Carburetor Icing

[Colonel Sanders]

I was doing some tailwheel training today. Just above freezing,
perhaps 2 degrees dewpoint spread. Ragged ceiling at 500AGL,
2 or 3 miles visibility in haze.

We were going to do some circuits. He has a Continental O-300,
and I asked the student what he had to look out for today, and
we got around to the subject of carburetor icing.

The conditions were ripe for it, and we all know the Continentals
make ice like a frigidaire. No less than Grant_McConachie
once had a very bad day because of carburetor icing - read his biography,
"Bush Pilot With A Briefcase".

Anyways, we taxied out and I pointed out that in these conditions,
with the throttle almost closed at idle, the carburetor could easily
fill up with ice during the taxi to the runway threshold.

So to ensure that the carb had no ice, we positioned for takeoff
and selected 1500 RPM, and full carb heat, and leaned the mixture
for max RPM. You always lean the mixture for max RPM whenever
you apply carb heat - especially in a descent!

We counted for 10 seconds, then all the knobs went into the dash
and off we went.

Now, the problem is that a Continental can build carb ice at wide
open throttle. Really. So, to deal with that, while he flew the airplane
I operated the carb heat and mixture in the circuit. I got the carb heat
on quickly, and also leaned the mixture for max RPM, and kept them
in that position during downwind, base and final to provide maximum
heat to the carburetor.

I could tell that it was working, because warm air was flowing into the
cabin, and it uses the same trick - a shroud around the exhaust, which
incidentally has to be removed every year so the exhaust can be inspected
for cracks.

Remember, an engine is just an air pump. You have to be able to get
air into (and out) of it, for it to make power!

My apologies to the old pros, who have heard all this before. Keep in
mind that kids these days have about as much familiarity with carburetors
as they do with vinyl records (78, 45, or 33 RPM).

PS My experience is that six-cyl TCM's are ordinarily very smooth
engines. This one runs more like a Lycoming, despite my playing
with mags (plugs, wires) and carb heat (fuel-air distribution). I
recommended that he do the rope trick as per Lyc SI 1425A,
even though it is a TCM - I am suspicious of lead in the valve guides.

[comfail]

Good tips on carb ice, especially this time of year with the ambient temps where they are. Thanks Colonel.

Your postscript really got my attention though. My O-300 runs rougher than the lyc O-320 im used to flying behind, I just had the exhaust valve guides reamed so I'm pretty sure its not valve(s) stuck. My AME noticed that the prop wasn't indexed correctly, and will correct this(currently being annualled). I have no doubt that correct prop/flange indexing is important, but I don't understand how this would change engine vibration level... Any ideas?

[Colonel Sanders]

It can change the balance if you put the prop on
the other way - that's probably what he's going
to try.

Always check plugs / wires / mags when you have
a slight roughness. Also, intake leaks, although this
is more noticeable at idle than WOT.

[Doc]

Carb Ice put me on the Conestoga Parkway! My student kept feeding in the power to maintain during slow flight. By the time he let me in on this, our 150 had become a glider. No harm, no foul. But it CAN get ya!
Don't know how many of you have Breezy time? But it's powered by a Volkswagen engine, the carburetor of which is a bleeding ice magnet! SOP....leave the carb heat ON full time.

[Colonel Sanders]

I had an O-200 fill up with carb ice in a full power
climb in an elderly buck fifty. The good news is
that it had 60 degrees of flaps.

[comfail]

Basically my understanding is that he is going to move the prop ccw two bolt holes from its current position. It's a vibration that starts around 2000 rpm and is pretty much gone by 2450-2500. Personally I think it is prop/engine balance issue but both mags are getting overhauled this go round as well so if it does improve I may not know for sure what it was. I will also get him to check the intake plumbing for leaks if he hasn't already.

More on topic, not sure if its just the weather we've had here out west or a Lycoming/Continental engine difference, but I've had more carb ice the past few flights in the Continental than in all 100 hrs or so that I have in a Lycoming.

[AirFrame]

I have no doubt that correct prop/flange indexing is important, but I don't understand how this would change engine vibration level... Any ideas?

It's never been adequately explained to me why this is either, but I do know that it matters as i've felt the change on more than one aircraft as the prop has been moved from one index to another. Over the years i've heard many reasons why props are indexed a certain way, many contradictory. The one that makes the most sense to me overall is the "ease of handpropping" argument, that says it should stop where it would be ready for handpropping if needed. I haven't heard a convincing engineering argument that explains any other positioning.

[rapid602]

Thanks for the tips fella's all good stuff .... I had a VW Engine in a homebuilt once and I guess I got lucky, I never got ice in it .... never got much heat out of it in the winter either.

Its amazing what you forget over time and take for granted sometime.

Thanks for the recall.

