Should I lean mixture while using carb heat?

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Oracle427
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Re: Should I lean mixture while using carb heat?

Post by Oracle427 »

I think two procedures are being mixed together here.

The first is the emergency of loss of engine power and there is a flow of things to check. The second is leaning for cruise and using continuous carb heat in cruise. In an emegrency with carb icing, you aren't going to be leaning out immediately to try and regain lost power, you just want that engine to maintain power so that it can pump heated air into the intake to melt the ice away. As Esa noted, once the engine turns off due to icing, you are pretty much never getting it back.

Fuel, Air and Spark is what an engine needs to make power (assuming it is still in airworthy condition)
So if you start loosing power check the things that control fuel, air and spark.

Fuel: Fuel tank selection, mixture and carb heat
Air: Throttle
Spark: Magnetos

Generally one wants to ensure the fuel selector is on a good tank, mixture is set appropriately or even full rich and carb heat is on full, then open up the throttle and check those magnetos.

If the engine springs back to life, wait and observe and check everything, best not to change anything else right away until you understand what is going on. Of course fly the plane all the while, make a course for a field to make an emergency landing and set up glide airspeed if required.

---

Now, as for leaning out with carb heat. This is normally done if running partial carb heat continuously in conditions conducive to carb icing. In the T6 it is important to avoid running the intake air in the red range as that can result in detonation. As previously noted, the heated intake air is less dense and it is therefore necessary to lean out the mixture to maintain a good fuel/air ratio for good economy and power. The power loss when running carb heat on some aircraft is significant, but not an emergency where one must immediately lean to maintain flight. You just don't want to take and takeoff or steeply climb in mountainous terrain without accounting for this as you will not be obtaining maximum performance from the machine.
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MkIV Hvd
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Re: Should I lean mixture while using carb heat?

Post by MkIV Hvd »

Good discussion...in my opinion, you should never lose the engine due to carb ice if you are paying attention. Sometimes it will accumulate at a greater rate than others, but there's always lots of warning in the form of falling manifold pressure before you'd actually have an engine failure. This engine is extremely reliable and predictable and a decrease in manifold pressure for any reason other than you having moved the throttle is a reason to suspect carb ice, especially while flying in cloud or any form of visible moisture. The procedure I learned was to select carb heat full hot until the ice clears, indicated by the the manifold pressure recovering to slightly less than it was before the event, at which point you can adjust carb heat to run in the green range on the temp gauge and continue to lean as you normally would.

I never really flew the airplane in what would considered carb ice inducing conditions in the real world, but the only time I ever used it for real very much got my attention!! It happened while taking off special VFR on a cool, humid, low overcast day and the manifold pressure started slowly decreasing for no good reason (throttle -tension correct and not creeping) just after the gear came up... :shock: Full carb heat solved the problem within a few (very long...) seconds, restoring the ability to breathe to the R-1340 and myself at the same time...
Last edited by MkIV Hvd on 26 May 2019, 18:34, edited 1 time in total.
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Re: Should I lean mixture while using carb heat?

Post by KarelPatch »

Sometimes things are pretty straightforward. Trust me, my carburetor will never freeze again. :roll:

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Re: Should I lean mixture while using carb heat?

Post by Oracle427 »

Never say never. :D
There is a 182 I fly that will sometimes ice up pretty badly in the time it takes to start, taxi to the active and begin run up. That’s just the way it is. Know your machine, know the environment.
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MkIV Hvd
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Re: Should I lean mixture while using carb heat?

Post by MkIV Hvd »

KarelPatch wrote: 26 May 2019, 15:14 Sometimes things are pretty straightforward. Trust me, my carburetor will never freeze again. :roll:
I'm sure!! :lol:
It's all part of a learning process...if you don't die during the process, its'a all good!
Oracle427 wrote: 26 May 2019, 15:41 Never say never. :D
There is a 182 I fly that will sometimes ice up pretty badly in the time it takes to start, taxi to the active and begin run up. That’s just the way it is. Know your machine, know the environment.
Two thumbs...exactly!!
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Re: Should I lean mixture while using carb heat?

Post by FlyingsCool »

I think a point is being missed here.

The original question was: Should the mixture be leaned in the event of carb ice? The thought being, well, we are often told that using full rich mixture cools the engine to some degree, and as we lean the mixture, the cylinder temperature will rise.

