The truth about Carb Icing?

[RudiGreyling]

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

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.

[lucky333]

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

[lucky333]

Here's the NTSB report I alluded to.

http://ntsb.gov/ntsb/brief.asp?ev_id...05X00452&key=1

John

[spooky212]

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.

[gmcjetpilot]

Originally Posted by RudiGreyling

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

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/communit...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? 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.

[osxuser]

Originally Posted by cytoxin

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.

[L.Adamson]

Originally Posted by osxuser

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!

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

[osxuser]

The Cessna 152, 172 (M model and on) and 177 all use Lycomings...

[garnt.piper]

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

[RudiGreyling]

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