Carb icing at takeoff power
Here in Oregon it's cool and wet--prime carb icing conditions today, just above freezing and just warmed up enough to clear out some low level clouds.  I'm scheduled for my private check ride in two days, and went to practice today. I used carb heat during taxi and paid close attention to the carb heat test during run up...I left it on for a long while, listening for roughness and watching for any increase in RPM.  No roughness, and maybe 20rpm increase but it fluctuates almost that much with wind, so I can't be sure. Certainly nothing major. Carb heat back on as I taxied onto the runway and prepared to practice a short-field take off. Line up at the threshold, nosewheel straight, brakes on. Power full, carb heat off, static RPM check, release brakes and fly! little 150, fly!

I climbed to 3000' (ground here is 200 feet) at full power and applied full carb heat as I reduced throttle to cruise. Within a few seconds the engine was very rough. Remove carb heat--roughness goes away. Apply--it comes back. Try leaning--no effect. Hmm.

This happened about 4 miles from the airport, at the edge of the delta. I can troubleshoot in the air, but there's an airport right there...I turned back and radioed tower asking to come straight back due to engine problems. Tower asks, do you want to declare an emergency? That sounds like a good idea, so I do. At this point I suspect carb icing but I know it's rough, and if it's icing it happened at full power on takeoff, not a good sign. Better to troubleshoot on the ground. I'm in glide range so no worries--left the carb heat on, throttle to idle, coasted down to the pattern and got a nice practice power-off 180. 

On the ground and where my wash won't bother anyone, I repeat the experiment. Full power for a while with carb heat off. Carb heat on... roughness.  In maybe 15 seconds it clear up.  Now I feel confident it was icing and I can get to the point of the story: should I take off again?  I'm not terribly worried about potential ice during cruise, but I'm not happy at taking off in this situation.  The PHAK and the manual call for carb heat off during takeoff. 

My decision was no. I don't know what's normal, but icing at full power wasn't in my training materials anywhere. I park it and start some research. You are now part of this research.
-how unusual is carb icing at full power?
-if you know or suspect that it's happening, would you take off?
-when during takeoff would you use carb heat, if at all?

Longingly thinking of fuel injection!
5 Replies
Very interesting problem. One other thought I hadn't seen here. Since the engine was in all probably running very rich during taxi and initial climb and then even more rich when the carb heat was pulled on, perhaps the plugs were somewhat fouled and cleared up over some amount of time. I have to say that ice is pretty far down on my list of suspected causes.

" You may have missed the part where I tried leaning, to no effect (and far more than 3 turns--there's no vernier on this one, I pulled it out an inch or more). "  If you have this problem again at cruise power (anything under 70% max), I suggest you pull the mixture until you get a hint of roughness (won't hurt a thing) and then enrich only enough to get smooth operation.

" I do tend to run little lean, but this proved my theory.  " How do you know this?  Everything I'm seeing you say indicates a mixture problem.
"tiny roughness (just the normal stuff for carb heat) "  This is not normal unless there was ice and should clear up in a few seconds. Roughness with carb heat on is typically an indication of an over-rich mixture.
" I used carb heat during taxi "  I would respectfully suggest that you don't do this. Carb heat air is typically UNFILTERED air, so anything you suck up will go directly into the engine. If you are worried about carb icing, as you might well be, pull the carb heat on while doing your run-up and that should remove any ice that accumulated during taxi. I know of no Continental or Lycoming engines that recommend using carb heat for take-off. Perhaps someone else might speak up if I'm incorrect.

Good question for Mike Busch, who really is an expert on these things.

I fly a Pawnee (O-540), Citabria (O-320) and RV-6 (IO-360)

Safe flying!
Thanks Chris! I passed the check ride :-)

Good thought!   I appreciate your point, and it's a good reminder not to fixate on one possible cause but to keep gathering data and testing hypotheses! Hard to do in the air, but that's why we train and share.  

That said, In this case I am confident that it was icing. You may have missed the part where I tried leaning, to no effect (and far more than 3 turns--there's no vernier on this one, I pulled it out an inch or more). I'm also used to this aircraft at similar temperatures and altitudes, and sustained application of carb heat was what cured the problem... I did it on the ground, and it was also verified by other pilots in the air. 

