburetor ice is part of this phenomenon, but there are other trouble spots that can leave you just as vulnerable.

Carburetor icing can affect flight operations in both VFR and IFR conditions. Impact icing is found in IFR conditions.

The dynamics of operating when induction icing is possible depends on the kind of fuel metering system the airplane has. There are three basic types: float-type carburetor, pressure carburetor and fuel injection. Keep in mind that there are several different kinds of fuel injection, each with its own nuances.

Whether the engine boasts a carburetor or fuel injection, its susceptible to ice. Airplanes have vulnerable spots in the induction system where ice can accumulate. In addition, ice can find its way to the air filter or collect in bends in the system and any of the critical areas of the fuel-metering accessories.

Impact ice is formed when moisture-loaded air that is at temperatures below freezing strikes and freezes on parts of the induction system. Any time you have airframe icing youll also have some degree of impact icing. Ice builds up on air scoops, heat or alternate air valves, intake screens and parts of the carburetor.

The danger zone for impact ice is flying in snow, sleet, rain or clouds when the temperature is around 25 degrees F. In this situation, impact ice can build up very rapidly, regardless of whether the airplane has fuel injection or a carburetor.

As an anti-icing move, the pilot needs to immediately use carburetor heat or alternate air to prevent the ice from accumulating enough to cut off the engine completely. When impact ice is suspected – and it usually shows up on the windshield and is visible on the leading edge of the wings – the pilot must pull the panel knob for carb heat or alternate air.

In some installations, the alternate air door is spring-loaded and actuated by engine suction, so its selection is automatic. Like carburetor heat, alt air will decrease engine power output slightly because the air is taken from a hot area inside the engine compartment. Trading a modicum of power for an engine that keeps on running is a good trade.

Dont be shy about applying the heat, either. Alt air and carb heat can only work if the engine is hot enough to provide warm air to melt the ice. Once the deicing capability is lost by a serious buildup of ice, it is almost impossible to regain it unless you descend into decidedly warmer air. If corrective action isnt taken soon enough, the alternate air doors may become blocked with ice. The selector valve can accumulate ice, too, and will freeze in position.

Alternate air and carburetor heat are both unfiltered sources (since the air filter is bypassed), which may allow dirt to enter the engine directly. Once you are out of the icing situation, the alternate air or carburetor heat control should be turned off.

Different kinds of engines are susceptible to different ice threats. An airplane with a float-type carburetor can suffer throttle ice if the throttle is partially closed, as in a descent, or at a cruise power setting.

The water vapor in the air condenses and freezes because of the cooling restriction caused by the throttle butterfly valve and the carburetor venturi. If the ice continues to build up, it can throttle the engine. You dont need visible moisture for this kind of icing, and this kind of problem can sneak up on you.

With a fixed pitch propeller, this shows up as a gradual loss of RPM and airspeed, unless throttle is slowly advanced. In an airplane with a constant speed prop, you normally wont find any change in RPM, but you will see a drop in manifold pressure and exhaust gas temperature.

In either case, performance will gradually decrease. The next indication of the problem will be the need to retrim to maintain altitude. The engine will start being rougher, usually followed by backfiring. Complete engine stoppage may occur. You can guarantee it by moving the throttle abruptly.

A related form of icing comes from the vaporization of fuel. Fuel cools as it expands between the carburetor and the engine manifold. That cooling can be enough to cause carb ice in float-type carbs even when the ambient air temperature is well above freezing.

Vaporization icing also can occur with pressure carburetors, when the fuel/air mixture reaches a freezing temperature. Pressure carbs are not as susceptible because the induction passages where fuel is injected are kept warm by engine heat. And in cases where the air is heated by a supercharger or turbocharger, or when fuel is injected directly into the cylinder, you generally dont have vaporization icing.

Keep in mind that vaporization icing can happen at temperatures from 32F to as much as 100F, as long as theres sufficient humidity. Relative humidity is the ratio between the actual water vapor a unit of air holds to the amount of water vapor it could hold.

A relative humidity of more than 50 percent puts you into the icing zone. The relative humidity becomes 100 percent when the temperature and the dewpoint are the same. When the temperature/dewpoint spread becomes 20F or less, the relative humidity is 50 percent or higher.

