Carb Ice

[aquanaut]

Okay guys, I need help here. I'm in Indiana, It's very cold, and I had what I think are carb ice problems today. Had some healthy cross winds today and thought I'd get some practice in. Temp was in the teens and wind was healthy. Basically just clear, very cold and windy. On my second landing I applied carb heat on downwind about 5 seconds before I pulled the throttle back for 1700 rpm and I left it on all the way in. I pulled to 1500 rpm on base and then adjusted as necessary for final. Just as I was entering ground effect the engine slowed itself and the throttle had no effect. With no option for a go-around I got lucky and pulled off a perfect crosswind landing. When I got the plane stopped I couldn't even taxi because the throttle had no effect. I checked everything I could think of (throttle, mixture, carb heat) to no avail. I pushed the carb heat off and it changed nothing at first, after about a half minute or so the engine began to respond to the throttle movement and I was able to clear the runway and taxi back to the hanger. I ran it for a few minutes at the hanger and It would be OK for a minute and then start acting up.
During the preflight I sumped both tanks and the carb drains and there were no signs of water. Can anyone out there shed some light on this? Do I need to change my use of the carb heat? Apply earlier etc, etc.? I did notice, the last time I flew (another cold day) I was cruising at altitude and heard one of those almost imperceptible, heart stopping, changes in the engine. I applied carb heat but didn't notice any change and flew the rest of the trip without incident. Everything I know about carb heat is information picked up here and there and I couldn't honestly stand up and defend any of it. So now I'll gladly listen to any comments, suggestions or advise. That's not to say I'll follow it blindly. But I'll sure listen.
Thanks,
Dave

[jrenwick]

Carburetor ice can form when the temperature in the carburetor throat is a few degrees on either side of freezing. Here are a couple of gauges that Aircraft Spruce sells:

They disagree about what the exact temperature range is. One calls it -10C to +10C; the other says -15C to +5C.

The carburetor throat will be colder than OAT because it's cooled by the reduction of airpressure in the venturi and the vaporization of fuel. So if you're operating in an OAT that's below freezing, the carburetor temperature may be well below the range in which ice can form. In that situation, adding carb heat might actually cause ice to form by warming the carburetor into the danger range.

If you have a carburetor temperature gauge, it can tell you if this might be the situation. From what you described, it sure seems to fit!

Best Regards,

John

[yukontools]

Are you using your carb heat while taxiing prior to t/o? I have gotten in the habit of doing so and I go carb heat cold prior to doing my CIGARS check. During run-up I will pull carb heat to insure a drop in RPM'S. Does it appear to be full power on t/o? If you are not getting a drop in rpm's during your runup check there may be a linkage bad that is not moving the flap appropriately. When you apply carb heat downwind did you get a rise in rpm? If so, this might indicate carb ice. I once read that during flights when pilots apply carb heat they don't leave it on long enough and if ice is present it won't all disappear. FWIW from me.

[blueldr]

Carburetor ice is a function of humidity rather than water in the fuel.

On a really cold day the carburetor heat system is hard pressed to provide much heat unless a very high power setting is pumping very hot exhaust into the heat exchangers. Not likely in the traffic pattern. I am not in the habit of using carburetor heat in the traffic pattern and can't recall ever having had a carburetor ice problem. I have, however, encountered it in cruise on humid summer days.

Also, remember that very cold air is incapable of holding much water in vapor form. The humidity is likely quite low.

I've never encountered carbureor ice in sub freezing conditions. It has always been in relatievly temperate or warm air in cruise configurations.

[pdb]

Carburetor ice is a function of humidity rather than water in the fuel.....

... I have, however, encountered it in cruise on humid summer days.

Also, remember that very cold air is incapable of holding much water in vapor form. The humidity is likely quite low.

I've never encountered carbureor ice in sub freezing conditions. It has always been in relatievly temperate or warm air in cruise configurations.

Blueldr:

From the description of the day (temp in mid teens) and clear (no visible moisture) its likely that the absolute and relative humidity were low...not usually the sort of conditions to cause carb icing.

However, the use of partial carb heat or the use of a heat box that is not operating well (worn seals, stretched cable, etc) in sub freezing air can warm air in the venturi up enough so that the few supercooled ice crystals in the air melt and then refreeze of the butterfly valve. (The Light Plane Maintenance Aircraft Engine Operating Guide has a good description of this..)

This is why lots of manuals recommend use of full carb heat whenever you think you might need it unless you have a carb temp guage and visually monitor the carb throat temps.

I would recommend that our friend have his carb air heat box checked to make sure its operating properly. The induction systems in our O-300s seem particularly prone to carb icing. Flying up here on the spring and fall when the temps are in the low 50's and operating in or near rain and/or clouds, carb icing is a problem. Its readily solveable but it requires your attention.

[aquanaut]

My mechanic and I checked the carb air heat box and everything is functioning correctly. The Carb Temp gauges are well worth looking into. During runup I noticed the normal drop in rpm when I applied carb heat. I always apply carb heat in downwind about 5 seconds before reducing rpm from full throttle, and then leave it on the rest of the way in. If I'm going around I push the carb heat in just before advancing to full throttle.

During the flight in question, I didn't notice anything out of the norm when I pulled on carb heat on downwind, which means the rpm probably dropped a little as it usually does.

blueldr: you said you're not in the habit of using carb heat in the pattern. Does that include very cold days? I always use it in the patten.