Much appreciated,

I don't think you had icing at all.  You say you took off at full power, no carb heat and everything was fine.  Then cut power a bit to cruise and added carb heat.  Then had engine roughness.  If you had icing it would have shown before the heat.  What you had was an over-rich mixture.  Notice that the roughness only occurred with carb heat in use.  Things were smooth until then.
To add perspective, I am in Civil Air Patrol and our squadron was assigned a rather elderly Cessna 182.  It was a good, honest flying aircraft but had a tired engine, ready for rebuild.  Two of our pilots took it out for an acquaintance flight and came back a little shaken.  "When we came in for landing, pulled the carb heat and the engine died.  We had to restart and go around.  The plane is awful".  I told them the same thing, too rich.  So I took the plane up myself and flew in as I would normally do, pulled the carb heat and lost some RPM and a little, tiny roughness (just the normal stuff for carb heat) and landed normally.  I do tend to run little lean, but this proved my theory. 
Why did you add carb heat if everything was going nicely?  At cruise throttle, icing is very rare.  I think flying around with the carb heat on is not a very good idea as it really is designed for emergency measures.  You said you were using carb heat to taxi.  Is that normal procedure?  I usually do some active ground leaning.
Here is an experiment.  Try the same takeoff and settings,  If you want more air go up to 5000'.  Pull power to normal cruise and see how it feels.  No roughness-add carb heat.  If you get that same roughness try leaning.  Not just a little tweak but 2 0r 3 wrist turns.  My wrist turn is about 1/3 turn of the mixture knob.  If things smooth out a little, there is your answer.  Remember to add a little mixture when you turn off the heat.
Good luck on your check ride.
Hi Ethan,

Glad that everything worked out and you were able to figure it out. And kudos to you for declaring the emergency, no reason not to when you don't know exactly what's going on, it can only bring more resources in to help you.

-how unusual is carb icing at full power?
I've never experienced carb ice at full power myself, but I've talked to a handful of pilots that have. We're taught not to really worry about it at full power, but from others experiences that doesn't seem to be true all the time.

-if you know or suspect that it's happening, would you take off?
The answer to this question is a judgement call. You have the tools to deal with carb ice if you experience it, but ultimately its your call after weighing all the other factors relative to your flight. In general, it wouldn't worry me too much to takeoff if I had detected carb ice during run up, provided I had taken sufficient steps to ensure I had cleared it out before attempting takeoff. Any real delay of the takeoff may warrant another check though. Ultimately it's your call, if you think the conditions are too prone to ice developing you can always scrub the flight or wait till conditions improve.

when during takeoff would you use carb heat, if at all?
I have to side with the POH on this last question. If it says carb heat off for takeoff, it should be off. If you develop engine roughness on climb out, you would need to run your immediate action items, which should include carb heat on. Again, its always your call whether to takeoff or not.

Again, glad everything worked out and I hope this helps with your research.

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Icing at full power is unusual, but not unheard of.  Get in the right corner of the envelope (see chart below), and it can happen even at full power, especially in the carb ice-prone Continental engine installations like your C-150/O-200 as well as the old C-172/O-300 and C-182/O-470 combinations from the 50's to late 70's.  The change to Lycoming engines starting with the O-320 in the 172 in 1968, then the replacement of the 150 by the 152 with the O-235, and the 182 going to the O-540 around the R-model dramatically reduced the susceptibility to carb ice of those aircraft.

But if it was carb ice, with your fixed pitch prop, you'd expect to see the problem develop as a loss of RPM along with roughness.  Pulling the carb heat would produce a drop in RPM along with roughness, followed when you applied the heat by an RPM rise to above the RPM at the time you applied the heat along with smoothing of operation.  Given that the problem occurred upon application of carb heat, not during normal operation with carb heat off, my first thought would be that there was no ice, but pulling full carb heat reduced the induction air density so much it put the engine over-rich.  However, you said leaning had no effect.  Therefore, my thought is that there's some sort of problem in the carb heat air path or the air box.  I'd definitely turn it over to the mechanic for further evaluation.