<HTML>Someone help me understand this. I've been told a carb temp guage would solve my icing problems. The device monitors the temperature of the air passing through the venturi into the carb and heat can thus be used before the temp drops below freezing. My question: What if the ambient (outside) temperature is already below freezing? Wouldn't a below freezing temperature in the carburter be normal? What good would the device be then?</HTML>
Re: carb ice
You are correct in a way,however carb ice is proportional to the Humidity in the air,Ive experienced Carb Ice quite often in the Summer,as opposed to winter time when it almost never happens.The prime purpose of a carb-temp guage,is so you can monitor the carb throat temp,and apply carb heat to ward off any formations of Ice in the venturi.The guage on 26G has a yellow arc which identifies the temp range at which carb ice is likely,and since its installation,ive not had another bout of Ice(through the diligent use of carb heat to keep the needle out of the yellow range).Since my plane has been REALLY susceptable to carb Ice in the past,The guage was worth EVERY penny.....</HTML>
Re: carb ice
<HTML>I personally disagree with the amount of value placed in Carb Temp guages. Firstly, a carb temp guage purports to display when temp conditions in the throat of the carb is likely to support the formation of ice. So what!
It doesn't tell you that you have ice. It doesn't tell you that you are developing ice.
Think of how all this works.
Air enters the carburetor. Fuel is atomized by the carburetor, which takes away latent heat from the carb. If moisture is sufficiently present, it MAY freeze to the throat of the carb thereby reducing the effective size of the throat/venturi area causing a loss of engine power. The engine power is usually detected (in fixed pitch prop aircraft) by a reduction of rpm. That symptom is always late in being detected because the engine power loss has already become substantial in order for the symptom to manifest itself noticeably.
What's wrong with this so far? Well, in order to keep the carb temp sufficiently high to avoid ice formation, you must use carb heat, which in itself robs you of engine power by causing a loss of "ram-recovery" in many airplanes, and creating a slight induction vacuum in ALL airplanes. It also causes an abnormally rich mixture which also steals power due to a loss of fuel-air ratio efficiency.
Also, a carb temp in the range that indicates a possibility of ice formation, doesn't guarrantee that other conditions will allow ice to form. For example, just as you said, if the outside air temp was substantially below freezing already, then any available moisture would ALSO be already frozen. Application of carb heat to raise carb temp could actually thaw the frozen moisture to liquid state, and then allow it to re-freeze inside the carb.
To give the short answer to this problem, if one wanted a single-guage answer to the problem,
I'd recommend instead of a carb temp guage, a manifold-pressure guage if one isn't already installed, and close monitoring of the MP guage if one is installed. If a power setting of say, 23 inches MP is set, then any reduction in that MP whatsoever would indicate a need to apply full carb heat momentarily to check for the possiblility of ice and to rid the carb of any ice if it were indeed present.
Additionally, a MP guage is useful for other purposes in flight. I once had a rough running engine at cruise, and I noticed that the MP was "vibrating" about 2 inches Hg, which indicated a stuck intake valve. Since I had just flown into heavy rain, I surmised that sudden cooling had shrunk the valve guide sufficiently to cause a valve to stick. (It was an almost new engine). Instead of making an emergency landing, I navigated out of the rain, and in only few moments the engine recovered completely and the MP was restored normally. When I got to the home base, I complained to the engine shop, who later confirmed that the valve-guides had been improperly reamed during assembly to the wrong overly-tight dimension. They corrected the problem and that engine never gave us any more trouble. So, in that case, the MP guage allowed a better diagnosis of an inflight problem.
There are other uses for the MP as well, i.e., detection of induction leaks, etc.
Hope this answer gives you some ideas and is found helpful.</HTML>
Re: carb ice
<HTML>Thanks both of you for the valuable input. Another question is raised: How accurate is the outside thermometer on my 150? Is it affected by wind chill? Summary:If OS reads say 40 and the carb temp begins to register below that and continues to drop, then I have an idication. If on the other hand, OS=25 then a low carb temperature is to be expected and moisture is already crystalized in air. It seems to me accurate reading of OS is essential. Also, MP is a sensitive measure of engine performance. Southeast OK is almost tropical this time of year so these concerns are very real, And what is going on when the engine surges during these episodes? The most recent episode which I belived to be icing I was just moving over the top of a cloud layer at 7500 and lean. The surging lasted down to 2500 before smoothing out.</HTML>
Re: carb ice
Being a somewhat low time pilot,Im going to leave most of youre question to George( I value his opinion,he knows his stuff),I suggest that you compare youre factory OAT guage with a known accurate device.The one in 26G is 31 years old,and while it does seem to be "close" I dont count on it to be right on the bucks.......My first loggable time was in 1974,and I finally completed the Private ticket in December of 2000...(got married,had kids,divorced, bought house etc)...but in all the various planes I flew during that time,Carb Ice wasnt ever a factor until we bought 26G.150s are supposedly more suceptable to Carb Ice that any other Aircraft(im told),however I know three other 150 owners,and they have relatively few problems with Ice.
Since we bought 5926G in such a deteriortated state,and had to do so much work on it to get it flying,I have a hunch that physical carachteristics of the specific Aircraft may play a part also....</HTML>
Re: carb ice
<HTML>OAT guages are only approximations in most aircraft. If you truly desire accuracy, consider a digital guage (such as one from DavTron. They even have one that alternately will display aircraft voltage, ...a very good thing in my opinion.)
