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Carb Heat in IMC

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Nov 29, 2001
Does anyone advocate the use of carb heat during operation in IMC, or does the engine temperature preclude any posibility of carb ice. If carb heat is used continuously, it would be wise to re-lean for this operation.

Any thoughts on the subject.
I would say it depends whether you are accumulating ice. If so the air intake may become blocked and alternate air is then required. If not then I would say no. Periodic carb heat checks may make you feel better though.

As always consult the owners manual first.

Ryan Dickensheets
The liklihood of carburetor icing is greatest when in visible moisture. If your carburetor air temperature is already in the ideal icing temperature range and you are in visible moisture, you stand a very high probability of getting carburetor ice.

If the outside air temperature is outside the typical icing range (plus five degrees C, to minus fifteen degrees C), the chance for carburetor ice is still very high. You can still encounter carburetor icing in visible moisture up to an outside air temperature of plus 95 degree F, or better (35 deg. C). At the lower end of the spectrum, I've had carburetor icing well into the negative numbers.

Without a carburetor air temperature gauge, you can only monitor the engine for signs of carburetor icing. If you intend to operate the airplane in visibile moisture, you should have a carburetor air temperature gauge. Without one, you're making an uneducated guess, which is never a good idea in aviation. Applying carburetor heat all the time may be putting your carb air temperature into an undesirable range, and may not be helping in the least. If it's too cold in your carburetor for icing, then adding carb heat only increases the temperature to a more ideal icing range. You don't need that. Not using it may keep it in or out of the icing range.

In large radial equipment, part of flying the airplane in instrument conditions was moving the carburetor air switches from time to time to keep carb air tmperature gauges where we wanted them. In our case, these were electrical switches that moved a door actuator on the back of the pressure carburetor. In moist conditions, we were always moving these, including on the ground, to prevent carburetor icing.

Should you use carb heat in the clouds? Absolutely. If it's warranted. Do you not know if it's warranted? Then what are you doing in the clouds in that airplane?

If you're flying carbureted engines in the clouds, chances are that you're flying single engine airplanes in the clouds. Single engine airplanes with one vacum pump, one electrical source, one engine. Do you think this wise.

Many moons ago I flew a 182 IMC. I experienced a very rapid buildup of carburetor ice, and on application of carburetor heat, the carb heat control came away in my hand Not only the carb heat control, but all the cable, which failed at the attach point to the carb air box. The engine failed, and I descended IMC without power until exiting the cloud base. I was unable to restore power, and landed on a gravel strip below. It was in mountainous terrain, and the gravel strip was in a valley which fortunately had approximately 3,000' of ceiling, and ample time to set up to land.

In the years since, I've had a lot of time to reflect on single engine IFR and single engine IMC, and much like single engine night cross country, I'm not a big advocate of the practice. In fact, I strongly encourage people not to do it. It's more than losing your engine. It's the single instrument power source, the single electrical power source, and so on. A lot riding on "one" with no backups. Additionally, if you're carbureted and have no way to manage your carburetor with respect to ice, then you're playing roulette. If you are guessing at your mixture, due to a lack of a multipoint temperature monitoring system, you're also playing roulette. If your avionics are marginal or not very good...you get the point.
Thanks for the responses.

My question dealt with a situation where you were flying a carbureted aircraft in IMC within the carb icing range of temps.(+5 / -15).

Those who regularly fly IMC under these conditions, do you usually:

A) Keep the carb heat on with a leaned-out mixture.

B) Look for icing indications such as RPM or MP drops then apply carb heat until cleared. Returning the switch to the off positon.

C) Perioidcally check for ice digestion by applying the carb heat at intervals.

Although I have the theoretical knowledge of what conditions cause carb icing, how to prevent it and cure it, I'm looking for advice from real-world experience encounters.

For those who have flown under these conditions, how often have you developed carb icing, and how helpful was it to use any of the above methods. What kind of RPM drops will you notice. My limited experience in IMC has been in and out of clouds and I have never encountered any indication of carburetor icing. Some instructors I have flown with in the past have stated that the engine is warm enough to really preclude any carb icing problems.
Well...here goes.

When I took my instrument checkride it was +7 at the surface. We were holding in IMC at 4000' and started to notice some ice (structural). In the descent, I thought "hey self, perhaps you have carb ice?" so I turned on the carb heat and to my surprise the RPM actually jumped up! I was expecting a rough engine, decrease in RPM then a rise...it jumped up...a few hundred RPM.

In my case, I suspect not carb ice (maybe that too but...) but structural ice on the intake air filter. When I went with carb heat, it was like alternate air and bam....a whole new engine.

Typically when I fly (now) in IMC (and it's not really +5 to -15 yet), I'll cycle the carb heat every so often so long as the temp gage isn't indicating that doing so will put me in trouble...nothing happens? Then okay...we're all sittin' in butter.

I wouldn't recommend leaving the carb heat on all the time. If you suspect carb ice, use the heat...if not, then don't.


Thanks mini.
That carb heat comes in handy as an alternate air source.

The procedure you used is very similar to what I've done in the past. Carb ice has always been in the back of my mind, well the front really, when in IMC. Although I've never seen or heard anything indicating ice, I've periodically apply FULL carb heat to see whether I had accumulated some.

I suppose that it is possbile to be in perfect carb icing conditons and not actually get any. I believe this may vary from model to model. Some more suceptiable than others.

Some engines as I understand it, have the carburetor strategically located close to a very hot part of the engine so as to preclude or decrease the incidence of carb icing. Anyone familiar with the makes and models or positioning of the carb. in reference to the engine?
NYCPilot said:
Thanks mini.

No problem!

The only solution I know of (and I haven't had the chance yet) would be a simple one.

...fly a jet :D

*sigh* back to dream-land...

Avbug makes a lot of good points. I never really trusted my carb. temp gages, and just run the carburetor heat full on or full off.

As for single engine IMC or night, things aren't much different than when you're VMC. On a sunny day, you always have an idea of where you can glide to if the motor quits. When the weather is down, know the terrain underneath you and how to make a single engine glide to a nearby airport, if you have one within range. This was important flying SE freight. Generally, if surface conditions were VFR, people who lost engines IMC were as likely to make it to an uneventful landing as people who were VMC. In other words, when you know where you are, you can descend through a cloud deck to VFR conditions and make an acceptable landing. If it's low IFR, however, you have to realize that an engine failure means you'll be on a deadstick approach (multiply the altitude you cross the OM by 3-5), or you're going to accept whatever terrain comes out of the mist at you. In this situation, the plan was always to aim the airplane at the friendliest terrain nearby, turn into the wind around 1000agl and prepare for impact.

In answer to Qs ABC:

When the carb. heat is required, it's on and leaned out. If in doubt, turn it on and lean the engine a little.

Although RPM and MP drop with carburetor icing, you really don't notice a sudden loss. Usually you wind up noticing that you've slowly pushed the throttles way up to maintain cruise power.

Yeah, I would regularly turn on the carb. heat to check for ice.

Flying patrol, I had carb ice issues a couple times. It was a bigger issue when we were flying skydivers. Flying freight in piston singles, we had injected engines, but you learned the value of knowing the terrain underneath you and having a good idea of where you were going to go if your engine puked.
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