Why is it carburetor icing has to be handled manually by the pilot, at least in a helicopter? I am hoping very soon to take my PL/H lessons, I have a little understanding of mechanics and how things work and this is one subject that puts the willies up me as a potential student helicopter pilot. Why can it not be an automatic response built into the engine? After all, this is the 21st century.

Also, I am assuming every motorized engine is the same, so why does a helicopter require a manual response to carb icing but a motor vehicle doesn't? Do we not notice it because the vehicle is on the ground?

  • $\begingroup$ Perhaps because most IC engines used in aviation were actually designed in the 1950s? AFAIK, few if any automobile engines used carburetors after about 1990. I think most carbureted engines dealt with the problem mainly by circulating engine coolant under the carburetor, so it operated at a higher temperature. $\endgroup$ – jamesqf Jul 21 '15 at 19:53
  • $\begingroup$ @jamesqf Once upon a time, I ran a 1983 VW Golf 1.8L. As you say, the carb had a water circuit from the engine around it, controlled by a thermostat similar to that of the radiator, but in reverse: it opened water supply when the temperature was low enough in order to heat up the carb body. This Pierburg carb was already on the 1976 vintage Golf 1.6L. $\endgroup$ – ALAN WARD Jul 21 '15 at 20:27
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    $\begingroup$ The R22 and R44 have mechanical interconnects between the collective position and the carb heat. As you lower the collective, the interconnect raises the carb heat, and vice versa. Voila, auto carb heat. $\endgroup$ – rbp Jul 21 '15 at 20:29
  • $\begingroup$ This asks not about aircraft, but why cars don't suffer from carb ice. The answer seems to support that. Off topic. $\endgroup$ – rbp Jul 21 '15 at 20:30
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    $\begingroup$ This may be the 21st century, but a very large part of the light GA aircraft and engines in service today date from the 20th :-) $\endgroup$ – Pondlife Jul 21 '15 at 21:28

Not every motorized engine is the same: For example, you would be hard-pressed to find a carbureted car these days.

The most similar kind of land-vehicle engine you'll find would be a motorcycle engine, and those do in fact suffer from carburetor ice:
- A discussion of carb icing on the Ninja 250
- A thread about carb ice on Bike Talk Forums

Older cars with carburetors were also susceptible to carburetor ice, and eventually included measures to avoid or mitigate it.
This could include passive changes such as mounting the carburetor in a warm part of the engine compartment (usually above the block, in a downdraft configuration), semi-active changes like "heat risers" which would feed warm air into the intake (usually behind the throttle plate, but occasionally in front of it), or fully-active systems like automatic "heat flaps" and "Time/Temperature switches" which do the same thing as the carburetor heat control in an airplane but without the driver's input or control.

These systems aren't usually fitted to aircraft as they add both weight and complexity, and can come with their own disadvantages (such as reduced power if the carburetor heat flap is in the "on" (heating) position during takeoff or on a go-around).

So why don't we hear about ground vehicles suffering from carburetor ice as much? Three main reasons:

  1. Carburetor ice is most likely to form at low power settings.
    Your car or motorcycle probably doesn't spend much time at or near idle power unless you're stuck in traffic: A few seconds coasting to a stop and then you accelerate away again.
  2. Carburetor ice is less dangerous when you're on the ground.
    One of the most common times you'll have a carburetor ice problem is during an approach/landing, with the engine at or near idle power for an extended period of time (this is why many aircraft call for carb heat during landing). Having your engine die at this point is a pretty serious problem, especially if you need to execute a go-around.
    The analog in a car or motorcycle is having your engine stall after spending an extended period idling (standing or stopped in traffic). Having your engine die when you try to accelerate away is inconvenient and will probably make other drivers honk their horns at you and say unkind things, but it's unlikely to kill you.
  3. There's nothing the driver can do about it.
    A driver doesn't usually have a "carburetor heat" handle to pull (except possibly in very old cars), so you can't turn around and say "Your engine stalled because you didn't do this." On top of that because the problem is usually more inconvenient than dangerous on the ground it's not likely to make news: "Driver had to sit on shoulder for 30 minutes while their carburetor thawed out. Hundreds inconvenienced by rubbernecking traffic." is simply not as newsworthy as "Airplane crashed!"
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    $\begingroup$ "A few seconds coasting to a stop and then you accelerate away again" and "more inconvenient that dangerous" - not always!! I had carb icing in a car in the UK a few times back in the 1980s. The usual result was that the throttle was locked into position by the ice, so (1) the car didn't slow down when easing off the gas, and (2) putting the car out of gear with the throttle locked half-way down over-revved the engine until it cut out. (Simultaneously switching off the ignition without locking the steering, while not hitting the car in front, seemed like it was to complicated to attempt). $\endgroup$ – alephzero Jul 21 '15 at 22:50
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    $\begingroup$ Re point #1, hereabouts (Sierra Nevada), it's quite usual to be at idle power for half an hour or so when descending a grade. $\endgroup$ – jamesqf Jul 22 '15 at 5:56

Cars can in theory suffer from this if humidity is high enough.

For what its worth not all planes are carbureted. A solid chunk of airplanes use fuel injection (mechanical) and FADEC control is quickly becoming popular. This eliminates the carb ice problem by eliminating the carb all together. Keep in mind that any system needs to be certified for the FAA. There is little reason for a company to spend money developing an anti ice carb system and have it certified when carbs are slowly going away and the ones we still service were certified years ago ("if it ain't broke don't fix it").

For what its worth it seems that there are sensor systems out there for it however I have never used one so I will hold back from commenting on how well they work.

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    $\begingroup$ I don't understand what FADEC has to do with carb ice. Fuel injection is the remedy. $\endgroup$ – rbp Jul 21 '15 at 20:27

It is because there is no reliable sensor that can detect when and how much ice is or could be present.

If there was some kind of super sensor that could tell if ice was forming in the throat of the carb, then maybe you could have a little computer and a solenoid that would actuate the carb heat, but this would require a lot of development and added complexity that has not been done yet. Also, what if the computer screwed up for some reason and your carb froze?

Another option would be to have the system automatically turn on the heat any time humidity and temperature reached certain levels. The problem with this is that if either of those (added) sensors failed then you could get iced. Also, the helicopter would be flying inefficiently constantly and burning excess fuel.

  • $\begingroup$ You're looking at the problem the wrong way round. The objective is not to detect ice. The objective is to prevent ice formation. If you can detect ice it is already too late. Ice formation is governed by three things, presence of moisture, air pressure and temperature. All three can be measured, but moisture and pressure sensors can be fouled by ice. So you measure temperature and you keep the system warm. $\endgroup$ – Peter Wone Jul 22 '15 at 9:30

Everyone is making the answer more complicated than it needs to be. The manufacturers can't build an automated carburetor deicing system for more than they could sell it for. Therefore, it's a manually actuated system.


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