Induction Icing and Other Obstructions
Rewritten and combined with article "Induction Icing"
The gasoline engine operates on a fuel/air mixture that is ignited by the spark plugs. Engines do not run when any of these elements are missing. Pilots know positively that they must refuel the aircraft on a regular basis if they want to fly without incident, but the possibility of losing the air part of the fuel/air mixture is not always considered and understood as well as it should be. Perhaps the personal experience of several individuals, and some facts about induction system icing can be used to help Flyer readers avoid an accident caused by lack of air for their engine.
Remember that any material that reduces or cuts off the flow of air in the induction system has the potential to cause a loss of power. A material failure of the air filter is one problem which is reported all too often. The filter is very necessary to keep dirt out of the engine; it must be inspected frequently and should be changed on some regular schedule. A filter which is several years old and has filtered the air during hundreds of hours of operation may be tired. One pilot reported that on turn up of the engine before takeoff, he could not get the static RPM that his engine and fixed pitch propeller should have produced. He wisely elected to return to the line and have the engine inspected. The air filter had pulled loose from its supporting frame and was lodged in the intake system where it was cutting off the air supply.
If this incident had occurred in flight, the engine would possibly not have been producing enough power to maintain altitude. Depending on the particular airframe, there are some options which might be utilized to regain some of the lost power. An alternate air system or carburetor heat system is designed into the induction system primarily to combat induction icing, but use of these systems may possibly help when intake air is blocked by other foreign materials. In some cases, just leaning the mixture may help to regain a little of the lost power.
Several years ago there was a reported loss of engine power in heavy rain. In that case, a paper air filter was being used. When saturated with water, the paper filter element became swollen so that airflow was impeded. In this case, the use of carburetor heat to bypass the filter and releaning to achieve a better fuel/air mixture were successful tactics that kept the aircraft flying until a safe, on airport landing could be made. We should keep in mind that it is not the ingestion of water through the engine that causes a serious loss of power; it is the reduced airflow.
Some pilots of aircraft that utilize a fuel injected engine believe that this engine makes them immune to induction icing. This is not so. Although the pilot flying with a fuel injected engine does not have the same threat of icing at the venturi as those with a carburetor, rain, snow, slush, and cold temperatures may cause a blockage (impact ice) to air flow in other parts of the induction system.
As an example, the pilot of a fuel injected single reported flying at 11,000 feet in light drizzle. The temperature was slightly above freezing and water readily ran off the windscreen. Although this would seem to be a no problem situation, the engine started to lose power. After consideration of the available options, the manual alternate air system was activated. The engine immediately regained power and flight was continued to the home base destination. After landing, the aircraft was taken into the hanger for examination. It was found that the air filter was covered with a layer of ice that had cut off the airflow. This is not an isolated or unusual case. When water is near freezing, movement of the water molecules may sometimes cause instantaneous freezing. This glazing over of the air filter is a known phenomena which pilots should expect and be ready to cope with. Again, bypassing the blockage of impact ice by use of alternate air proved to be a successful tactic for this pilot.
The most subtle and insidious of the airflow blockage possibilities is probably refrigeration ice, known more commonly as carburetor ice, that forms in the vicinity of the "butterfly" or throttle plate. Unfortunately, there are many pilots who are not fully aware of what carburetor ice can do or what to do about it when it does occur. An indication of this are statements made by pilots involved in power loss accidents who have said that they tried carburetor heat, found it did not work, and then returned the control to the cold position. Carburetor heat does not provide instant relief when applied after ice has formed in the carburetor. Once heat is applied, it should be left on until engine power returns. Left uncorrected, ice accumulation in the carburetor may cause complete engine stoppage.
Every pilot who flies an aircraft powered by a carbureted engine should be thoroughly educated about carburetor ice. They should know that under moist conditions (a relative humidity of 50% to 60% is moist enough), carburetor ice can form with any outside air temperature from 20o to 90oF. It is most likely in the 30o to 60oF range. Temperatures in the carburetor can drop 60o to 70oF (refrigerator effect) as a result of fuel vaporization and the carburetor venturi effect. It also happens that carburetor ice forms more readily when the engine is operated in the lower power range. It will form while taxiing and this makes it very important to check engine power before takeoff and to remove the ice if necessary. Care should be taken to avoid dusty or dirty conditions when utilizing carburetor heat on the ground.
Next, it is imperative that the pilot recognize carburetor ice when it forms during flight. The loss of power that occurs will cause a reduction of RPM when flying with a fixed pitch propeller, and a loss of manifold pressure when a controllable pitch propeller is used. In either case, a loss of altitude or airspeed will occur. These symptoms may sometimes be accompanied by vibration or engine roughness. In any case, it is a good idea to consider carburetor ice as the cause of any unexplained power loss during cruise flight.
Once a power loss is noticed by the pilot, immediate action should be taken to eliminate ice which has already formed in the carburetor, and to prevent further ice formation. This is accomplished by applying full carburetor heat which will initially cause a further loss of power (perhaps as much as 15%) and, possibly, engine roughness. The additional power loss is caused by the heated air that is being directed into the induction system. Heated air makes the mixture richer and also melts the ice which then goes through the engine as water. The throttle may be advanced and the mixture may be leaned to help get some of the lost power back, but immediately after the application of carburetor heat the pilot must be patient and keep the airplane flying until the ice has completely melted and normal power returns. How long this will take depends on the severity of the icing, but the pilot should expect a delay of 30 seconds to several minutes. Under the circumstances, this period of time will be stressful and always seems longer than it really is, but the knowledgeable pilot will not retreat from use of carburetor heat. Carburetor heat should remain in the hot position until power returns.
In conditions where carburetor ice is likely to form, the pilot may use heat during cruise to prevent the formation of ice in the carburetor. It is also appropriate to use full carburetor heat, if needed, to prevent icing when operating at low power for instrument approaches, or for flight in the traffic pattern. Unless the aircraft is equipped with a carburetor air temperature (CAT) gage, and very few general aviation aircraft are, use of full carburetor heat is recommended. An unknown amount of partial heat can actually cause induction ice in the float type carburetor. This may occur when moisture in crystal form in the incoming air that would ordinarily pass through the induction system without any problem is melted by the partial heat. This moisture then freezes when it comes in contact with the cold metal of the throttle plate.
Whenever carburetor heat is used in the landing configuration, and a go-around or touch-and-go takes place, there are some important steps for the pilot to remember. The throttle must be advanced and the carburetor heat lever placed in the cold position. The order in which these steps are accomplished is not too important, but both must be done. Leaving the carburetor heat on during a go-around will result in a loss of power that could be critical at low altitude and low airspeed.
Do not use carburetor heat for takeoff or climb with a Lycoming engine as it is not necessary, and it may bring on detonation and possible engine damage. An exception to this rule might be justified in extremely cold weather conditions such as those found in the Arctic, and these conditions require a special knowledge to accommodate operation under such extreme conditions.
A review of the material discussed in this article should help pilots to cope with reduction of engine power when it is caused by loss of intake air for combustion. A thorough understanding of the air intake system and the knowledge to competently deal with induction icing are essential to safe flight in general aviation aircraft. Pilots are encouraged to enhance the safety of their flying by knowing what to expect and what steps to take when the air flow to the engine is cut off for any reason.