Let me say right from the start I am aware of many aircraft powerplants that do not make use of carburetor heat. This is in large part because many aircraft powerplants do not require a carburetor in order to meter fuel.
Turboprop aircraft are a fine example of this exception. As are turbojets, turbofans, and, of course, high-bypass turbofans. Oh, I guess I should include fuel-injected aircraft engines too. And electric powerplants. They may be few, but they are growing in number.
Now that we have the exceptions out of the way, let’s get to the point.
Carburetor heat is a gift from the engineers who created it. Without carb heat I would have disappeared into the woodlands of New England, or Florida, or somewhere in between long ago. That simple knob on the panel of my Cessna has revived my ailing engine on more than one occasion.
It could be said that I may owe my very life to carb heat. Hip, hip, hurray.
Then again, carb heat can be the anchor that holds us back. Anyone who has attempted to takeoff from a relatively short runway on a high-density altitude day knows of what I speak. Without verifying the carb heat is turned off, and I mean fully off, that pilot has felt the sweaty palms and experienced the lackluster performance of an engine that’s putting out far less power than expected.
I’ve committed this sin myself. While flying with a friend in my 1940 Piper J-3 Cub my right foot did not find the carb heat actuator and press it fully closed as it should have.
The Cub was pulled through the air by an anemic, but reasonably acceptable, Continental 65-hp engine. That powerplant was mated to a wooden prop. As we rose into the air with a blistering climb rate of nearly 125 feet per minute, my friend called back to me, “Are you sure you untied this thing?”
His suggestion that I had left the tie-down ropes attached wasn’t far wrong. Losing a hundred RPMs or so during a critical phase of flight is no small thing. I pressed my right foot into that carb heat control with more force than was probably necessary. That did the trick. Our climb performance went from pathetic to not very good. A much-appreciated improvement, under the circumstances.
That’s the downside of carb heat. For all its wondrous magic, that simple control comes to us at a terrible price. And that price is performance.
The basics of the system are fairly simple. When we pull the carb heat control to “on” the temperature of the air flowing into our intake system jumps upward considerably. This air is taken, at least in part, from the heat building up in our muffler. The muffler is surrounded by a length of sheet metal formed into a closed tube. From that closed tube an outlet flows hot air in the direction of the carburetor. If everything goes according to plan, the ice that was building up will be eradicated and our engine will begin to run normally again.
None of that happens instantaneously, however.
Consider this error in the use of carb heat. A pilot suspects carburetor ice is forming. They come to this conclusion because the engine’s RPMs are dropping and the powerplant has begun to run rougher. To correct this issue the pilot engages the carb heat only to find the engine runs even rougher and the RPMs drop even lower.
This is normal. It’s to be expected. But if the pilot doesn’t know to expect this reduced performance, they may do the truly stupid thing. They turn the carburetor heat off, essentially sealing their fate. That ice is unlikely to go away on its own. Rather, it may well get worse.
Sometimes you have to accept reduced performance for a time in order to get back to the level of power you were expecting before the problem popped up. Apply carb heat and put up with the shaking for a few minutes. Eventually the RPMs should increase and the shakes smooth out.
One of the more entertaining carb heat misunderstandings I’ve encountered as a CFI occurred during the pre-takeoff run-up of a Cessna 182. I was checking out a pilot who was new to the additional horsepower and constant speed prop of the C-182. Since it was an older carbureted model, he wisely leaned the mixture fairly aggressively for ground operations. There’s no point in loading up the plugs with gunk prior to flight.
At the hold short line we did a run-up, as the checklist suggested we do. Everything was fine and dandy until we got to the point where my client pulled on the carburetor heat to verify it was working. The anticipated response should have been to see a slight drop in RPM. But that’s not what happened. The RPMs went up.
Hmmm, that’s a head scratcher. My client was befuddled.
Knowing how the system works, any system, makes all the difference when flying at altitude.
Pulling on the carb heat brings hot air into the engine’s intake. Hot air is less dense that colder air. Adding less dense air to a fixed flow of fuel results in a richer mixture flowing into the combustion chambers, hence, an increase in RPMs.
The problem wasn’t with the carb heat. Rather, it was rooted in my client’s leaning of the mixture for ground operations without enriching it again to perform the run-up. He was essentially enriching the mixture without touching the mixture control. He was doing it incorrectly via the carb heat.
It’s been said that everything has a purpose. And that may well be true. But as pilots it would be advisable for us to truly understand what the purpose of every knob and switch in the cockpit is and what sort of mischief we might find if we misuse those knobs and switches.
It’s something to think about.
Thank you for a very good article.
Leaning during ground operations is great for the engine, and at the low power settings required for taxi, you can’t hurt the engine by being too lean. A safety feature is to lean “so aggressively” that the engine will “Quit” if you forget to increase mixture before doing the run-up.