The best way to eliminate, or avoid induction system ice, is to understand how the induction system works. When you place a float-type carburetor next to a pressure carburetor, you notice that the fuel jet on the float type carburetor is ahead of, or below, the throttle butterfly. Its easy to see that the fuel is being sprayed into the carburetor venturi, where it vaporizes with a large temperature drop – and has the best chance to form ice.

In the pressure carb, the jets squirt the fuel farther downstream in the channel, away from the venturi. And that accounts for why there is less venturi icing in this pressure carburetor system.

Float-Type Carb Ice
The float carburetor is a low-pressure carburetor. It uses a reservoir, or bowl, from which fuel is siphoned into the engine through jet nozzles in the throat, or venturi. This is where the fuel/air mixing and the vaporization temperature drops take place. The level of the fuel in the bowl is controlled by a valve on an arm, connected to a float, similar to a toilet water tank installation.

In a float-type carburetor, fuel is vaporized, which absorbs heat from the air and causes rapid cooling in the induction system. More heat is lost due to the high air velocity, which causes a low pressure area through the carburetor venturi.

The temperature in the mixing chamber may drop as much as 70F, below the temperature of the air that is coming in. Even if you are flying in 90 F air, the temperature will drop far enough to be below the freezing point of water. If the air contains a more than average amount of moisture, you get ice in the carburetor venturi throat and at the throttle butterfly.

If this ice buildup is not eliminated, it can rapidly grow to a point where a serious drop in power output will result. The engine stops through fuel starvation.

When the conditions appear to be right for carburetor ice, the pilot needs to apply full carburetor heat before the ice forms. Once confronted with the warning signs – the unexplained engine roughness or power loss is a red flag. Pull on full carb heat immediately and leave it on.

As the ice melts, the engine may get rougher as the water goes through the engine. It will smooth out, as long as you dont panic and remove the heat.

Using partial heat is a tricky procedure, unless the aircraft is equipped with a carburetor air temperature gauge. You can actually heat air that is too cold for carb ice into the danger zone.

Extremely cold air generally has very low relative humidity. The moisture that is present consists of ice crystals, which pass through the air intake system without problem. You can be sure that any moisture in the air is frozen when the outside air temperature is 14F or below.

Partial heat can actually cause induction ice by melting the ice crystals, which then refreeze farther back in the venturi. Your airplanes POH will confirm that you must use either the hot or cold position – no in-between settings.

Roughing It
Using full heat will first cause a loss of power and some engine roughness. This is partly due to the melting ice and partly due to the fact that the warmer air is less dense and therefore the mixture is rich. The temporary roughness of the engine and the moderate power loss induces some pilots into turning the heat off, which then allows the icing to continue. Rest assured that the roughness and power loss is not doing damage to the engine.

There will be some power loss due to the water content, but the roughness can often be smoothed out by leaning the mixture. Just remember to enrich it again when you turn the carb heat off.

The key to beating carb ice is knowing why it happens and how carb heat can solve the problem. Without that understanding, youre just pulling knobs and hoping for the best.

A float-type carb system heater uses the heat muff method of heating the air going into the carburetor venturi. The carburetor heat lever taps the heat (through linkage) from a block of air trapped in a metal chamber, or muff, that is wrapped around the engines exhaust manifold. As long as the exhaust manifold is hot, heated air is available from the muff to melt the ice in the carburetor venturi.

If the engine and exhaust manifold gets too cold, as in a prolonged descent with low power in cold air, there is not enough heat available for de-icing. Numerous pilots have been embarrassed to find that, after a long descending glide at idle power, that the engine only sputtered when they advanced the throttle. Going to carb heat now wont do much good because the exhaust manifold isnt hot enough to warm the air in the manifold heat muff. The ice doesnt melt and the pilot finds himself flying a quiet, heavy glider.

Pressure Carbs
The pressure carburetor has no reservoir or bowl like the float carburetor. The fuel is metered and delivered, under fairly high pressure, from a fuel pump to a jet or jets near the throttle butterfly. This is very much like the automobile throttle body injection system, which is used by many automobile manufacturers.

Most pressure carburetors have an automatic mixture control. Any application of carburetor heat on the ground will affect the mixture control unit unpredictably for a brief period. If for some reason the pilot has used the carburetor heat on the ground, possibly during the run-up check of carburetor heat, takeoff must be delayed for at least 2 minutes. This will allow the automatic mixture control unit to re-stabilize, and avoid an erratic fuel flow.

