Carb Ice?

[poadeleted20]

What were they. I mustve missed them.

The main one is not bypassing the air filter.

So, you disagree with the use of carb heat as a preventive in a Piper. What about the POH language I quoted above? Are you now saying don't follow the POH?

Depends on which model, but many of them just say "as required."

Not one person on this thread supporting the use of carb heat as a preventive suggested doing something "for no other reason."

Actually, that was the original question -- doing it just because the OP had been taught to use it in a different type with a different recommendation.

 

[redtail]

I think what I'm getting is... Be flexible in my thinking and application of carb heat in the Piper "as required".
In other words, don't religiously use it as in the Cessna, but use it as a preventive measure when the conditions are conducive to carb ice. So on some days I'll be using it PROACTIVELY and others, REACTIVELY.

 

[hindsight2020]

Anybody care to comment on the potential for carb icing on departure while at takeoff power? Seems like the discussion usually centers around descents with the power back, but I've heard of c182s having problems dealing with carb ice on departure. I don't have a 182 poh handy but it seems that could be quite the coffin corner, since even the application of car. Heat while on climb out would further reduce power available, thus exacerbating the potential for a crash. I like to believe that scenario would be a result of icing buildup at idle on the ground, otherwise consideration would have to given to taking off with carb heat on, which I know it has never been a procedure in any poh I've seen and is actually heavily discouraged.

 

[Headspace]

Arguably you could say that if the recommendations lean towards turning the carb heat on more often, more people might forget and leave it on during takeoff, especially when practicing multiple takeoffs/landings. Which is worse than just having unfiltered air going to the engine.

 

[midlifeflyer]

The main one is not bypassing the air filter.

Hmm. I didn't realize that using carb heat in a Cessna did not bypass the air filter.

Depends on which model, but many of them just say "as required."

And if they do? You seem to be equating "as required" with waiting until icing occurs. I've never seen a formal definition that supports that viewpoint. "Conditions conducive" seems to fit the "as required" mantra just as well.

 

[midlifeflyer]

Arguably you could say that if the recommendations lean towards turning the carb heat on more often, more people might forget and leave it on during takeoff, especially when practicing multiple takeoffs/landings. Which is worse than just having unfiltered air going to the engine.

With all of 10s (maybe hunrdreds) of thousands of hours spent by people all over the world in Cessna trainers, I have never heard of that being a significant problem.

Although interestingly, the 1973 Cherokee 180 manual indicates the potential for lack of power in a go-around as the reason for its POH recommendation.

 

[midlifeflyer]

I think what I'm getting is... Be flexible in my thinking and application of carb heat in the Piper "as required".
In other words, don't religiously use it as in the Cessna, but use it as a preventive measure when the conditions are conducive to carb ice. So on some days I'll be using it PROACTIVELY and others, REACTIVELY.

I think what you are saying makes a lot of sense and fits very nicely with the "As Required" in the POH.

 

[EdFred]

Hmm. I didn't realize that using carb heat in a Cessna did not bypass the air filter.

It does in the Pipers.

 

[midlifeflyer]

It does in the Pipers.

Noted. I understand that. But my question was whether there was anything different in that respect between Pipers ans Cessnas that would support not using preventive carb heat in a Piper. Hence my limiting my question whether carb heat doesn't bypass the filter in a Cessna. I always thought the bypassing of the air filter was standard for all carb heat installations so that it can also be used in case of filter problems (like alternate air) as well are carb ice.

 

[Cap'n Jack]

Anybody care to comment on the potential for carb icing on departure while at takeoff power? Seems like the discussion usually centers around descents with the power back, but I've heard of c182s having problems dealing with carb ice on departure. I don't have a 182 poh handy but it seems that could be quite the coffin corner, since even the application of car. Heat while on climb out would further reduce power available, thus exacerbating the potential for a crash. I like to believe that scenario would be a result of icing buildup at idle on the ground, otherwise consideration would have to given to taking off with carb heat on, which I know it has never been a procedure in any poh I've seen and is actually heavily discouraged.

My understanding is that at high power, such as takeoff, the airflow is sufficiently high that icing is less of an issue- by the time the air cools enough to form ice, it is already inside the carburetor. Not to say icing can't happen during takeoff. I had the same question as a student pilot- and was answered the same as I have.

 

[poadeleted20]

Hmm. I didn't realize that using carb heat in a Cessna did not bypass the air filter.

