Sport Pilot Talk

Ok, here is a question for you "seasoned" northern flyers. During training I as always told to pull the card heat (if equipt) when at low RPM and in the pattern. No one ever really clarified on long cross countries when or if you need to sporadically use it in cruise just to be safe at sub freezing OAT. The POH I'm reading says "as needed" which is pretty vague as well. I know some ROTAX have carb heat, some do not. Most other engines: if they have a carb, they have carb heat. So since I'll be flying this weekend on a long xcountry, in a plane equiped with carb heat I figure I'd ask. My training tells me it's unlikely to form carb ice at higher RPM, but I'd like to get a second opinion since the POH is less than helpful, and I've "heard" of it happening at full throttle.

It kind of depends on what your flying, and the weather conditions. A Cessna 182 with the Continental O-470 for example is very prone to carb ice. Most have a carb temp gauge, and you use carb heat to keep the carb in a safe range on the gauge. For a Lycoming powered Cessna 172 carb ice is less likely, though I did get a little I think one time when the temp was right at freezing and light precipitation. I have never experienced it in a Rotax powered airplane, but the folks from across the pond say it can happen.

just pay attention, and if you see a 100-200 reduction in RPM pull the carb heat and see if that was the cause. Just don't wait until the engine quits before you pull the carb heat.

We teach to use carb heat for Every approach/landing. There is a fairly rare condition (real cold) where carb heat can raise the temperature in the carb enough to cause it to ice... Watch your OAT.

Enroute? Yes if needed and I have had to do this with Continental engines. (actually had one ice so bad on the TO roll it would not climb until heat was applied)

If you notice an RPM drop, add the carb heat.

I get the overall sense from the OP that carb ice is mainly a problem at cold temperatures, i.e. below freezing.

Such is not the case:



Note it can be 100° F OAT and one can still get carburetor ice with enough moisture in the air.

In fact, at really cold temps, carb ice is rarely a problem, since cold air can hold less moisture than warm air.

Fasteddie, no I am fully aware it can happen at just about any temperature with humidity, I was mainly referring to this particular flight where temps are expected to be much colder than I'm used to here in Louisiana. Usually we don't see 20's at ground level often, higher up it will be even lower temp so that's unusual for these parts. Very early morning = less turbulence, but much colder than I'm used to!

Just a "for instance" occurrence. Last may, after leaving Sheboygan (WI), ground temps about 60F, I flew about 2 miles out over Lake Michigan, and I guess the increase in humidity caused some ice to form. The O-200 gave plenty of warning, slowly losing some rpm (maybe 200?). Carb heat fixed it. Left it on until I was well back over land. I didn't see much of a decrease in performance with carb heat, so if in doubt, whip it out.

Interesting, I'm in the central Florida area (high humidity of course) and never experienced carb ice nor have I ever used my carb heat on my 912.

Correction, I use it during my run up checklist to confirm that it works.

ShawnM wrote:I use it during my run up checklist to confirm that it works.

What are the indications that verify it is actually working?

On Remos it was a slight drop in rpm, about 10-20 rpm.

ShawnM wrote:Interesting, I'm in the central Florida area (high humidity of course) and never experienced carb ice nor have I ever used my carb heat on my 912.

Correction, I use it during my run up checklist to confirm that it works.

Somebody familiar with Rotax, help me here. But from what I understand, The carbureted Continental O-200 is more likely to develop carb ice than the Rotax 912, which I think has some kind of circuit to prevent that, and is fuel injected? Also, on the O-200, run-up carb heat indications are a slight decrease in rpm, and the CARB F gauge shows an increase in temperature (Cessna Model 162 POH page 4-18).

I was taught to fly in my Sportstar that has a ROTAX 912ULS. We were always trained to look out for carb ice. Check the Carb Heater on run up etc etc. However, we were also told that it can be somewhat rare in the 912ULS due to the location of the carbs. That said, I will pull Carb Heat on when descending from height, and even the manual states to use it in the circuit.

I guess the only problem is remembering to turn it off if you have to go around - but if you practice this a few times, it becomes 2nd nature.

HAPPYDAN wrote:

ShawnM wrote: The carbureted Continental O-200 is more likely to develop carb ice than the Rotax 912, which I think has some kind of circuit to prevent that, and is fuel injected?

To be clear, only the new 912is is fuel injected. Those, by definition, cannot get carb ice since they have no carburetors. Injected engines usually still have an alternate air source, either manual or automatic, in case the induction system gets iced over or otherwise blocked.

In 10 years and about 500 hours I have never used the carb heat in my Sky Arrow other than to test it at runup. It’s worth mentioning that when carb heat is in use, the air entering the carbs is unfiltered. Not a big deal once in a while, but in general it’s better for the engine that the incoming air be filtered of particulate matter. Extended, unnecessary use of carb heat could theoretically increase engine wear and shorten engine life.

As an aside, I only recall getting carb ice once in my flying career. I was flying some students around on short flights in a J3 Cub. South Florida, so not cold at all. On one flight, I got the impression the throttle was creeping. I had to keep nudging it forward to get the same rpm. When I finally had full throttle and still had lowered rpm and performance, carb ice dawned on me and I applied carb heat. The engine stumbled, then died momentarily before coming back. Whew! On landing, there was actually frost on the outside of the carb and it was quite cold to the touch.

Wm.Ince wrote:

ShawnM wrote:I use it during my run up checklist to confirm that it works.

What are the indications that verify it is actually working?

I get a small RPM drop that tells me that hot air is now flowing into the carbs.

Wm.Ince wrote:

ShawnM wrote:I use it during my run up checklist to confirm that it works.

What are the indications that verify it is actually working?

Bill, on your CTSW it is more of an alternate air rather than carb heat. Most carb heat set ups pull heated air from around the exhaust. This heated air is normally enough to cause enough change in air density to see a drop in engine RPM. The CTSW just pulls air from inside the engine compartment. While it is certainly warmer than outside air the temperature differential is not enough to indicate a drop in RPM when tested.

HAPPYDAN wrote:

ShawnM wrote:Interesting, I'm in the central Florida area (high humidity of course) and never experienced carb ice nor have I ever used my carb heat on my 912.

Correction, I use it during my run up checklist to confirm that it works.

Somebody familiar with Rotax, help me here. But from what I understand, The carbureted Continental O-200 is more likely to develop carb ice than the Rotax 912, which I think has some kind of circuit to prevent that, and is fuel injected? Also, on the O-200, run-up carb heat indications are a slight decrease in rpm, and the CARB F gauge shows an increase in temperature (Cessna Model 162 POH page 4-18).

The type and location of the carb on the Continental make it more susceptible to carburetor icing, than the dual carb set up on the Rotax.