Power Struggle

Left: Float-type carburetor working normally During normal operations, air enters the carburetor through the air inlet -- at the bottom, in this case. The venturi's constriction creates an area of low pressure; fuel is introduced. The throttle valve at the top controls the flow of the fuel/air mixture into the combustion chambers. Right: Float-type carburetor with ice forming If the intake air is relatively cool and especially humid, carburetor ice is more likely. Reduced air pressure at the venturi throat creates a temperature decrease; so does vaporization of the fuel. If conditions warrant, ice generally will form in the area of the throttle valve.

Every pilot knows that temperature is an important factor in flying.

But consider these situations that you may not have thought about.

It's one of those beautiful days. The sky is clear. Visibility is unlimited. You've just touched down for a stop-and-go on a shortish runway and haven't yet pushed your carburetor heat knob to the Off position.

Cancel everything else until you do, because it's probably too hot for you to take off safely with carburetor heat applied. Hot air temperature will seriously rob your engine of power.

Here's a real-life accident report to support that contention. "The pilot reported that when he taxied to takeoff, he used carburetor heat to preheat the carb (sic) because of the high humidity. While taking off, he noted the aircraft would not gain airspeed after liftoff or climb sufficiently to clear obstacles. He turned to avoid a hangar, but the aircraft subsequently hit trees and crashed. A post-accident examination...revealed that the carburetor heat was in the full On position. No preimpact part failure or malfunction of the aircraft or engine was evident. The pilot reported the temperature was 95 degrees."

With the outside air temperature at 95 degrees Fahrenheit, and a typical temperature drop inside the carburetor of about 60 degrees, the temperature of the air entering the cylinders would have been 35 degrees F. That's OK, but when you add full carburetor heat (at a guaranteed minimum increase of 90 degrees F), the takeoff temperature of the air being drawn into the cylinders in this accident was a blistering 125 degrees F! That's what the engine had to work with.

Here's the other side of that carburetor heat coin. Visualize yourself in solid instrument meteorological conditions at 8,000 feet with cruise power set. ATC has just directed you to begin your descent for an IFR approach to minimums. Temperature and dew point on the surface are both 75 degrees F, and it's raining cats and dogs. In this case, you could very well lose engine power at some point in the approach if you don't apply full carburetor heat before you reduce your power to descend.

That's because you may be inviting your engine to ice up with a reduction of power under these conditions. The carburetor icing probability chart accompanying this article (see p. 40) clearly shows that, under the circumstances stated, there is an excellent possibility of serious icing after you reduce power -- even with the ambient temperature that high. The air is so moisture-laden that your carburetor will probably ice up if you don't use heat.

So which is it?

In the first scenario, using carburetor heat likely would preclude a successful takeoff. In the second, under a very different set of circumstances, not using carb heat would be the pilot's downfall. What's the secret of proper carb heat use?

Let's look at another situation: You're in cruise flight at 10,000 feet msl. Would it surprise you to know that your engine has lost at least 30 percent of the sea-level horsepower advertised by the manufacturer -- just by virtue of the fact that you're flying at 10,000 feet? You can verify this by looking at your aircraft's performance charts and seeing the maximum percentage of power available at that altitude. If your engine is due an overhaul, there might be an additional 10-percent power loss. If it's humid, knock off even more.

In this example, the Cessna 172 engine that you believe has 180 hp in reality would be developing less than 120 hp at 10,000 feet. That assumes, of course, that the outside temperature (OAT) is around 23 degrees F (standard temperature at 10,000 feet). If the OAT is higher than standard, your power loss would be even more. That's because density altitude is affected by temperature, and the temperature is above standard in this case.

Talking temperature

Brit and koch charts The carburetor icing probability chart (top) graphically depicts the conditions under which carburetor icing is most likely. Originally intended to illustrate the effects of increasing density altitude for airplanes without performance charts, the Koch Chart (above) compares the relationship of various temperatures and altitudes to takeoff and climb performance computed at sea level.

It's critical that you understand density altitude, because your normally aspirated engine will lose 3 percent of its power for every 1,000 feet that it climbs. Separate from density altitude, but closely related to it, is our main topic, temperature, a phenomenon that affects everything we do in an airplane: takeoff, climb, cruise, descent, landing, and even ground operations. The hotter it is, the greater power and performance will degrade. Every few degrees that the temperature is above standard increases density altitude about 1,200 feet.

Hot, when combined with humid can mean problems. Look again at the chart at top left. With high humidity comes the danger of carburetor icing, even with the outside temperature as high as 90 degrees F.

Regardless of temperature, if the humidity is above about 50 percent (meaning that temperature and dew point are within about 10 degrees of one another), weird things can start happening to what we call "power" if we aren't careful.

In his book, Proficient Pilot, long-time flight instructor Barry Schiff explains that "if the ambient temperature is 80 degrees F and the relative humidity is 90 percent, power loss due solely to the presence of water vapor (humidity) is 6.5 percent."

