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Induction system icing 

 

Induction system icing is a potential problem in many reciprocating engine aircraft. Of primary concern to the pilot during an induction icing event is the loss of power, and if the icing is not eliminated, eventual engine failure.

This month I’ll cover the different types of induction systems, their pro and cons in terms of the induction icing hazard, and the methods a pilot can use to combat induction icing in each.

Since all reciprocating engine aircraft use some sort of induction system, one would think that all are equally susceptible to the icing problem. However, this assumption is a bad one, as there are few general rules to guarantee to the pilot that a particular aircraft will be more likely to have induction icing problems than some other aircraft type.

Even the engine type is not necessarily a good guide as to the icing potential. So what information can the pilot use to identify potentially ice prone aircraft? As it turns out, the best place to start this discussion is by examining the specific types of induction system.

There are two main types of induction systems, carburetor and fuel injection. However, sub-types are worth considering, as there are differences between float-type and pressure carburetors. Also important is whether the engine is supercharged or turbocharged. But regardless of the type of induction system, the piping that gets the inlet air to where it needs to go is also a factor in determining induction ice potential, so I’ll talk about that as well.

Although the type of induction system determines which types of ice are most likely to occur, if at all, there are three types of icing common to the light aircraft we fly: 1) impact ice, 2) throttle ice, and 3) fuel vaporization ice; the last two sometimes categorized jointly as refrigeration icing.

Since impact ice is perhaps the easiest to understand, I’ll deal with that first. Impact ice is more-or-less synonymous with structural ice in the way that it forms. Basically, anytime the atmospheric conditions are right for structural ice (as when you see it building on some part of the aircraft) it also has great potential to adhere to various elements of the induction system such as the air scoop, intake screens or filters, and any other components that may be directly exposed to the free airstream.

This means that ice observed on the outside is a sign that ice may be adhering to the air inlet area and continued flight in such conditions could lead to significant induction blockages. For this reason, aircraft are designed with an alternate means of getting air, a topic that I’ll talk about later. Ice can also form in bends of the induction system piping with similar effects.

Throttle, or butterfly valve ice, and fuel vaporization ice occur due to the temperature drop when conditions are moist (approximately more than 50% relative humidity) and ambient air temperatures range from about –7 to 32 degrees C.

Colder air is generally not moist enough to support icing to the point where it’s a problem, and warmer temperatures are equally less susceptible since the temperature drop in the venturi is no longer sufficient to freeze the moisture.

Bear in mind that both throttle valve and fuel vaporization icing are a problem for carbureted engines, but only throttle valve icing is a potential problem for fuel injection systems since fuel is not introduced into the venturi section of the regulator unit as with a carbureted unit.

The term “induction system” implies that more than a single piece of hardware is responsible for getting air to a location so that it can be mixed with fuel to make the engine run. In some cases, relatively simple piping routes air from outside the engine compartment to inside, where it can then be used.

Most induction systems use a filter at some point in the air inlet part of the system to keep debris, bugs, etc. from potentially becoming lodged further downstream in more critical areas. More about this air inlet piping later, but it suffices to say that what type of fuel/air mixing device that piping leads to is one of the main determinants of induction system icing.

In many lower powered aircraft the induction piping leads to a carburetor, with higher performance engines most commonly being fuel injected. However, there is no firm rule that says a large engine needs to be fuel injected, nor that a small one needs to be carbureted. For example, Lycoming 360 cu in engines come in both the carbureted and fuel injected styles.

In any event, most publications will compare and contrast float-type carburetors with fuel injected engines. And everyone agrees, for this particular comparison, that the carbureted engine is more susceptible to icing than the same engine if it happened to be fuel injected. The reason why this is so, is fairly academic.

The cooling that takes place, and leads to icing in the first place, is a function of the MIXTURE of air and fuel. Note that “mixture” is in capitals. That’s because icing is less of a problem in the absence of fuel. There is no “mixture” of fuel and air in the fuel injected engine up until the point where the fuel/air passages are kept warm enough by the engine heat. And fuel vaporization icing is generally precluded in the case where the fuel is injected directly into each cylinder or the air is heated by a supercharger or turbocharger. The air from these later two are too warm to support icing, but the pilot still needs to be concerned about impact icing.

When considering the Induction icing potential of a carbureted engine it’s important to differentiate between a float-type carburetor and the pressure-type carburetor. While a float carburetor is most common, it also suffers greatly from throttle and fuel vaporization icing.

