Carb Ice Discussion

Sinistar said:
The MP makes for a fantastic indicator...but you have to fly level :)
Click to expand...
That’s exactly how I caught the induction icing on the Twin Beech most recently right before Christmas.

Flying along in IMC and rain and noticed the left engine MP was starting to sag. My Twin Beech has Bendix Injection (no actual carb). Pulled on the manifold heat solved the problem before the family noticed anything.
 
Capt. Geoffrey Thorpe said:
Here is the classic picture of temp / dewpoint and carb ice probability
nyc02fa025_1.jpg
:

The thing is, absent visible moisture, raising the temperature with carb heat moves you horizontally to the right - any heat moves you to a reduced risk. The only exception is if there is some form of water (rain, snow, ice, cloud) that you can evaporate to increase the absolute humidity, and even then, you have to raise the dew point faster than you raise the temperature in your induction system.
Click to expand...

So really what he said just wasn't so for any practical application...

If engine is running about how much heat does carb heat add?
 
Tom-D said:
It varies per engine exhaust setup. the 0-300 runs about 250 degrees
Click to expand...


Well unless some engines are extremely cooler than that, that kind of blows a hole in what that examiner told me! You would have to be flying with ambient below -150F for his theory to be right... I suppose one could argue it may not be needed in bitter cold, but to argue it could put a "too cold for ice build up" carb into any sort of "danger" zone of temps would take a day far too cold for me to go flying! lol.
 
Tantalum said:
Right, but you lose what, 100 RPM? I'll take that over an engine failure.. and it's not like you would run it with it always on..
Click to expand...

There are two risks. Sometime, do a circuit and note the actual RPM in the climb. On the next circuit, set the throttle so that the RPM never rises above an RPM 100 less than you saw on the first circuit. It's scary, and it's one of the things we used to teach: a reduced-power takeoff to convince the student that every rev matters. And we always checked the tach in the takeoff roll to make sure we were getting as much as the engine was capable of giving.

The other risk: elevated induction temperatures raise the risk of detonation. At full power detonation is more likely, so on takeoff we'd use carb heat only if absolutely necessary. Since carb ice is less likely at wide-open throttle, the need for carb heat would be rare indeed. You'd be in IMC, basically, or you'd have accumulated a bunch of carb ice during warm-up and taxi.

Some installations make an awful lot of carb heat. The 180 comes to mind. Some of those would nearly quit when you pulled full heat. The mixture would get way too rich, so you sure didn't use full rich in the approach with full heat on.
 
When instructing practice forced landings/approaches (PFLs) we taught the student to clear the engine every 1000 feet in the glide. The syllabus never explained why, so we had to expound on it. That exhaust system gets cool in the glide and can't contribute much heat if it becomes necessary. Running the engine to 2000 RPM or so for ten seconds (which is a long time) heats it up and since the carb heat is on anyway, it warms the carb and removes any ice that has formed. That "clearing" of the engine also helps the student reach that field he's aiming for. Ha.

We had one airplane, a Lyc-powered Citabria, that iced up on a PFL, and the simulated approach turned into the real thing. It landed in a snowy field and we had fun getting it out to an adjacent road. See, even a Lyc can get you. Clear that engine.
 
Huckster79 said:
Well unless some engines are extremely cooler than that, that kind of blows a hole in what that examiner told me! You would have to be flying with ambient below -150F for his theory to be right... I suppose one could argue it may not be needed in bitter cold, but to argue it could put a "too cold for ice build up" carb into any sort of "danger" zone of temps would take a day far too cold for me to go flying! lol.
Click to expand...
The 0-300 dedicates 1 muffler to de-ice the carb, Many others place a muff on a pipe.
 
Dan Thomas said:
There are two risks. Sometime, do a circuit and note the actual RPM in the climb. On the next circuit, set the throttle so that the RPM never rises above an RPM 100 less than you saw on the first circuit. It's scary, and it's one of the things we used to teach: a reduced-power takeoff to convince the student that every rev matters. And we always checked the tach in the takeoff roll to make sure we were getting as much as the engine was capable of giving.
Click to expand...
Yeah that's a good point.. in any plane I've flown always do the full power check. Are you making the RPM? And if you have manifold pressure gauge are you getting what you should. I think most newbie pilots on one of their first solo "pattern work" flights leaves the carb heat on in their little 172N and doesn't realize it until they can't figure out why the engine sounds different and the climb sucks

But.. I guess my point was beyond just flying around with the thing on all the time. I know in the Pipers they told us to not really use it unless you needed it because it bypassed the air filter.. so it was really there just for emergency.. and something with the pipe being different so it doesn't collect ice as easily? Although I've never seen that second part corroborated anywhere

But.. if you are in conditions ripe for carb ice.. than why not just use it anyway..?

....but to further belabor the point, this just another thing that makes me shake my head with how painfully low tech our airplanes are

Cheers, oh and happy 2019!
 