Six and a half percent doesn't sound like much, but if you've inadvertently left on the carburetor heat for takeoff (expert consensus says that's another 15 percent or so) you've lost more than 20 percent of your power.

On top of that, frozen water in the carburetor throat can reduce engine power to zero -- if no air can enter the cylinders it's a sure bet the engine will quit.

The Koch chart

Now is a good time to consider the Koch chart (lower left). It's been around a long time. Originally intended to illustrate the effects of increasing density altitude for airplanes without performance charts, it shows the relationship of various temperatures and altitudes vs. takeoff and climb performance computed at sea level. Increases in both altitude and temperature can be seen to critically decrease performance.

Seasonal altitude disorder

Although power versus carburetor heat is frequently discussed, relatively few pilots pay enough attention to the subject.

There's a popular but deceptive notion that carburetor ice and carburetor heat are wintertime problems associated with colder temperatures. As a matter of fact, my last carburetor icing/heat-related engine failure happened right after takeoff on a beautiful, cloudless 85-degree day.

Inside the carburetor

Carburetor icing can happen anytime. That's why FAR 23.1093 requires an almost instantaneous temperature rise of at least 90 degrees F when carburetor heat is applied in most typical general aviation applications. That's a big temperature rise. But it's necessary because, in the event the carburetor ices up, you need to be able to get rid of the ice now.

Let's say you've computed takeoff performance for a temperature of 80 degrees F, but inadvertently leave the carburetor heat on. Even considering the 60-degree "drop," your engine is working on an air temperature of 110 degrees. So the lesson here is pretty simple: Don't take off with the carburetor heat on.

The impact of summertime temperatures

Regardless of where you fly, summer normally means higher temperatures to begin with. Sea-level temperature on a "standard" ISA (International Standard Atmosphere) day is 59 degrees F. When the temperature is higher than that, performance starts to degrade. At temperatures anywhere above standard, engines lose power because the air is less dense.

Combine reduced power with short runways and danger lurks just around the corner for pilots who tend to disregard POH data. If you're operating on a 10,000-foot runway at sea level, you probably couldn't care less. But a shorter strip at a higher density altitude can eliminate the performance margin -- or reverse it.

Short and/or high runways mean problems in the summertime if you're not alert. Forget to return the carburetor heat to Off after the runup, or descend for a touch and go without returning the carburetor heat to Off, and you've just written a prescription for trouble.

The pilot of a 75-hp homebuilt aircraft in Idaho had just completed several touch and goes on a clear 53-degree-F day. Visibility was 30 statute miles. After liftoff on his last circuit, he clipped a fence after losing control when the aircraft refused to climb. The pilot confessed to inspectors that he "forgot to turn off the carburetor heat during the landing roll and before the application of power." Incidents like this one can happen to any pilot, at any time.

Typical carburetor heat problems are two sides of the same coin.

Clear set-ups like the ones described in this article are caused by many factors. You might never have heard of a carburetor icing probability chart unless you knew about it from accident reports.

If you have, you might have seen the chart -- or one of several variations -- printed so small that you couldn't use it effectively or even study it.

When you study the carburetor icing probability chart accompanying this article, consider how and where you fly and develop a feel for when icing is possible during your normal operations. A little "what if" on the ground could spare you trouble in the future.

Even with a healthy supply of modern fuel-injected engines and high-performance airplanes coming off assembly lines nowadays, there will continue to be plenty of normally aspirated airplanes and their carburetors out there. And they will continue to be involved in accidents, because of operator carelessness, ignorance, or uncertainty about what to do with the carburetor heat knob. Newer airplanes and advanced technology have not eliminated the carburetor ice/heat hazards.

Helpful tips

Here are a few tips for avoiding common carburetor heat pitfalls:

• Think about what you are doing 12 months of the year. If you haven't given much thought to the effects of temperature on flying, now is a good time to start.
• Realize that high temperatures (regardless of what causes them) seriously affect engine performance.
• Understand that carburetors can ice up any time humidity is high.
• Never take off with the carburetor heat in the On position.
• Understand and use the carburetor icing probability chart.
• Understand and use the Koch chart to develop a feel for the effects of altitude and temperature on takeoff and climb performance.
• Above all, know the airplane that you fly. Study the POH and other available literature. Some models seem more prone to carburetor icing's effects than others. If you have any questions, discuss them with your flight instructor.

While it's not always possible to compute the precise effects of a lot of what we've discussed here, it is important that you generally understand what's happening and follow the few simple rules listed above. They'll go a long way to keeping you safe.

Wally Miller is president of an aviation training, consulting, and marketing firm in Monument, Colorado. He is a Gold Seal CFI who has been instructing for more than 30 years and flying for more than 40.

Want to know more?

Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.