[Colonel Sanders]

I've had more carb ice the past few flights in the Continental than in all 100 hrs or so that I have in a Lycoming

The airframe installation can make a difference, but fundamentally Continental tends to hang the carburetor out in the breeze. This keeps it cooler and might make more power, but is more prone to icing because a carburetor is essentially a refrigerator - the evaporation of fuel in the venturi cools it off.

However, Lycoming said screw that. It bolts the carburetor directly to the oil sump and runs the intake tubes through the crankcase. So, as soon as the oil warms up, it raises the temperature of the carburetor. This is bad for power - the fuel-air mixture expands - but is good for fighting carb ice.

Do remember that on the first takeoff of the day, the oil is still cold on a Lycoming, so you are still vulnerable to carb ice at that time. Inflight icing is still possible with a Lycoming after the oil warms - remember, every airframe installation is different - but is not common, in my experience over the decades with Cessnas and Pipers.

That said, all the airplanes in my hangar are fuel-injected!

[rapid602]

Colonel ..... you have forgotten more about aircraft and flying than I will ever know. Have you written any books with your bits of wisdom... If you haven't I suggest you do and call it GOOD STUFF TO KNOW and I will purchase the first two copies. Seriously. If you have written any where can I buy two.

[Colonel Sanders]

Thanks - I have tried to start doing that here:

http://www.pittspecials.com/articles.html

There's an instructor in the USA that did that - you may find it worth a read:

http://www.whittsflying.com/web/index.htm

[Big Pistons Forever]

I've had more carb ice the past few flights in the Continental than in all 100 hrs or so that I have in a Lycoming

The airframe installation can make a difference, but fundamentally Continental tends to hang the carburetor out in the breeze.

I have heard more than one pilot say you don,t have to worry about card ice in Lycomings. However the 2 worst airplanes for carb icing I have flown were the Apache and the Islander, both with Lycomings.

The bottom line is, for aircraft with carburated engines, the first thing you should do with any unexplained reduction in engine power or rough running with a free airtemp between minus 15 and plus 25 is put the carb heat on. I have flown my little Grumman AA1B (Lycomings O235) for over 10 years on the West Coast. Even idling on the Plus 5 with 100 percent humidity Winter days, I have never had any ice. In fact the only time I ever had carb ice was cruising at 4500 feet on a nice clear Fall day with an OAT of 9 degs, go figure.

[rapid602]

Interesting thread .... got me thinking to my early days, and thought that with all that can go wrong with carb ice, its surprising that it doesn't give more lower time pilots and students a bad afternoon. Not that they are bad pilots, there is so much going on when you are starting out, and no experience base to draw upon for the old GET OUT OF TROUBLE SOLUTION. I rememeber flying around building time and would pull the heat about every ten minutes or so, and if there wasn't much change I pushed it in. I didn't know any different.

Anyway FLy Safe All.

[jump154]

Interesting thread .... got me thinking to my early days, and thought that with all that can go wrong with carb ice, its surprising that it doesn't give more lower time pilots and students a bad afternoon. Not that they are bad pilots, there is so much going on when you are starting out, and no experience base to draw upon for the old GET OUT OF TROUBLE SOLUTION. I rememeber flying around building time and would pull the heat about every ten minutes or so, and if there wasn't much change I pushed it in. I didn't know any different.

Anyway FLy Safe All.

It is a pity that one cannot demonstrate carb ice in a safe manner - the above situation sounds rather like me, checking every 15 minutes as I was taught, using carb heat when instructed to as a precaution, but having never experiecned it for real it's all rather unknown and scary to me.

[Shiny Side Up]

It is a pity that one cannot demonstrate carb ice in a safe manner -

Most motorcycles I've had will build carb ice fairly readily if you're tough enough, or insane enough, or both to ride them in reasonably wet conditions. In most cases the fix is to rig yourself up some "carb heat" by baffling in some warmer air from somewhere. You'll get the textbook responses from the engine as it builds ice, you melt it and the engine chokes through the water.

One of the other best things I've seen to help is the carb temp guage that can probably be installed on anything, but I've only ever seen it on the O-470 engines in their C-180 and C-182 installations (but is probably elsewhere). Any bit of moisture in the air seems to give you a little bit of ice if the temp is in the right arc on the guage and again one can see textbook responses from the engine with the application of heat to remove. Not sure if I would ever go purposefully seeking out a demonstration, though one hardly needs to with the 180 I find.

[robertsailor1]

In my experience fall days at plus 8-12 can be the easiest days for carb icing. As it gets colder the air can't hold the moisture so your less apt to get a real strong dose compared to temps above zero.

[Old Dog Flying]

BPF: I've had plenty of carb icing with my AA1A 0-235 (HC)..and I too live in Lotus Land...so why the difference. I've had it after start and taxiing and in cruise.