Huckster79 said:
Well unless some engines are extremely cooler than that, that kind of blows a hole in what that examiner told me! You would have to be flying with ambient below -150F for his theory to be right... I suppose one could argue it may not be needed in bitter cold, but to argue it could put a "too cold for ice build up" carb into any sort of "danger" zone of temps would take a day far too cold for me to go flying! lol.
Click to expand...

That's not what the carburetor sees. I heard the system needs to be capable of increasing carb temp by delta_90F (not 250) when ambient is 30F for normally aspirated engines. These spam cans seldom make it. More than likely they hover in the 40-60F delta at the carb by the use of CH on a good day, some people have suggested as low as 20-30F delta (experimental setups), but this is all anecdotal.

The reason you should dismiss the notion of really cold air being raised to the degree it re-freezes moisture that otherwise would pass thru the carb unstuck, is that at those OATs the content of moisture in unsaturated air is insufficient to cause problems even if it stuck to the carb body by partial upstream pre-heating. The only outlier case would be going through visible frozen moisture (cirrus clouds) and heating it up to the exact degree that it enters the target zone. Tall order.

The more likely scenario is that of partial carb heat use in ambient temperature conditions with enough endemic moisture to cause problems in the first place. Partial heating doesn't get the air hot enough to get far enough away to the right of the target zone, so that when it goes through the carb body induction cooling drop (up to 70F delta) it still freezes and sticks with impunity, whereas it wouldn't have at all if you had pulled the CH knob all the way. That's why most of these ancient blind setups warn against partial carb heat, and proffer the more prophylactic all-or-nothing mantra.

I think the economic argument to run around with these things in 2018 is stupid, and the FAA is at fault for this. If I could modify an induction system in-place, it wouldn't be really all that expensive. But because of the FAA, I have to go through an entire expensive side quest just to one-off (STC, 337, field approval A/P potato whatever -- I don't care about pedantic distinctions without a difference) what is and would be a simpleton change on the experimental side of things. If primary non-commercial had been allowed, retrofitting these induction systems wouldn't be so artificially uneconomical.

But I stopped tilting at that windmill when I sold my warrior II (I did have one small instance of carb ice on takeoff after a long chilly nighttime taxi, and a less than robust CH runup in hindsight) and just went to a fuel injected aircraft. Haven't looked back since.
 
Last edited:
I’m a student at 17 hrs and every morning we go up we need carb heat right at start up, and taxi, and usually run up... I find that chart humorous since most mornings we fly out it’s about -10f or so, that chart doesn’t show any ice below 10/10. We fly a 182P and carb heat is used more than prop control. We fly out of GUC which tends to be a rather cold spot, guess I’m in the minority, but I’m sure not afraid of the carb heat!
 
Huckster79 said:
That has always been on my mind, On most checklists I see apply carb heat as not the first item for after engine failure, in my mind it should be first due to that, and the fact it is a split second operation, trimming to glide may take more than 1/4 second as an example, and while trimming to best glide your mufflers are going to be cooling off the whole time.

]
Click to expand...
Do them simultaneously. Pitch up with on hand, carb heat with the other. You can go through the entire engine failure procedure like that, and trim when you get a chance.
 
Tantalum said:
....but to further belabor the point, this just another thing that makes me shake my head with how painfully low tech our airplanes are
Click to expand...

It's what we get for flying airplanes built 40 and 50 and 60 years ago. If we were driving cars that age we'd also be dealing with hard starting and a bunch of other stuff like the need to replace ignition parts every 12,000 miles. And the carbs in those cars would ice up if it wasn't for the automatic, thermostatically-controlled carb heat system that kept the incoming air at around 70°F, and the carb base was heated by exhaust gases in the manifold beneath it. All of that cost considerable power, but in a car it didn't matter. Cars in the 1950s, and some '60s, still had manual chokes and the driver had to know how to use them. Automatic chokes were trouble enough when they showed up.

STC'd injection retrofits would be nice, but they'd cost as much as an old airplane. We're too cheap for that. Most of us can barely afford to fly the clunkers we do. New airplanes are out of the question. It's no wonder that the E-AB registrations rise every year.
 
Dan Thomas said:
It's no wonder that the E-AB registrations rise every year
Click to expand...
I think therein lies the ticket. Even a new million-ish dollar Cirrus / Bonanza / Mooney / Baron / Malibu is going to have the engine be the most low tech component that needs constant babying and fiddling while the threat of an engine failure, or at least a costly premature overhaul, is always looming

Granted, when the market is as small as it is there's no surprise that the growth is as slow as it is and the costs as high as they are
 
vegabond1 said:
I’m a student at 17 hrs and every morning we go up we need carb heat right at start up, and taxi, and usually run up... I find that chart humorous since most mornings we fly out it’s about -10f or so, that chart doesn’t show any ice below 10/10. We fly a 182P and carb heat is used more than prop control. We fly out of GUC which tends to be a rather cold spot, guess I’m in the minority, but I’m sure not afraid of the carb heat!
Click to expand...
I'm just speculating here, but I've had a rough running engine at sea level in -15F before in my 180 and the issue wasn't carb ice. It was running rough because I physically could not get the mixture rich enough for such dense air. The carb heat has the effect of enrichening your mixture because less dense air is entering the engine, and that is what smoothed out my engine. I believe this may be happening in your 182 as well.
 