The pressure carburetor can still be found on some aircraft so I felt it worthwhile to discuss the differences. The main difference being that the fuel in a pressure carburetor system is introduced downstream of the venturi and throttle assembly of the carburetor, thus reducing the fuel vaporization icing problem.

In the float carburetor, the fuel is introduced at the worst place in terms of pressure drop and cooling (in the venturi itself) and so fuel vaporization exacerbates this problem making icing even more likely.

So, in recap, float-type carbureted engines may be susceptible to not only impact icing, but also icing at the throttle valve (also called the butterfly valve) and fuel vaporization icing. Pressure-type carburetor engines may be susceptible to impact and possibly throttle valve icing. Fuel-injected engines, as well as supercharged or turbocharged engines are generally only susceptible to impact icing.

That being said, bear in mind that there are wide differences in the frequency or potential for “carb” ice between aircraft with float-type carburetors due to the piping of the induction system as well as where the carburetor is mounted. For example, in my experience, the Beechcraft Sundowner and Sports had far more “carb” icing problems than the Piper Archer IIIs even though they all have the Lycoming O-360 engine.

Since they’re the same engine, icing potential is apparently a matter of the particular carburetor installation, where it is installed, and the induction system piping to that carburetor.

What to do, and what not to do when you get ice

When dealing with non-turbocharged carbureted engines, regardless of the type of ice, the first symptoms will be a reduction in RPM and possibly rough running engine. The immediate action is to apply full carb heat.

Since the introduction of warmer air will reduce the air density, a subsequent reduction in RPM is to be expected. If the problem is indeed due to carb ice, then you can expect an increase in RPM shortly after applying carb heat. Once the RPM stabilizes, turn off carb heat and monitor RPM for additional ice events.

If the carb heat needs to be frequently applied, it may be best to simply leave it full on until landing. However, partial carb heat should never be used unless the induction system includes a carburetor air temperature gauge. In these installations, the gauge will show where the carb temperature should be and so the pilot may fine-tune the amount of carb heat added to keep the mixture in the “non-icing” range.

The hazard of applying partial carb heat without a gauge is that it is possible to worsen the carb-icing situation. Above all else, also be sure to follow the aircraft manufacturer’s recommendation for using carb heat.

The engine manufacturer’s manual for a specific engine is not a good source of guidance for carb heat use in a specific aircraft so stick with the aircraft manual instead. Realize that what is recommended in one type of aircraft may be quite different than for another aircraft.

Prior to each flight, carb heat should be tested during the engine runup by bringing it full on and noting a nominal RPM decrease. The amount of decrease is usually not more than 100-200 RPM. If no drop is noted, ground the airplane for inspection and find another airplane to fly, or cancel your flying plans for that day!

In a fuel injected aircraft, you won’t get carb ice per se, but impact ice can have the same basic impact; loss of power as seen on the tachometer in the case of fixed pitch prop aircraft, and on the manifold pressure gauge for constant speed prop aircraft.

In aircraft with automatic alternate air, you may never realize that it has activated during an icing event. Many aircraft that advertise automatic operation of the alternate air also have a backup by means of a lever in the cockpit. Regardless of whether the system is supposed to be automatic, if there is no other explanation for a power loss, apply alternate air to the full on position using the manual alternate air lever. If impact ice was the problem, power should return to normal.

The alternate air will most likely need to remain on unless there is some indication that warmer flight temperatures might have melted the icing from wherever it was at in the system.

Prior to flight during the engine runup, if a manual alternate air lever is provided, check it for full range of motion but don’t expect to see any changes on the RPM or manifold gauge. If you do see a change, something is likely wrong and you should not fly until having it checked by a mechanic.

Since there is no power change noted when checking the alternate air, what the pilot is checking is freedom of movement of the lever and little else. Realize however that there is no guarantee that the alternate air door will open automatically during flight, nor that the manual override will function as desired! Not an ideal system design from a feedback standpoint, but thankfully they are pretty robust (although a colleague recently told me of a friend he knew that had the alternate door mechanism fail and so could not remedy induction ice problems during flight!).

So there you have the basics of induction icing and its impact on the various types of induction systems for reciprocating engine aircraft. As always, the bottom line regardless of the type of fuel/air induction system on your aircraft is to 1) know what type it is, 2) what kind of icing to expect, if any, and 3) know how to remedy an induction-icing event. 

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.