The issue that I see with this was mentioned above, but, from my interpretation given there was much discussion after that was said the advice seems to have been ignored. What was said is this:
Carb ice occurs in the carburetor and has nothing to do with the air/gas mixture. It is accumulated moisture in the carb throat that freezes under certain conditions and restricts the flow of air (and gas) through the carburetor, effectively choking the engine.

On the other hand, leaning the mixture has a tendency to increase the temperature of the Cylinders. But increasing the temperature of the cylinders is not going to help heat up the carburetor. Not only that, leaning the mixture further is likely the last thing you want to do as the carb ice is already leaning the flow of gas and air for you, you want as much gas and air as you can get as you are losing power.

I have not completely worked out the physics on this, and there may be an effect I'm ignoring having to do with pressure and temperature and gas mixture, but it seems to me the answer is... no... you should not lean the mixture further than it was at the altitude you are at.

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Re: Should I lean mixture while using carb heat?

Post by Oracle427 »

Well carb heat takes very hot air and forces it directly into the carburetor. I've seen carb throat temps of around 135F with full carb heat at low power and 65F at higher power. This is RW on on O-470.

The effect of that very hot air is immediate melting of ice, as in right now! :)

I have seen the effect of leaning on carb throat temp as cylinder heads temps come up over the course of around 10 minutes. Usually they go up by a degree, maybe two with the cowl flaps fully closed. I suspect the hot air in the cowling heats the carb body and some of that heats up the throat. The throat temp will usually remain will below 32F all throughout. No Ice is going to melt here.

Bottom line is the engine will be very dead if the carb heat isn't applied very quickly after detecting carb icing.
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Re: Should I lean mixture while using carb heat?

Post by FlyingsCool »

Well, yes,... in fact, if you think about it, adding carb heat would automatically lean the mixture, as the air would expand and effectively become "more air" as it heated up, more so than the fuel would expand... so, again, no, I wouldn't lean the mixture when I added carb heat.

(The original question asked if the pilot should lean the mixture when they added carb heat. There was never any question about whether or not carb heat should be added when the effects of carb ice appeared).

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AKar
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Re: Should I lean mixture while using carb heat?

Post by AKar »

FlyingsCool wrote: 30 May 2019, 22:44 Well, yes,... in fact, if you think about it, adding carb heat would automatically lean the mixture, as the air would expand and effectively become "more air" as it heated up, more so than the fuel would expand... so, again, no, I wouldn't lean the mixture when I added carb heat.
You've got it wrong way around.

As you heat the air, its density goes down. Mixture is the ratio of air and fuel masses. Carburetor applies the amount of fuel approximately in proportion to the air volume, unfortunately, and that essentially remains constant. Therefore the net effect of applying carburetor heat is less air, but more or less the same amount of fuel. So the mixture goes rich. That's the idea behind the question whether one should lean the mixture while using carburetor heat, not any secondary effect on CHTs.

(This is also the point of having intercoolers in turbo-/supercharged engines.)

-Esa

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Re: Should I lean mixture while using carb heat?

Post by FlyingsCool »

AKar wrote: 31 May 2019, 03:33
FlyingsCool wrote: 30 May 2019, 22:44 Well, yes,... in fact, if you think about it, adding carb heat would automatically lean the mixture, as the air would expand and effectively become "more air" as it heated up, more so than the fuel would expand... so, again, no, I wouldn't lean the mixture when I added carb heat.
You've got it wrong way around.

As you heat the air, its density goes down. Mixture is the ratio of air and fuel masses. Carburetor applies the amount of fuel approximately in proportion to the air volume, unfortunately, and that essentially remains constant. Therefore the net effect of applying carburetor heat is less air, but more or less the same amount of fuel. So the mixture goes rich. That's the idea behind the question whether one should lean the mixture while using carburetor heat, not any secondary effect on CHTs.

(This is also the point of having intercoolers in turbo-/supercharged engines.)

-Esa
Ahh, you're right, ok, that makes sense to me. He did talk about the cooling effect of higher fuel/air mixture in the cylinders, but your explanation makes sense to me. I had forgotten carb heat heats the air before it enters the carburetor, not in the carburetor.

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