Carb Ice Questions

[MichaelFarley56]

Greetings Everyone,

I wanted to spend a moment and bring up a subject that not talked about much, but I recently had a few discussions with some good homebuilder friends here in central Ohio about carb ice conditions in homebuilts. It’s got me thinking about the possibility of carb ice conditions in our airplanes, and even more specifically, what can be done about it if any of us encounter these conditions, so I wanted to bring up the hypothetical discussion and see what you all have to say about it.

Before I continue, I know and agree that the possibility of us getting carb ice is very remote given the design of the AeroInjector (no venturi and no butterfly valve to build up ice), and the location of the carb (inside the cowling); despite this, I have talked to several Sonex pilots who have experienced situations while flying that may have been attributed to carb ice. These conditions include slight engine roughness and slight loss of engine power, but no one can positively say what the causes were. I know I myself have performed carb tuning engine runs on the ground with no cowling and have experienced condensation on the bottom of the Y pipe intake, frost on the outside of the AeroInjector, and a partial jamming of the throttle slide; all this goes away and never happens with the cowling on, however.

In any case, just for the sake of conversation I wanted to float the idea by everyone on here that, while the possibility of carb ice in a Sonex is incredibly small, there’s still a small possibility of it happening. I’m curious who all here has actually installed a carb heat setup, and if so, what your impressions are of how well it works.

Finally, I’m curious if we as a group can possibly come up with a strategy on how to deal with carb ice, just in case someone is flying along and they start to encounter a rough engine or a loss of power. Call me crazy but, in my opinion, it seems like cycling the throttle may be a good idea just to help clear out the carb and intake system of any contamination. In other words, if you’re flying along and your engine starts to run rough, pull the throttle back to almost idle, up to full throttle, then back to cruise and see if that eliminates the roughness.

Does anyone else have any thoughts, experiences, or recommendations they’d like to add?

[daleandee]

I would never suggest that any carb, injector, or induction system would never be susceptible to icing. Yet I flew a slide carb for nearly 200 hours and trusted that it was safe to do so. I can't say that any of the issues I had were ice related. Some slide carb manufacturers do insist on a carb heat system and other countries require it.

Now that I fly a Corvair with a Marvel Schebler I'm much more concerned with the issue of possible carb ice. I do have carb heat installed and use it religiously any time I reduced power (unloading the engine) i.e. slow flight, descents, approaches to land, etc, and any time I see conditions where carb ice are favorable (temp/dew point spread or visible moisture). Most Corvair flyers use the rule of thumb when landing ... "see the windsock, pull the carb heat."

There have been a few crashes over the years where no reasonable cause could be found. I too have run engines (both two and four stroke) and noted frost and ice build up on the carb and intake. Many pilots don't realize that the OAT don't have to be freezing for carb ice to form, in fact many a nice day with a clear and sunny afternoon have brought carb ice concerns.

With all due respect I'm not certain I would clear carb ice exactly the way you describe as I'd most like open the throttle wide open first, and then reduce it a bit. Also keep in mind that carb ice will generally cause the engine to run richer due to the lack of air entering the engine. When first pulling on carb heat the engine can get rough as it ingest the ice in the carb. At that point some will think they have made the problem worse and push the carb heat off. Some recommend that once you have pulled on the carb heat, leave it on until you're on the ground. I won't pretend to know how others should handle the situation if they should experience it. But I do think this is a good topic for discussion.

Still below the freezing mark with snow on the ground here in the Carolina's ...

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
166.7 hours / Status - Flying
Member # 109 - Florida Sonex Association

[peter anson]

My Sonex is fitted with a Jabiru 3300 engine with the stock Bing carburetor so not immune to carb icing and I believe I have had one carb ice incident. I was on cruise, 20 minutes from our departure airfield carrying a passenger and in humid enough conditions so that we were dodging rain squalls when the problem showed up as an occasional "misfire", sharp enough to be felt through the floor, and becoming more frequent. I tried flipping one of the ignition switches and the motor started to die so we turned back to the departure point and I opened the throttle. The problem immediately disappeared so I rechecked the ignition and all was well. We turned back to our destination and had no further problems. The misfire has never reoccurred so my assumption is that it was caused by ice in the venturi and the "misfire" occurring when bits of ice broke off and went through the engine. I did not apply carb heat at the time because it did not occur to me to do that first and the problem cleared so quickly that there was no need to try anything else. At the time I thought it was caused by a problem with the ignition but I was unable to find a fault and have had no further problems.
I'd go along with Dale's idea of opening the throttle first, if only because that's what worked for me.
I run a Jabiru air cleaner which incorporates a valve for carb heat and is fitted with paper element automotive filter. Like all these things it was fiddly to fit but seems to work very well. I always use it on descent and in the circuit until established on final.
Peter
Sonex 894
344 hours

[radfordc]

I’m curious who all here has actually installed a carb heat setup, and if so, what your impressions are of how well it works.