Barney C-GFXH

[rapid602]

Hey Colonel or some of you very wise individuals very experienced with carb ice ....

How many pilots do you think may have had carb ice during taxi and thought it was just the plugs fouling from being too rich ???? I would bet it happens more than we knowor think. I would think many pilots probably myself included would think plugs first.

[rapid602]

Hey Colonel or some of you very wise individuals very experienced with carb ice ....

How many pilots do you think may have had carb ice during taxi and thought it was just the plugs fouling from being too rich ???? I would bet it happens more than we knowor think. I would think many pilots probably myself included would think plugs first.

[Beefitarian]

I'm not very wise in the ways of carb ice and tend to fly 172s in Alberta. So I rarely have seen the roughness just after applying carb heat that would indicate ice. This of course has me in a nice sense of false security. "I never get carb ice." I think we had a bit in Maui that carb heat cleared during the run up. When I suggested it must be common, the instructor said mostly at higher altitudes. I think he assumed I was asking about airframe icing. I forgot to ask more to clear that up in the debrief chat/visit later.

It is a pity that one cannot demonstrate carb ice in a safe manner -

Most motorcycles I've had will build carb ice fairly readily if you're tough enough, or insane enough, or both to ride them in reasonably wet conditions.

I think mine creates rain. I can take it out on the nicest day ever and soon it is raining.

Off the topic of carb ice and on the topic of smooth running engines I spotted a beauty of a model A last week. I waddled over to talk to the driver and thought it must have been a V-8. I don't know what sort of exhaust he had but it idled super smooth. It was the original four cylinder. He claimed he only added an overdrive so he could get out on the freeways of Utah. Wonderful car, typically I didn't take a picture.

[jump154]

It is a pity that one cannot demonstrate carb ice in a safe manner -

Most motorcycles I've had will build carb ice fairly readily if you're tough enough, or insane enough, or both to ride them in reasonably wet conditions. In most cases the fix is to rig yourself up some "carb heat" by baffling in some warmer air from somewhere. You'll get the textbook responses from the engine as it builds ice, you melt it and the engine chokes through the water.

Might have had that on my Honda 400/4 I used to ride all year in the UK - btu then whenever it rained the effects were probably masked by the constant switch from 2/4 cylinders as the coil shorted out.....
I've heard the FV1200's at the race track make ice like crazy at the early spring races.

[Shiny Side Up]

Might have had that on my Honda 400/4 I used to ride all year in the UK - btu then whenever it rained the effects were probably masked by the constant switch from 2/4 cylinders as the coil shorted out.....
I've heard the FV1200's at the race track make ice like crazy at the early spring races.

My Bandit builds ice pretty predictably. The oil cooled engine doesn't heat up the air that goes into the intake sufficiently at the single digit temperatures.

[iflyforpie]

My Volvo 240 actually has carb heat--the shroud around the exhaust looks remarkably like a 172's. The good old British side-draught carb doesn't really get the engine heat like a traditional down-draught carb.

If I'm going to get carb ice here, it is usually on the ground. Spring and fall bring fog and I often depart right after it has cleared with a minimal temp/dewpoint spread (when you get prop vortices off a 172, you know it is nearly 100% humidity). Carb heat, full throttle static, carb heat in and fine tune the mixutre (I am never full rich) and off we go.

[Colonel Sanders]

My 1970 Camaro 350 had carb heat! There was a vacuum-operated flap on the air cleaner intake snorkel which pulled in air from a flex hose running to a shroud over one of the exhausts. Sound familiar?

Of course, I tossed all that in the trash when I put an Edelbrock aluminum Performer intake manifold and a Holley carburetor on it. Didn't have the electric choke. Didn't hook up the manual choke, either - pulled all that crap off, too, which made cold starts and the warming up interesting - had to run it off the accelerator pump until it warmed up.

God, I loved that car. Torque rocks. Even with a ridiculously high axle ratio, it laid rubber as it if was specifically designed to torture-test tires by shredding them. I always wondered what it would have been like with a 4:10 ring and pinion.

[LousyFisherman]

All I can say is the first time you have carb icing i'ts really scary.

Southern Alberta, C150, 6000 ft, engine noise changes,
RPMs slowly decrease, brain freaks, hand pulls carb heat, engine goes rough,
brain is caught between OH NO we're going down, while in the background
a tiny little voice is saying this is good.

30 seconds later, engine runs smoothly, RPMS are back to normal.
life is good

Happened again 15 minutes later, fixed it, raised altitude to 7500 ft, no more icing.

I am extremely happy my first experience with carb icing was while in cruise,
rather than takeoff or landing. Now it is just something that sometimes happens
given certain conditions.

LF