Again, thanks for the input.
Dave

[4stripes]

Apply carb heat for the entire downwind leg. 5 seconds prior to reducing throttle may not remove any carb ice that had formed prior to this point. Carb ice does not only form at low throttle. You may have had a good chunk prior to your carb heat activation. Once you reduce throttle, carb heat works less efficiently, as the engine is producing much less heat.
Cheers Eric

[flyer170]

I have done both so to speak 1. Carb heat on entering down wind and turn it off after turning final in case of a go around.
2. Now I don't use carb heat at all unless I suspect carb ice, other than checking it for drop prior to take off.
Does anybody use carb heat to see if they have not leaned out to far. Pull carb heat and there should be some drop if you are on the right side. I know you can use rpm setting and egt also but jusy wondering.
I would like to hear from others also on the use of carb heat.
Bob

[N2865C]

I have done both so to speak 1. Carb heat on entering down wind and turn it off after turning final in case of a go around.
2. Now I don't use carb heat at all unless I suspect carb ice, other than checking it for drop prior to take off.

I prefer to turn the carb heat on on downwind and push it in with the throttle in the event of a go around. It becomes second nature with some practice. I would not like to find out I had developed carb ice while on final during a forced go-around.

After run-up I leave the carb heat on until takeoff per the Operators Manual unless there will be a delay in departure. The range of carb ice is broad http://www.aopa.org/images/asf/ePilot/nyc02fa025_1.jpg and I would rather err on the side of safety.

[pdb]

I prefer to leave the carb heat on and push it in with the throttle in the event of a go around. It just becomes second nature with some practice. I would not like to find out I had developed carb ice while on final during a forced go-around.

After run-up I leave the carb heat on until takeoff per the Operators Manual unless there will be a delay in departure. The range of carb ice is broad. http://www.aopa.org/images/asf/ePilot/nyc02fa025_1.jpg and I would rather err on the side of safety.

I think it's very good practice to use the carb heat as you describe on landing.

Flying up here on wet fall days...the worse case scenario as your graph clearly shows, I can also ice up on the ground after the run up while at idle waiting to take the runway. Using full carb heat for a few seconds on the takeoff run will clear the ice out and then pushing it restores full takeoff power.

Thanks for the graph..its a nice illustration.

[Roesbery]

Probably not your problem, but might be worth checking the carb bowl drain for water. Have a container that can catch any fuel and or water as you remove the plug. Then replace the plug and saftey it. Any ice will need to be thawed first. Have had that happen. Also one time took off of a bush strip at -12F and wouldn't develope full power untill 15 or 20 minutes into the flight, found out later the electric fuel boost pump sump had ice in it which melted from engine heat. It didn't allow full fuel flow while frozen. Course a stock 170 won't have a electric fuel pump but something to keep in mind for the higher HP installations.

[AR Dave]

Doesn't sound like Carb Ice to me! Anytime I've had Carb Ice, when I pulled the Carb Heat out, I knew it immeditely as the chunk of ice was chewed up. That's in cruise at 2450 rpm's. With carb ice, the mixture control gets trapped, where you can't lean or enrich without the engine dying. At least that's the way it's been in my experiences.

Not only do I apply Carb Heat on down wind, I'll pull it every once in a while on days when the carb ice needle is in yellow. Why wait until the engine is choking off to discover it.

So is your engine running good now?
Are you leaning your engine?
Getting all the condensate out of sumps?

[GAHorn]

Carb ice can occur even on clear days in sub-freezing temperatures. The concept of carb ice not occuring in sub-freezing temps is a result of visible moisture being already frozen in most cases , and therefore already falling out of the atmosphere....(otherise any visible moisture will be super-cooled droplets.) In other words, just because it's not visible...doesn't mean there's not moisture content in the air.
A further drop in temps usually will precip it out... but a warming...such as passing thru a carb-heater system....will prepare that moisture for re-freezing within a cold carburetor. In very rare circumstances carb ice can be acquired even in sub-freezing temps.
But, in the absence of further info it's difficult to determine if Dave actually encountered carb ice or not. I am inclined to doubt it for several reasons.
One: Carb ice normally would be indicated in the case of slow reductions of available power. In the case of a pilot continuously decreasing power during approach to a landing (after having flown during cruise or traffic patter work) it's unlikely that the application of carb heat with carb ice already present, would have failed to manifest itself without a sudden drop in rpm followed by a recovery of some of that rpm.
Two: In the case of carb heat application prepratory to landing subsequent to a low-power descent into the traffic pattern, it's possible that carb ice may have become present... but application of carb heat should have caused a stumble or rought running engine shortly upon heat application.
Three: Without further info by what is meant by "I ran it for a few minutes at the hanger and It would be OK for a minute and then start acting up."... it's pretty difficult to diagnose this event. Does "OK" mean that it responded to throttle application?... Or does it mean it idled smoothly?... Or does it mean that while idling it "acted up" by backfiring, smoking, shaking, and belching fire thru the carb?

My first inclination is to believe the carburetor accelerator pump is malfunctioning and that the removal of carb heat in such cold temps (effectively leaning the mixture) prior to application of throttle, manifested itself as an engine that refused to accelerate.

[jatkins]

I decided to install an STC'ed carb air temp guage in my C 170B , after thinking about , all the time you are running with carb heat on , you are using unfiltered air for your engine. My thought was with a carb temp guage I can select carb heat when it is required , and us as little unfiltered air as possible.

[4stripes]

Dirty air is not a problem when flying. We are not like auto's that trail other vehicles dirty wake.
Use carb heat religiously, as carb ice has brought down many perfectly serviceable aircraft.
Cheers Eric