I installed the carb heat system designed by the Sonex factory....it's called a "cowl". All the heat from the engine flows down through the cowl to the carb and ensures no icing. The only downside to this system is that you can't turn it off.

I don't think there has ever been a failure of this system to prevent in flight icing. I seem to remember someone installing a temp probe at the carb intake and reporting temps well over 100 F all the time.

[Onex107]

I have experienced carb ice with a Continental O200, both on the ground and in the air. With the Marvel Schleiber? carburetor the first indication is the rpm dropping slightly. Carb ice can form at almost any combination of temp and humidity and the butterfly in a carburetor is the sharp edge it forms on, and if it is in the idle position, almost closed, it will stop the engine . Google carb ice and get the graph depicting the ice formation curve. In both cases the heat is turned on full, if you have it, and can be left on, except for take off where you need full power. The time flying I was near the cloud bases and I presume the dew point temp. On the ground it was during taxi. I expect, with the AeroInjector, the ice would form on the square edge of the slide opening causing the engine to go richer. I should think going wide open would dislodge it. The slide is probably not as warm as the Injector body. But if that's the case, ice, I'd expect it to happen again until you change altitude or location.

[radfordc]

I expect, with the AeroInjector, the ice would form on the square edge of the slide opening causing the engine to go richer. I should think going wide open would dislodge it. The slide is probably not as warm as the Injector body. But if that's the case, ice, I'd expect it to happen again until you change altitude or location.

Nice theory...but has anyone seen it happen? I only had a little over 400 hours on my Aerovee when I sold it but for sure never noticed anything that seemed like icing. I don't remember anyone else talking about getting icing.

The purpose of a carb heat system is to bring warm air to the carb to prevent/cure icing. It seems that the Sonex design ensures that warm air is at the carb all the time.

[Sonex1517]

Getting the thread back on track, I am also curious to hear of the experiences of others.

I am well aware of what we have been told regarding carb ice and the AeroInjector, and I have a cowling installed. That does not mean I cannot get carb ice, and it certainly should not prevent the conversation.

Personally, I tend to be far less likely these days to believe what I am told by a manufacturer or someone without first hand experience.

[MichaelFarley56]

Nice theory...but has anyone seen it happen? I only had a little over 400 hours on my Aerovee when I sold it but for sure never noticed anything that seemed like icing. I don't remember anyone else talking about getting icing.

Well Charlie, I have nearly the same number of hours on mine and I’ve never experienced what I believe is carb ice either, but I know there’s people on here who have experienced suddenly rough running engines while in cruise for no apparent reason. I’m not saying I know what the reason is but if we can all discuss the possibility in an intelligent matter, just maybe someday it could keep someone from making an off airport landing. To me, that makes the discussion worth while, especially when there’s more than one NTSB report with an “engine failure for unknown reasons” as the cause.

There are foreign government agencies that require carb heat. We all have plans that include installation on our brids if we choose. I just thought it would be worthwhile to discuss.

[WaiexN143NM]

Hi all,
We have the rotec tbi installed on our jab3300 on our waiex. The rotec install instructions require carb heat. Maybe for the lawyers too, and to comply down in australia. Weve never used it, havent had too yet, the planes down in tucson. We arent there in july or august when the monsoons hit. The rest of the year very dry.
I remember reading an accident report years ago, about a kr2 near minneapolis , in the fall on a cold wet day. The airplane had a vw motor and a posa carb. Its a slide carb, like the aeroinjector, or rotec. The engine quit and the airplane made a forced landing in a field hit some trees. Pilot ok , but plane was totaled. Official report listed carb ice as the cause for the engine to quit.

We have a heat muff on one exhaust stack, scat tubing, and an air box. Control cable runs into cockpit to knob near flap handle area.

I also wondered why jeff schulz's first sonex crashed, recent jab 3300 install, aeroinjector or bing? Maybe jeff can chime in and update the story.

It's an easy thing to add. Better to have it handy if you need it.

Fly safe!!
WaiexN143NM
Michael

[sonex892.]

I run a bing on a Jabiru 3300 and have a carb heat setup. On very rare occasions, I have experienced rough running only whilst warming up prior to flight. Definitely icing as application of carb heat soon sorted it. Revs drop a bit then it comes back to life. In 280hrs of flight I have never noticed any symptoms of carb icing whilst flying.

Even if I had an aerocarb I would fit an airbox and use the cool air for performance and still have the heat available for safety.