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The truth about Carb Icing?

RudiGreyling

Well Known Member
Hi Guys,

I need to poll the truth about carb icing and RV?s.

I need to start selecting an engine soon and are back and forth between Carbs and FI. I do not have the experience on Lyco?s and been reading a lot, but getting conflicting information. I have experience on Rotax 2 strokes engines and carb icing is not a really a big problem there, due to the way the carb operates, i.e. sliding piston, no butterfly valve to gunk up.

I read here on the forums that Carb icing is 100% pilot error and avoidable :confused: . I have also read in the recent kitplanes issue, in the article about accidents and safety, that Cessna has ditched carbs in all their line-ups primarily due to carb icing :confused: . I?m confused and do not have any experience to draw from so I open up the question. :rolleyes:

How real is carb icing on RV?s specifically and how avoidable is it?


Thank you, Kind Regards
Rudi

PS: I?m posting this under the 'RV General discussion' topic, since the Firewall Forward Section does not cater for Carbs, only for FI at this stage.
 
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Its not a big deal

Like all other things aviation, you understand it and manage it. In general, RVs are no more or less succeptable to carb ice than any other plane, except as the builder, you are responsible for ensuring that your carb heat setup is sufficient and as the pilot, properly used.

One thing to consider is that the standard Van's heat muff is not particularly good (not enough heat rise). The NTSB investigated one forced landing (RV-6?) with carb ice suspect and found the standard muff provided 'negliable' temperature rise. Vetterman and others have good alternatives.

Whatever setup you wind up with, be sure to measure the heat rise that the carb heat provides to be sure its working. Be sure to install the heat muff along the exhaust pipe before any cabin heat muffs. Carb temperature probes and guages are available and can provide good peace of mind.

Fuel injection eliminates the carb ice problem but costs more and has its own issues (more complex, hot starts etc) which are again easily managable when understood.

Regardless of the various merits of either system, I wouldn't let the spectre of carburator icing deter you from chosing to go that way.

There's lots of stuff on the web too.

Carb Icing Chart


John
 
I understand that the severity of carb icing really depends on a number of factors such as humidity, outside air temperature, model of engine... some being more prone to icing than others. Make sure the heat muff you use is effective (not Vans). Carb heat tempurature guages are useful. Pilots have been dealing with carb icing for decades and it's really no problem if understood and your carb heat is used effectively. There are lots of books on the subject.
Don't let Cessna's fuel injection plans throw you.... it's probably all liability driven. Full FADEC engine control will probably be next, training wheels after that.

Mike

RV7
Tail done - Just closing up wings.
 
Relax just be aware

RudiGreyling said:
I?m confused and do not have any experience to draw from so I open up the question. :rolleyes:

How real is carb icing on RV?s specifically and how avoidable is it?


Thank you, Kind Regards Rudi
It is VERY real and very avoidable. I suggest you Google carb ice and read all you can to brush up on the cause, conditions and defenses. Also check this forum, there are a few good threads on the topic. AOPA has a ice write up and there is a ton of info on the web. Here is one:

http://www.vansairforce.com/community/showthread.php?t=8702&highlight=carb

First if you have this level of attention to avoiding carb ice when you fly as your post indicates, you want to understand, than you chance of carb ice is NIL. Most carb ice accidents are from pure pilot complacency, lack of training or poor procedures or no clue.

It's true RV's are less likely to get carb ice because they have Lyc's which in general with their heated sump and warm carb are more resistant (NOT immune) to carb ice. Add the fact our cowls are tight and the exhaust pipes go aft near the carb, good for carb ice RESISTANCE, not good for vapor lock, but that is another issue. Note, I said resistance to carb ice. RV's VAN AND DO AND HAVE HAD CARB ICE incidencies. Engines like the Continental O-470 in the C-182 are very prone to Carb ice due to the fact the carb and induction tubes are not on the hot sump. The good news it is not common, but there are things you can do to make the chance of it happening to you basiclly zero.