With a pressure type carburetor, ice is seldom collected in VFR weather. In this design, the fuel nozzles are placed farther away from the venturi throat and the throttle butterfly, where the temperature drop is the largest. IFR flight conditions are most likely to affect the pressure carburetor.

Fuel Injection
Fuel-injected engines sense airflow and pressure, then meter fuel on demand. The fuel delivery lines to each cylinder connect through a flow divider (sometimes called an injection spider), usually located on top of the engine, which allows the fuel to arrive at the individual cylinders in correct timing.

Of course fuel-injected engines have no carburetor, so they dont have the carburetor ice problem. However, ice crystals and snow can melt and reform as ice around the injector nozzles, which can result in lost power. Because the nozzles are close to the source of the engines heat, however, the potential for icing is slim.

Injected engines do need air to breath, however. Once conditions develop that favor structural icing, the air filter and intake passages can be blocked by ice. You can expect a loss of power, or even engine failure, if the air intake system is cut off. There is a remedy.

Alternative Lifestyles
An alternate air source can be selected that bypasses the filter. This source provides intake air from an area not prone to ice buildup, such as inside the nacelle or cowling.

Alt air uses a system of baffles that collects warm air from around the engine and directs it to the carburetor or intake manifold. Some alt air systems are manual and require the pilot to active them. Others are automatic, involving a spring-loaded door that opens if the engine cant get air through the filter. Heres another case where the Pilots Operating Handbook can give details of operation.

Like carb heat, alternate air creates a richer mixture. And just like in a carb heat situation, leaning the engine will help it run smoothly at cruise power. If you are in the traffic pattern or any other low power situation, however, leaning the mixture may not be practical.

Running Hot and Cold
If you experience a go-around situation, youll need full power, and that means getting the carb heat or alt air to the cold position. Using heat for takeoff or climb may bring on possible engine damage from detonation.

At power settings of 75 percent or lower, however, Lycoming tests have shown theres no danger of detonation or other engine damage simply from running carb heat or alternate air. If you doubt that, think of how hot the air coming out of a turbocharger is. The amount of heat generated by carb heat or alt air is small by comparison, and yet detonation and induction system problems are not the source of turbocharger worries.

A far more important concern is keeping the heat available for when you need it. That means keeping engine power high enough to supply heat to the exhaust manifold and the engine compartment. Otherwise, the carburetor heat or alternate air wont be available when you need it.

If youre IFR and ATC wants an accelerated descent, throw out all the drag you can, including gear, flaps, speed brakes and anything else you might have, and keep some power on. If the controller indicates youre not descending fast enough, tell them unable and make it their problem.

Generally youll just get vectors to a wider or longer pattern. No harsh words will be spoken, and no phone calls will be requested after landing.

Mixing it Up
Sometimes the line between icing in the induction system and no icing boils down to the position of the mixture control. A lean mixture can increase the temperature in the induction system just enough to prevent icing.

Fuel injected engines do avoid the problem of icing at the venturi. However, wet snow can block the induction air filter system despite the alternate air, and can even foul the alternate air baffling in extreme weather conditions.

Induction system icing has occurred in fuel injected engines in light, powder snow. If a power loss is experienced in a fuel injected, supercharged engine when flying through light snow precipitation, it is usually be the result of induction system ice.

Importantly, the wrong pilot reaction to power loss could make matters worse. Remember, fuel is used to cool the engine at high power settings during hot weather. If the pilot puts the mixture to full rich and advances the throttle, the refrigeration action increases and the injector nozzles can ice up.

This kind of dangerous power loss can be avoided by leaning the engine close to peak exhaust gas temperature and holding on to the already established cruise power, not increasing it. The leaning should be accomplished after the use of carburetor heat or the alternate air.

The typical turbocharged powerplant is not affected by induction system ice, except in extreme conditions. The high temperature of the induction air when the compressor is running prevents icing. Slush snow can still create a blockage in the air filter area, which can be a threat, if alternate air is not available.

Although engine ice isnt a problem thats limited to cold-weather flying, the colder ambient air makes correcting for the ice more difficult. Understanding the phenomenon will make it easier to prevent the problem and to solve it should your engine get strangled by ice.

Also With This Article
Click here to view “Carb Ice Probability.”
Click here to view “Avgas on the Rocks.”

-by Raymond Leis

Raymond Leis is a CFII and ATP with more than 23,000 hours.


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