I didn't say it did not. I said that one reason for not using it unnecessarily is to not bypass the filter unnecessarily.

And if they do? You seem to be equating "as required" with waiting until icing occurs. I've never seen a formal definition that supports that viewpoint.

I do not believe I said that at all. To me, "as required" means "when appropriate based on aircraft, conditions, and power setting.". It does not mean "any time you are less than cruise power in any carbureted aircraft."

"Conditions conducive" seems to fit the "as required" mantra just as well.

That's a reasonable statement.

 

[midlifeflyer]

I didn't say it did not. I said that one reason for not using it unnecessarily is to not bypass the filter unnecessarily.

Ok. I was undder the impression this was the context of the conversation:

OTHO, if someone comes up with a reason that makes the results of use of carb heat at low power settings in a Cherokee any different than in a Cessna 172 in terms of engine health and safety, that would be something to seriously consider.

And as Ed pointed out, there are valid reasons for not using carb heat when it isn't needed.

My mistake.

 

[EdFred]

I didn't say it did not. I said that one reason for not using it unnecessarily is to not bypass the filter unnecessarily.

Ok. I was undder the impression this was the context of the conversation:

My mistake.

I've read Ron's quote 3 times, and I still don't know what he's saying for sure. Quadruple negative = what?

 

[bluerooster]

Having flown a Cherokee for the first time, last week and getting checked out in....now I'm really confused

Do I follow the POH with regard to carb heat or the NTSB

Failure to follow which can end up with yer butt in a sling?

My guess would be the POH.

On another note; Our Cherokee 180, positively hates carb heat, unless it's needed, then it loves it. And Owners manual sez to not use it unless you need it. Kinda like th' early 182 manual saying something to the effect of, if you don't need full power, don't use it.

 

[moonshine]

Here's another question

I fly a Diamond 40FP that sports O-360-A4M engine (carb, vertical induction, non-filtered carb heat) and the carb heat comes off of a PowerFlow exhaust - very powerful carb heat. About 100 rpm drop on run-up.

The POH and checklists (here's the checklist - http://www.diamond-air.at/fileadmin...F_G1000/DA40_F_G1000_Checklist_Edit_15_A4.pdf ) say the following:
- check on run-up
- on for descent/approach
- on final - mixture rich, carb heat off
- after landing - carb heat off
in emergency
- rough engine - carb heat on
- windmill engine start - carb heat on
- powered engine start - carb heat on
- unintentional icing - carb heat on

All of the above is nice and dandy in theory, in practice any time I apply carb heat and not pull the mixture back significantly (oh, 1/2 to 2/3 back) the powerful blast of hot air enriches the mixture to the point of the engine starting to run somewhat rough.
Hence I either put it on and pull the mixture back when there's visible moisture, or just don't touch the thing.
Any other suggestions? I'm in central Florida if that helps.

 

[moonshine]

So the engine is the same as Archer II and III - and I believe those are the Pipers that aren't supposed to get any carb ice?
Somehow never had any experience whatsoever with the actual carb ice, except one very brief moment with a Bing carb Jabiru engine on a hot moist summer day, descending out of 8500'

 

[poadeleted20]

So the engine is the same as Archer II and III - and I believe those are the Pipers that aren't supposed to get any carb ice?

No such animal, other than the Pipers with fuel injection, and that doesn't include any of the Archers (at least not without a big STC).

 

[Dan Thomas]

Noted. I understand that. But my question was whether there was anything different in that respect between Pipers ans Cessnas that would support not using preventive carb heat in a Piper. Hence my limiting my question whether carb heat doesn't bypass the filter in a Cessna. I always thought the bypassing of the air filter was standard for all carb heat installations so that it can also be used in case of filter problems (like alternate air) as well are carb ice.

A Cessna's carb heat bypasses the filter. This is to allow continued engine operation if the filter should become plugged with snow or ice or whatever.

Besides, dirt is only a problem on the ground, unless you're flying in a dust storm, and few of us do that.

Boats almost never have air filters.

Dan

 

[Dan Thomas]

Anybody care to comment on the potential for carb icing on departure while at takeoff power? Seems like the discussion usually centers around descents with the power back, but I've heard of c182s having problems dealing with carb ice on departure. I don't have a 182 poh handy but it seems that could be quite the coffin corner, since even the application of car. Heat while on climb out would further reduce power available, thus exacerbating the potential for a crash. I like to believe that scenario would be a result of icing buildup at idle on the ground, otherwise consideration would have to given to taking off with carb heat on, which I know it has never been a procedure in any poh I've seen and is actually heavily discouraged.