The down side of RV's is they tend to have poor basic carb heat design. In part that is because there are so few carb ice accident with RV's, so even the poor stock carb heat has worked. That does not mean it is good. The original stock Van's carb heat is just open the filtered air box (FAB) into the cowl area, in theory to draw warm air from the cowl. That may be fine if you are in mild conditions and use it early or proactively as a preventive measure. Also later the optional "stove pipe" Van now sells is better, but still marginal and a small improvement in my opinion. Most have gone to a dedicated heat muff for carb heat. You can find them at Spruce Air or Wicks (see thread above). If I lived and only flew in SW Arizona I might not worry about it. However for a IFR bird or a plane I planned on doing X-C I would install a more substantial Carb heat muff than what Van offers in their catalog. It may be overkill?

Last line of defense is a CAT or Ice Man probe. Carb air temp (CAT) can improve you awareness of potential for carb ice. The "Ice Man" probe is a optical device that actually accumulates ice in the carb throat and gives a aural and visual warning. It does not measure temp it "sees" the ice. They tend to be very sensitive and can go off all the time, in even very mild conditons, but it will let you know early, way before ice accumulates on the carb throat or butter fly. Some find it annoying and others like it. There is CAB ICE where the engine chokes off and a light internal frost of the carb which comes and goes. The ICE MAN is super sensitive but the best device if ice detection. I am not recommending it, but do value the CAT gage. It does not tell you if you have carb ice but tells you if your carb air temp is in a critical range. What many RV'ers find is the air in the carb is often very warm for the reasons I mentioned at the beginning (RV's are carb ice resistant but not immune).


Carb ice is a rare problem, but can be deadly. If you're going to do idle descents from 8,000 feet, on a humid day with ambient temps in the critical range, expect to get carb ice. If idle descents are your thing, you may have a (bad) surprise someday. Even if you use Carb heat with Van's marginal capacity it may not be enough at idle power. I never use idle power for long descents and use carb heat if there's ANY DOUBT that carb ice conditions might be more likely. When I plan a decent I start down earlier and keep the power up (21" of map). You can put a carb ice chart on your panel that gives you temp verses relative humidity and carb ice severity. Here is a typical chart (click to enlarge):


I only got carb ice on my RV-4 on taxi out, that I was aware of. I say aware of because you would be surprised how "frosty" your carb gets sometimes. However there is a big differnce in engine choking ice build up on the carb venturi and butterfly and a light frosting. When I flew in the humid area Northwest, doing run-ups, I noticed after applying carb ice engine RPM picked up after 10-20 seconds, even with a constant speed prop since it was below the governor range. When the carb heat was turned off the RPM was higher than I started with. Hummmm. People don't leave the carb heat on long enough to do any good. Same in flight, KNOW WHAT TO LOOK FOR. You don't just pull the carb ice out and shove it back in. You pull it out and note the MAP drop (constant speed) or RPM drop (fixed prop) and WAIT. Look for either MAP or RPM to go back up. Same on the ground, look for the effect of carb heat just as you apply it, after it has been on for awhile and after you turn it off. You don't necessarily want to fly around with carb heat on all the time, especially take off. If you do fly with carb heat on continuously as a preventive measure, don't forget to re-lean, since you are now richer with the warm air. Also after you turn carb heat off, you may need to re-lean.

I used the Piper method of carb heat verses the Cessna method. The Cessna method called for carb heat ON for the entire approach and landing. The piper method is more of a check, turn it On than Off, when arriving on initial approach / pattern, i.e. check for carb ice affects. Depending on conditions (dry/hot) that could be just a quick "does it work" to a 2 minute leave it on the whole time (wet/30-70F). You have a OAT gage right? Well if you see water, like rain, mist, fog or standing water or snow on the ramp, that is visable mosture and high humidity.

