Carb Heat during Cruise

[James331]

One should be able to tell the difference between dust and moisture in flight.

What’s dust have to do with flying in saturated air?

Take another look at what you said and what I said.

 

[Dan Thomas]

What’s dust have to do with flying in saturated air?

Take another look at what you said and what I said.

I said an air cleaner does nothing at altitude if there's no dust in the air. It was a response to the concern that carb heat delivers unfiltered air, so its use has to be limited. What's your deal with the IMC?

 

[Skip Miller]

@Dan Thomas I wasn't aware of that. Does the Continental O-300 also route the carburetor output air flow through the oil in the crankcase as the Lycoming does? In the Lyc, I would speculate that this fuel/air path also heats up the fuel/air considerably.

For those interested, below are line drawings of two engines.

Left side: Lycoming 320/360

Right side: Continental O-200 (? I think)

You can see that the Conti oil sump is not in close contact with the carburetor. -Skip

 

[James331]

I said an air cleaner does nothing at altitude if there's no dust in the air. It was a response to the concern that carb heat delivers unfiltered air, so its use has to be limited. What's your deal with the IMC?

You were talking about pilots needing to check weather (which I’m sure most here had no idea about) and talking about dew point spread before flying a carbed plane, I asked what about flying IMC in a carbed plane

 

[Skip Miller]

For those interested here is a picture of the oil sump of a Lycoming O-320 showing the piping which carries the fuel/air mixture from the carburetor (center) to the cylinder intakes. Fill this with hot oil and the fuel/air mix will gain some warmth. -Skip

 

[Kenny Phillips]

I was told in no uncertain terms by all the instructors I've ever had to either have a carb heat fully on or fully off never in between
the logic being that you'll heat the air up just enough that ice will form further downstream and you will be screwed...

If you've ever seen that part of an engine you'd see it's unlikely that you could consume enough ice to have a problem downstream of the carb; it would take quite a massive amount, as there may be a plenum and four or six spigots. The carb is the issue; at cruise, especially down low, there's a much smaller gap to fill.

 

[Tantalum]

If you've ever seen that part of an engine you'd see it's unlikely that you could consume enough ice to have a problem downstream of the carb; it would take quite a massive amount, as there may be a plenum and four or six spigots. The carb is the issue; at cruise, especially down low, there's a much smaller gap to fill.

Thanks, and I don't doubt that's true, but if a 100 or 200 RPM difference was enough to impact the safety of flight then I think there are other issues at play, plus I've always been able to recover some of the RPM loss by leaning.. for example if Weekend Warrior up thread is unable to climb with carb heat on then that needs to be looked at

 

[midlifeflyer]

Before landing checklists: engage carb heat

After landing checklist: close carb heat

Every single carbed plane is the same that I’ve seen.

At what point during landing do you close it? Just asking because you have much more experience than
me. When do you close it? I can see the potential advantage on a go around.

Guess you've never been in a Cherokee in which the checklists say car heat only when needed, not as a low power preventive (although there are arguments about that).

To answer the when question, typically on short final. Too little time to develop icing, not a lot of airflow, and all set to go for the go around. It is a technique which some also use for cowl flaps,

 

[midlifeflyer]

@Dan Thomas I wasn't aware of that. Does the Continental O-300 also route the carburetor output air flow through the oil in the crankcase as the Lycoming does? In the Lyc, I would speculate that this fuel/air path also heats up the fuel/air considerably.

For those interested, below are line drawings of two engines.

Left side: Lycoming 320/360

Right side: Continental O-200 (? I think)

You can see that the Conti oil sump is not in close contact with the carburetor. -Skip

True, but the Piper recommendation is not about that. It is about the location of the carb heat control.

 

[Stewartb]

For those interested here is a picture of the oil sump of a Lycoming O-320 showing the piping which carries the fuel/air mixture from the carburetor (center) to the cylinder intakes. Fill this with hot oil and the fuel/air mix will gain some warmth. -Skip

The carb is external to that and carb ice happens in the carb. Heating the intake air upstream of the carb doesn't serve any advantage and in fact robs power, which is why cold air induction is so popular with the exp guys, including me.

 

[Dan Thomas]

@Dan Thomas I wasn't aware of that. Does the Continental O-300 also route the carburetor output air flow through the oil in the crankcase as the Lycoming does? In the Lyc, I would speculate that this fuel/air path also heats up the fuel/air considerably.

For those interested, below are line drawings of two engines.

Left side: Lycoming 320/360

Right side: Continental O-200 (? I think)

You can see that the Conti oil sump is not in close contact with the carburetor. -Skip

Your pictures are correct. Most of the small Continentals are as shown, but the O-300 looks like this from the underside:

That round hole in the sump, surrounded by the four studs, is where the carb is attached.

The air spends very little time in the passage through the sump and isn't going to pick up much heat. The Lycoming's passages within the sump are longer and the air might get a bit warmer. Warm air will reduce the amount of oxygen reaching the cylinder, but it also helps vaporize the fuel, which, as it evaporates, cools that air a lot. On very cold days (Canadian cold) you can get an engine to run much better with some carb heat. Gasoline is like any other liquid: it evaporates quicker at higher temps. Liquid fuel in the cylinder (droplets) is a big waste.

 

[Dan Thomas]

You were talking about pilots needing to check weather (which I’m sure most here had no idea about)...

§91.103 Preflight action.
Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include—

(a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts.......

Canada:

602.72 The pilot-in-command of an aircraft shall, before commencing a flight, be familiar with the available weather information that is appropriate to the intended flight.

....and talking about dew point spread before flying a carbed plane, I asked what about flying IMC in a carbed plane

IMC means no temp/dewpoint spread, so carb ice is extrememly possible. Might need carb heat all the time in some airplanes. If the temp is very cold (below -20°C) the moisture is frozen and won't ice up; using carb heat in that case can melt it and cause icing in the venturi.

 

[James331]

§91.103 Preflight action.
Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include—

(a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts.......

Canada:

602.72 The pilot-in-command of an aircraft shall, before commencing a flight, be familiar with the available weather information that is appropriate to the intended flight.



IMC means no temp/dewpoint spread, so carb ice is extrememly possible. Might need carb heat all the time in some airplanes. If the temp is very cold (below -20°C) the moisture is frozen and won't ice up; using carb heat in that case can melt it and cause icing in the venturi.

Did you think people on this site don’t know they have to check weather?

But legit carb ice is nearly always possible, so check it once in a while, during cruise I’d pull it even if I didn’t think I was getting any ice to check, no ofcourse extreme cold is another matter and the line of FIKI also starts coming in around there at times too obviously.