Not many airplanes will ice up at full throttle. The pressure drop in the carb is at its minimum with WOT, so the temp drop is less. Ice on takeoff is often formed during a long taxi, not on the takeoff itself. It's one reason why a pilot needs to know the temp/dewpoint spread and what it will mean for that carburetor.

But that requires thinking.

Dan

 

[Cap'n Jack]

Not many airplanes will ice up at full throttle. The pressure drop in the carb is at its minimum with WOT, so the temp drop is less. Ice on takeoff is often formed during a long taxi, not on the takeoff itself. It's one reason why a pilot needs to know the temp/dewpoint spread and what it will mean for that carburetor.

But that requires thinking.

Dan

Could you please walk me through that?

I thought airflow through the carb is at it's highest at WOT, so the air goes through the highest pressure drop, causing the most adiabatic cooling. I'd also think there was higher fuel flow, causing the most evaporative cooling as well.

 

[MAKG1]

Could you please walk me through that?

I thought airflow through the carb is at it's highest at WOT, so the air goes through the highest pressure drop, causing the most adiabatic cooling. I'd also think there was higher fuel flow, causing the most evaporative cooling as well.

He's confusing manifold pressure with venturi pressure.

Venturi pressure drop is a function of airflow and is greatest at WOT. Manifold pressure drop ("vacuum") is near zero at WOT. If manifold pressure drop were a factor, it would affect fuel injected engines as well. They all have throttles, but don't have venturis.

The reason most engines don't idle at WOT has nothing to do with pressure drops. It's that the engine is making a lot more heat.

 

[hindsight2020]

He's confusing manifold pressure with venturi pressure.

Venturi pressure drop is a function of airflow and is greatest at WOT. Manifold pressure drop ("vacuum") is near zero at WOT. If manifold pressure drop were a factor, it would affect fuel injected engines as well. They all have throttles, but don't have venturis.

The reason most engines don't idle at WOT has nothing to do with pressure drops. It's that the engine is making a lot more heat.

So what you're saying is that the reason carburetors ice up at idle instead of WOT is not because of the greater venturi acceleration and cooling exhibited at WOT, which is theoretically MORE conducive to icing than the venturi acceleration exhibited at idle, but because the ambient heating of the engine compartment at high power settings yields a higher temperature around the carburetor components than is yielded at low power operation. Is that correct?

Otherwise, strictly from a venturi cooling point of view, WOT operation causes a greater drop in temperature at the carburetor than closed plate (low power) operation and thus would make WOT operation more prone to carb icing, which of course goes against our experience flying behind carbureted engines.

 

[midlifeflyer]

A Cessna's carb heat bypasses the filter. This is to allow continued engine operation if the filter should become plugged with snow or ice or whatever.

Of course it does.

 

[Dan Thomas]

He's confusing manifold pressure with venturi pressure.

Venturi pressure drop is a function of airflow and is greatest at WOT. Manifold pressure drop ("vacuum") is near zero at WOT. If manifold pressure drop were a factor, it would affect fuel injected engines as well. They all have throttles, but don't have venturis.

The reason most engines don't idle at WOT has nothing to do with pressure drops. It's that the engine is making a lot more heat.

Pressure drop, and the consequent temp drop, is due to acceleration of the airflow in two places: the venturi, and the restriction caused by a partly-open throttle plate. At low power settings this small gap creates a very large acceleration and temp drop, exacerbated by vaporizing fuel, and is therefore the first place ice will appear.

Dan

 

[N801BH]

A Cessna's carb heat bypasses the filter. This is to allow continued engine operation if the filter should become plugged with snow or ice or whatever.

Besides, dirt is only a problem on the ground, unless you're flying in a dust storm, and few of us do that.

Boats almost never have air filters.

Dan

Boats are required to have Flame Arresters... Those are usually more restrictive then normal air filters..

 

[Dan Thomas]

Boats are required to have Flame Arresters... Those are usually more restrictive then normal air filters..

Not nearly. I had an inboard with a Chev 283, and you could see between the arrester's plates. The only purpose of that thing is to absorb the heat from any backfire gases, thus quenching the flame. Outboard motors don't even have an arrester. Most have nothing or an intake muffler.

Dan