Rudi, relax but from now on when you listen to ATIS and you hear the temp dew point spread is 0, 1, 2, 3 or even 5 to 10 degrees (F), consider carb ice. If you live in Phoenix, carb ice is not as likely as Seattle. Remember the idea is prevent it. If you get carb ice and the engine is choked off there is no more heat to melt it off after the fact. It's a real issue and I think it contributes to about 17 accidents a year, but this is totally avoidable with some thinking on the pilots part. Add a line in your check list, Got Carb ICE? :D Just remember keep your engine HOT with some power you will avoid all but the worst conditions. Also consider a carb temp gage. Bottom line the RV is NO MORE likely to get carb ice than any other carb'ed Cessna or Piper and may be even less; however if you get severe carb ice the RV may have less carb heat to get out of trouble than a Piper or Cessna.
 
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cytoxin said:
it is very real and probally happens more than we know. however on a lyc it is not as likely as some others because the carb is mounted to the sump and stays warm...it is still possible though. the piper doesnt mandate carb heat unless required ( whatever that means ) the cessna however reccomends any time below 1700 rpms. different engine designs.. one on the sump one not. i will be using a carbed clone engine and have a heat muff. i have heard vans heat deal is not very good.
Incorrect, some Cessna's that recommend carb heat (152, 172, 177) out of the green arc also have the carb mounted to the sump, the difference is in the induction system before it gets to the carb. Pipers generally have a longer induction system, which leads to the air getting heated more before it enters the carb than does a Cessna. The RV's general have a similer short system, and should definitely consider using heat when operating in low RPM ranges.

I personally consider Van's FAB a junky design, with the carb heat running through the filter first and such, that filter absorbs quite a bit of the heat that would normally go straight to the carb. I personally would only go with FI on any airplane I have the choice on, and FWD or rear facing FI at that.
 
osxuser said:
Incorrect, some Cessna's that recommend carb heat (152, 172, 177) out of the green arc also have the carb mounted to the sump, the difference is in the induction system before it gets to the carb. Pipers generally have a longer induction system, which leads to the air getting heated more before it enters the carb than does a Cessna. The RV's general have a similer short system, and should definitely consider using heat when operating in low RPM ranges.

Since I have not personally seen the carb setup on the Cessna aircraft you refer to, I'm just quoting the following:


http://www.sacskyranch.com/crbice.htm

Your Continental aircraft engine forms carburetor ice at a higher outside air temperature than your Lycoming engine since the carburetor on a Continental engine absorbs less engine heat. Continental mounts the carburetor to the intake pipes away from engine heat. Lycoming mounts the carburetor to the oil pan where it absorbs heat from the engine oil. One side effect of forming carburetor ice at a higher air temperature is that the warm air holds more moisture than cold air; therefore, you have a potential for greater carburetor icing with Continental engines than with Lycoming engines.

Personally, I figure the risk of leaving carb heat on, and resulting loss of power, during a high altitude mountain flying go-around, is greater than the chance of carb heat. I'll stick with the Piper type recommendation. Never liked those Cessna's anyway! :D

FWIW, this argument goes right along with what causes lift, and pitch or power for speed. No exact answers, and never will be.

And BTW, I've flown that exact airplane that was in the NTSB report regarding carb ice. It's also totally re-built. I don't think Van's used the newer 2" tubing connector that allows additional heated cowl air through the sides, in those days.

L.Adamson
 
Carb Heat

On my RV-4 I built my own exhaust muff that wrapped around both exhaust tubes that run in front of the sump (crossover exh system). Then a short piece of SCAT betwqeen the heat muff and the FAB alternate air inlet. Quite simple and light. I also mounted a Radio Shack indoor-outdoor thermometer in the cockpit with the external sensor in the airbox. During my 40hr testing period I recorded as high as 90?C in the airbox. I think FAR Pt23 requires around 70?C at 75% power. The thermometer has since failed, but I have confidence in the amount of heat available. I use it frequently in possible icing conditions. Don't let carb icing put you off a carb, its is easily manageable. If you go the Ellisonm TBI route, you should have carb heat also.

