Conditions Conducive to Carb Ice
Most airplanes do not have a relative humidity indicator, which is very important for knowing if you are likely to get carb ice, and at what power setting. So how does a pilot, without an on-board meteorologist, know when they are in these conditions?
The first step is understanding when carb icing conditions are likely to occur, and then being vigilant in using induction heat when they do occur. Take a moment to study the chart on the next page. The data is from a NASA study of carb ice accidents back in the 1980s. Note that the temperature and humidity range covers a lot of the kinds of conditions we fly in throughout a good part of the year.
The worst conditions for carburetor ice are also the wettest, where the temperature and dew point are equal at 100-percent relative humidity (RH). Now, think about where we fly when we have a cloud ceiling. As VFR pilots, we tend to fly as high as we can without going into the clouds or busting minimums. In a stable atmosphere, the RH climbs with altitude until you hit the cloud base where it is, by definition, saturated (i.e., 100-percent RH).
Carb Heat Systems
The primary way to avoid having engine problems due to induction icing is to use carb heat. In most airplanes, when you pull the carb heat knob, a flapper door opens and the engine pulls warm air through a heat exchanger (also called the carb heat muff) that surrounds your exhaust system. The standard that has served us well for testing a new carb heat system is a 90 F heat rise, at 75-percent power, at 30 F outside air temperature.
This heat rise test is done fairly frequently as part of the new Supplemental Type Certificates for exhaust systems, and often fails because the system hasn’t been well maintained. Typically when we dig into the causes, a leak in the air box is revealed, which allows cold ram air coming through the air filter to leak around the flapper valve, diluting the hot air and thereby cooling down the heated air from the carb heat muff.
Some airplanes have an automatic spring-loaded door that opens by engine suction in the event that the air filter becomes plugged. When you are flying through snow, it’s common for the air filter to become plugged with impact snow (especially wet snow). The condition of the door is important because if it doesn’t open, the engine will quit (this happened to me personally in the clouds over the Cascade mountain range). When the door opens, you’ll notice a slight power loss due to the intake air being warmer and sheltered inside the cowling. It’s possible the door can stick, so it’s a good thing to check it periodically, especially at annual to make sure it opens properly.
Bring the Heat!
Far more issues arise from not using carb heat, especially on descent at lower power settings, than using it too often. Using carb heat doesn’t hurt the engine, except in truly rare conditions. The only exception might be in extremely dusty conditions with severe visibility restrictions, or if you get caught flying through volcanic ash where the air filter is very important and is bypassed by most carb heat systems.
If you are pulling your carb heat knob and not seeing a significant rpm drop, the system could be leaking, or your cable could be broken. Leaky air boxes are remarkably common. One frequently asked question is, “how much drop should I expect?” The manual is the best place to start for answers, and many say to expect a drop between 75 and 150 rpm. If I was getting less than 75, I would probably have a chat with my mechanic.
If you are burning auto fuel, some research has caused Transport Canada to advise extra caution regarding picking up carb ice. It appears that aircraft burning auto fuel may be more susceptible to carb ice than aircraft burning 100LL, due to increased evaporative cooling caused by the higher vapor pressure in auto gas.
Set your power deliberately at a specific rpm or manifold pressure and keep those gauges as part of your scan. It also helps if you choose a specific altitude for cruising, so you can tell if an rpm drop is due to ice or because you are climbing.
If you wait to pull carb heat until the engine has actually quit, it will be too late for the heat exchanger to melt the ice. That means the likelihood of getting power back is pretty low if you don’t catch the ice buildup early. Sometimes descending to a lower altitude where the air is warmer works, but terrain has to allow for that. The bottom line is if you think you might be getting ice, pull carb heat, watch for an rpm drop, which is followed by a rise. The engine might run rough for a little bit.
If you are in the temperature and humidity envelope in the chart, you are in induction icing conditions, even if it isn’t raining or snowing. That means you should suspect carb ice, and take the steps necessary to prevent an engine failure. Based on accident reports and information from pilots and flight instructors alike, there may be a lack of knowledge about how prevalent icing conditions are. As a result, pilots aren’t using carb heat downwind in some airplanes when power is reduced, even when they are in the blue area of the chart.
Some pilots suggest that applying carb heat at low power on approach may impact the power needed for a go around, especially on off-field landings. Based on the accidents and the science, however, the bigger risk is not having ANY power due to carb ice on the go around. You get 95-percent of the power with the heat on anyway, and it only takes a second to push the heat closed.
Dave Swartz is the Senior Engineer at the Anchorage Aircraft Certification office and an active general aviation pilot and airplane owner.
This article was originally published in the January/February 2017 issue of FAA Safety Briefing magazine.