Dan Thomas said:
And the carbs in those cars would ice up if it wasn't for the automatic, thermostatically-controlled carb heat system that kept the incoming air at around 70°F, and the carb base was heated by exhaust gases in the manifold beneath it. All of that cost considerable power,
Click to expand...
The motor to control the valve on the riser was vacuum operated so that when you put your foot to the floor, it would close due to the lack of vacuum and you would automatically get cold air.

And, vacuum operated windshield wipers - those were great - try to pass on a two lane in the rain - step on the gas, vacuum goes away, wipers stop, can't see ****...

My first car was from the 50's and had a manual choke - we've come a long way baby...
 
Mtns2Skies said:
I'm just speculating here, but I've had a rough running engine at sea level in -15F before in my 180 and the issue wasn't carb ice. It was running rough because I physically could not get the mixture rich enough for such dense air. The carb heat has the effect of enrichening your mixture because less dense air is entering the engine, and that is what smoothed out my engine. I believe this may be happening in your 182 as well.
Click to expand...
At 7000 feet MSL? Seems unlikely.
 
Mtns2Skies said:
Let's go ahead and reread my post.
Click to expand...
I wasn’t discounting your experience, I was doubting your theory for the other guy - that’s at 7000 feet.
 
Salty said:
I wasn’t discounting your experience, I was doubting your theory for the other guy - that’s at 7000 feet.
Click to expand...
Whoops I misunderstood. Yeah I didn't see that GUC was at 7000MSL.
 
Yup, some good elevation to start with! But at -20F this morning the da was around 4000
 
Mtns2Skies said:
I'm just speculating here, but I've had a rough running engine at sea level in -15F before in my 180 and the issue wasn't carb ice. It was running rough because I physically could not get the mixture rich enough for such dense air. The carb heat has the effect of enrichening your mixture because less dense air is entering the engine, and that is what smoothed out my engine. I believe this may be happening in your 182 as well.
Click to expand...
The carb's jets are sized to deliver enough fuel at such temps even at sea level. If the engine isn't getting enough fuel, the mixture control rigging is off, or the carb is incorrect.

Another possibility: Summer and winter gasolines have different Reid vapor pressures. Summer stuff is less volatile--it doesn't evaporate as quickly--so that vapor lock risk is reduced in hot weather. Winter fuel has a high vapor pressure so it will vaporize properly in the cold. If you have a tankful of summer stuff and go flying in cold weather you can get roughness just beccause the fuel isn't vaporizing well in the manifold and cylinder, so more fuel is required to support combustion. Carb heat vaporizes it and enriches it.
 
Dan Thomas said:
Here's an article outlining the skills needed to drive many years ago. The author suggests that we'd all be better drivers if we had to master old technology like this: https://driving.ca/auto-news/news/could-you-drive-your-grandparents-car-today
Click to expand...
Back in the early '70s, our CAP squadron obtained a couple of military surplus M38A1 Jeeps (same basic model as the CJ-5). These were built in the '50s.

I was put to work training my fellow cadets to drive them (I owned a '51 CJ-3A). The stick shift issue wasn't bad... still a lot of manual transmission cars back then. Manual choke was no big deal then, either.

What really foxed them was the starters were controlled by foot pedals. Not an issue...if the Jeep was sitting level. A bit more of an issue if the jeep was on a hill, since you had four pedals (throttle, brake, clutch, starter) and only two feet to work them.

Ron Wanttaja
 
Also in the 1970s, I had a 1953 Chevy whose starter was controlled by a button on the dashboard. One night, some poor sap tried to steal it, but he apparently couldn't figure out how to start it. He had disassembled the ignition switch in an apparent attempt to figure out how to engage the starter!
 
On the subject of carb ice ...

Don't let anyone tell you that you can't get carb ice in a Lycoming O-360 in Arizona at 70% cruise power. This morning I flew my C-172N/O-360-A4M from Goodyear to Yuma at 4,500' (44F OAT). About half an hour after takeoff, power was set to about 21.5"/2520 rpm (I have a manifold pressure gauge, though it's a fixed-pitch prop). I noticed rpm on the digital tach drifting down, as was the manifold pressure. Even the EGT was going down a bit.

I pulled carb heat. About thirty seconds later put it back in, and voilá, there was that missing 100 rpm and half an inch of manifold pressure.

It had rained last night, and the air was still soggy. There was a zero temp/dewpoint spread at GYR when I took off (it was clear, but ground fog was starting to gather on nearby ag fields), so conditions were right for it.
 
You must log in or register to reply here.
Back
Top