RV-4 VH-PIO
250hrs
 
Hi Guys,

thank you very much for your posts and being RV specific.

I have done my searching and googling before, but it was not RV specific. I have never had any instruction behind a lycoming yet...

Due to these posts it means that carb engines are not excluded from my engine choice.

Thank you, Kind Regards
Rudi
 
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Heat muff

Hi Rudy,
We have a 180 Lyc on the 6A and I see little or no RPM drop whenever I apply carb heat since we have Van's "stovepipe" heat system. If I were you, I'd definitely go with a heat muff on the other side exhaust for a more substantial heat rise in the carb throat. For little or nothing more, costwise, its a much better way to go.
Regards,
 
Alternate carb heat boxes

Rudi,

Check out this thread from last year.

George (gmcjetpilot), posted some really good links to alternate carb heat boxes.
 
Rudi:

After 40 years of flying without a single instance of carb ice (that I can recall), I started flying a Waco (Jacobs engine) and now an American Traveler (150 Lycoming). Carb ice has become a constant nag.

I know that it is controlable. But, there are better systems. I know of no advantage of a carb. Injected engines are proven and reliable.

So long as I can afford it, I'll put an injected system, and have at least one problem less to worry about.

Jim
 
Some advantages of a carb

jimpleasants said:
Rudi:

I know of no advantage of a carb. Jim

Advantages of a carb:
1. Less chance of vapor lock (No fuel lines running over hot cylinders)
2. No high-pressure, high-dollar fuel pump in your cockpit
3. Simple, with few moving parts -- extremely reliable
4. No microscopic orifices to clog
5. No fuel filter to clog
6. Starts easily when hot.
7. No complicated fuel return lines to run
8. Costs at least $1500 less
9. Continues running at extremely low fuel pressures
10. Less likely to start an engine fire (no tiny, rigid, high-pressure fuel lines to break).
11. Is never mentioned as contributing to engine/prop combo restrictions or RPM limitations.
12. Does not require tuning and adjustment while you're also trying to test fly your new airplane (most FI systems found in RV's are installed after-market).
13. Certificated

Disadvantages of a carb:
1. Carb ice, under extremely rare conditions (at least when we're talking about Lyc's installed in RV's).
2. Can't go negative G (but neither can the engine without an inverted oil system).
3. Can't go "lean of peak" (if that's a good thing)
4. Engine does not feel as smooth (as if a four-cylinder engine with pistons the size of coffee cans can ever be said to "feel smooth").
5. Depending on the engine and tuning, may produce slightly less horspower.
6. Can't be used with a FADEC system.
 
jonbakerok said:
Advantages of a carb:
1. Less chance of vapor lock (No fuel lines running over hot cylinders)
Wow, with the higher pressures involved with FI, I'd think there is a smaller chance of vapor lock. And a properly operating FI system won't vapor lock in normal curcumstances.
2. No high-pressure, high-dollar fuel pump in your cockpit
3. Simple, with few moving parts -- extremely reliable
4. No microscopic orifices to clog
5. No fuel filter to clog
6. Starts easily when hot.
7. No complicated fuel return lines to run
8. Costs at least $1500 less
9. Continues running at extremely low fuel pressures
10. Less likely to start an engine fire (no tiny, rigid, high-pressure fuel lines to break).
11. Is never mentioned as contributing to engine/prop combo restrictions or RPM limitations.
12. Does not require tuning and adjustment while you're also trying to test fly your new airplane (most FI systems found in RV's are installed after-market).
Our's has needed a bit of tuning in the time we've have it on the Cardinal.
13. Certificated
Many FI systems are as well
Disadvantages of a carb:
1. Carb ice, under extremely rare conditions (at least when we're talking about Lyc's installed in RV's).
Be that as it may...
2. Can't go negative G (but neither can the engine without an inverted oil system).

3. Can't go "lean of peak" (if that's a good thing)
it is
4. Engine does not feel as smooth (as if a four-cylinder engine with pistons the size of coffee cans can ever be said to "feel smooth").
5. Depending on the engine and tuning, may produce slightly less horspower.
6. Can't be used with a FADEC system.
Also add to the list of disadvantages, uneven fuel distribution to the cylinders, leading to uneven CHT's and EGT's, leading to the inability to run LOP reliably. And the chance of water collecting in the float bowl.
 
Carb heat on takeoff?

Just ran across this accident in which the NTSB seems to recommend carb heat on takeoff.

https://www.youtube.com/watch?v=IEikGJJhanA

http://www.kathrynsreport.com/2018/08/aeronca-7ac-champ-n65hm-fatal-accident_25.html

I've always tested to make sure carb heat is working during the pre-flight, but never knew that it should be used to really clear the carb before takeoff, particularly during a longish ground run at idle.

No idea if this problem would happen on a typical carbureted RV engine, but thought I'd flag it as I was not aware.
 
I'm installing an ARP carburetor ice detector on my RV-3B. I've got one on my Cessna, had it for years, and it definitely works fine. With it, carb ice is a non-issue.

When the light comes on, I pull the carb heat on till it goes off again.

My only connection to the product is being a thoroughly satisfied used.

Dave
 
I'm installing an ARP carburetor ice detector on my RV-3B. I've got one on my Cessna, had it for years, and it definitely works fine. With it, carb ice is a non-issue.

When the light comes on, I pull the carb heat on till it goes off again.

My only connection to the product is being a thoroughly satisfied used.

Dave

Hi Dave, have you seen carb ice during times when you didn't expect it? Traditionally it's only supposed to happen during descent to landing under "high" humidity conditions, not under full power. But it seems this accident was caused by idling before takeoff and then a sudden engine failure due to ice.
 
Traditionally it's only supposed to happen during descent to landing under "high" humidity conditions, not under full power. .
This is untrue. I used to own a 182 (a known ice maker) and saw carb ice during a climb in IMC, and low altitude cruise over the bay (both high humidity). In both cases there was an obvious decrease in power. The issue with low power descents is that the engine could be losing power without any obvious symptoms.
 
Keep an eye on your manifold pressure, if you haven't moved the throttle and the manifold pressure changes more than likely ice is forming if straight and level.
 
Hi Dave, have you seen carb ice during times when you didn't expect it? Traditionally it's only supposed to happen during descent to landing under "high" humidity conditions, not under full power. But it seems this accident was caused by idling before takeoff and then a sudden engine failure due to ice.

Oh, yes.... Once in a while, not often, in cruise (it's a Cessna, need I say more?) and at least once immediately after my run-up, which had been normal. That particular time I was directly downwind of a large lawn sprinkler. Interestingly, I've never gotten it in the pattern except that one time, and I hadn't actually made it to the runway yet then. I figure carb ice is where it is - and I can deal with it.

Caveat, this is a Continental O-470 in a Cessna, not a Lycoming O-320 in an RV. But I figured that with the device, I wouldn't be concerned about carb ice.

The "universal" style lets me locate the switch, knob and light where I choose, which is handy in my little RV-3B cockpit.

Dave
 
This is untrue. I used to own a 182 (a known ice maker) and saw carb ice during a climb in IMC, and low altitude cruise over the bay (both high humidity). In both cases there was an obvious decrease in power. The issue with low power descents is that the engine could be losing power without any obvious symptoms.
I agree - it seems like carb ice can happen at any time, which is why I raised the point. Not sure everyone is trained to watch out for it. I know I was not trained for this - but could be just me.
 
Installing a carb temp gauge in an RV is straightforward as it is dependent on the sensor fitting the carb --- same carb as is found in a Cessna. And you won't need some form 337 or other paperwork to do it, unlike on a certified plane. Put it in and make a note in your logbook(s).
 
In 30 years of flying I've gotten carb ice I think 3 times. That's in around a couple of thousand hours of flying behind a carburetor.

Of the 3 times I recall, once was in flight in a C152, once in flight in a C172, and once on taxi in a Taylorcraft. The Cessnas were both Lycoming, but I have to confess I don't remember what engine the T-craft had, I expect it was a Continental A-65.

Other that an odd flight here and there, I've been fortunate enough to get to fly pretty much nothing but new fuel injected Cessnas for the last 8 years and I like them, so it was never a question for me. I'm not afraid to fly a carburated airplane, but I'm going fuel injected in my RV7.
 
In 30 years of flying I've gotten carb ice I think 3 times. That's in around a couple of thousand hours of flying behind a carburetor.

Of the 3 times I recall, once was in flight in a C152, once in flight in a C172, and once on taxi in a Taylorcraft. The Cessnas were both Lycoming, but I have to confess I don't remember what engine the T-craft had, I expect it was a Continental A-65.

Other that an odd flight here and there, I've been fortunate enough to get to fly pretty much nothing but new fuel injected Cessnas for the last 8 years and I like them, so it was never a question for me. I'm not afraid to fly a carburated airplane, but I'm going fuel injected in my RV7.

That’s funny, in 20 years, I’ve experienced carb ice twice.. must be a “once every 10 year” thing. First time was in a Continental O-200 Cessna 150, full throttle climb out at night, high humidity summer night.. I felt the engine didn’t seem to be pulling as hard, I pulled carb heat during my climb, the engine almost quit while chugging the ice! A few moments later, it felt more normal, carb heat off and it roared to its 100hp life. This happened several times on climb out, and I resolved it with partial heat on climb out. The problem seemed to clear up at higher altitudes, I presume the humidity was on a lower altitude band. The other time was on a Lycoming O-235 in an Alarus CH-2000 aka CH2T. I thought (and still feel) that lycoming engines are almost immune to carb ice, but that Alarus would get ice all the time on the ground! I tossed it up to that particular design, as I’ve never seen a Lycoming have carb ice before or since.
 
Not reading all the posts but will offer my experience:

Carb heat can happen in a Lycoming. I've seen it a few times in my RV-6, with its O-320-E2D.

The stock "snorkel" carb heat tube is next to useless, in providing heat to the engine. Every RV i've been in that has one shows zero response on the RPM gauge when you turn on carb heat during a run-up, which suggests it's not doing all that much.

A "muff" style collector is needed to bring in enough warm air to make a difference. I replaced the stock snorkel with a Robbinswings muff and now get a clear drop in RPM during a run-up... Some days up to 50 rpm.

I have used carb heat on the ground to clear ice... But i've never had it in flight that i'm aware of. Having learned on a Continental I am always ready to turn it on if I get a cough in humid conditions though.
 
Carb ice Rv4 standard airbox with small tube and duct

Ive flown my RV4 all over in many weather conditions
I have picked up ice on taxi out ( probably the worst case) and picked it up in cruise. In each case closing the flapper cleared the ice. I was skeptical that that small tube would clear it but it worked everytime for me.

Use the standard detection method and pay attention when your are flying or taxiing.

Cm
 
Carb temp

O360 carburetor temperature is normally at 127 highest it will read. Outside temperature was -8 C, 13,500 VFR on top. Carburetor temperature would very a lot down to 96 add carburetor heat and it would go up a couple of degrees, but that’s all. No real change in engine performance. I have a Vetterman exhaust on a RV6A. Did I have carburetor ice?
 
.... Outside temperature was -8 C, 13,500 VFR on top. ... Did I have carburetor ice?
According to this diagram, unless the humidity at that altitude was higher than 80%, it is not likely that you had carb ice.

FAA Icing temperature humidity diagram.jpg
 
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