Consider this little known fact,...airplanes less than 7,000 lbs are not even required to have OAT guages when they are certified! It's an optional peice of equipment, ....so how much money do you think your mfr. spent to include it? Despite the extravagant costs to replace these cheaply made little devices, they are usually no more than within 5 degrees of accurate.
Also, keep in mind that OAT has very little to do with carb ice unless it's well below freezing. Carb ice can occur at any time, and usually unexpectedly, anytime moisture is present in the atmosphere. (i.e., almost always, unless you live on the other side of the arctic circle).
So, unless you're willing to give up a little engine performance continuously by using almost continuous carb heat to keep a carb temp guage in the green, ...I'd again suggest to install/use a manifold pressure guage, which will truly tell you when the ice is actually being formed (by displaying a gradually falling manifold pressure.) Blast it with full carb heat, watch the MP return to the previously set level, turn the carb heat off, (change altitude if it just repeats over and over again), and enjoy full power and best fuel economy and unfouled spark plugs. My 2 cents.</HTML>
Re: carb ice
<HTML>Oh, yeah, your other question, re: surging.
Although carb ice may have been what you were experiencing, (I wasn't there to observe exactly what you were seeing) , you might be interested to know that a certain amount of surging occurs whenever any propeller driven engine is run through clouds that are heavily moisture-laden.
I've seen this become especially noticeable in turbine equipment such as King Airs (and other PT-6 powered aircraft with torque-meters). The widely-varying differences in densities of the moisture-laden clouds that occur even from cloud-to-cloud will impart varying loads upon the propeller. A particularly sensitive tachometer or torque-meter will display this as large power changes, and under extreme conditions, the pilot may notice the engine power fluctuations.
The safe thing to do, and always the first thing to do, with carb equipped engines, is to apply full carb heat, so that at least you know what an ice-free carb runs/sounds like.
Re: carb ice
I have been reading your questions and comments about carbuerator icing with much interest. As an A&P mechanic perhaps I might help to shed a little light upon the subject of carbuerator icing.
First, some physics. The carbuerator, as mentioned by one respondant, primarily combines an atomized mixture of fuel and air for induction into the intake plenum. It accomplishes this in much the same fashion that a wing produced lift, that is via the Bernouli Principle. When a fluid is accelerated through the venturi its velocity is increased and its pressure is decreased. This pressure differential causes the fuel to be drawn from the carbuerator float bowl into the airstream. One of the byproducts of this exchange of energy is the extraction of latent heat from the airstream. That loss in temperature is approximately 30° Farenheit.
The atmospheric conditions most likely to induce carbuerator icing are an ambient air temperature of 59° to 63° Farenheit with a relative humidity of 50% to 60% or greater.
In the winter the air is usually, but not always, drier than the summer. This means two things. First, the relative humidity is insufficient to produce icing conditions. Second, that moisture entrained in the air will most likely pass through the venturi of the carbuerator as ice crystals and as such will not adhere to the surface of the venturi.
If I had to choose a single instrument to warn me of incipient icing of my aircraft's carbuerator I would most likely prefer an accurate OAT sensor. That's only personal preference, though.
The one bit of advise I had hoped to see, but did not, is this:
Whenever carbuerator icing is suspected the pilot should apply maximum carbuerator heat, and leave it on, until the pilot is certain that the threat of icing no longer exists.
If partial carbuerator heat is applied or if carbuerator heat is withdrawn the pilot risks either a recurrance of carbuerator icing, or having their carbuerator choked completely by a large mass of ice that is now impossible to remove. That means an off-field landing looms in someones future, along with the attentions of the FAA.
Thank you for reading my little missive.
Safe and happy flying. Matt the A&P</HTML>
Re: carb ice
<HTML>Hi, Matt! Re: your comment that you didn't see anyone recommend FULL carb heat, ...
I'll refer you to my first response, "apply full carb heat momentarily to check for the possiblility of ice and to rid the carb of any ice if it were indeed present."...
And also my next response, "Blast it with full carb heat,"...
And also my last response, "The safe thing to do, and always the first thing to do, with carb equipped engines, is to apply full carb heat"...
Which one of those did you miss? <grin>
Also, the exchange of energy from a high pressure zone to a low pressure zone is not the reason for the loss of latent heat in the body of the carburetor,...It's the evaporation of fuel from liquid to gaseous state that requires heat and is the source of escape of latent heat into the vaporized fuel, leaving behind a chilled carburetor. (Exactly the same principle in vapor-cycle air conditioners when liquid freon is sent by a compressor through an expansion valve, i.e., venturi, and changed into gaseous freon,...resulting in a loss of tempurature.)
It is also incorrect to state that there is a particular range of OAT likely to result in carb ice-forming temps inside a carburetor. The tempurature level of the carb body is most affected by the AMOUNT of fuel being vaporized in terms of specific gravity, i.e., weight. This will vary with the type of fuel, 87 ROM autogas, 87 Oct Avgas, 100LL Avgas, and 100/115 Oct Avgas all have different capacity to absorb latent heat because they all have different vapor-pressure levels and different amounts of aromatics in their composition. (For example, 100LL has Toluene in it, 100 Oct Leaded Avgas does not.) The point is, there is no method available to the pilot in the cockpit to accurately determine whether or not ice formation is actually occuring until he either observes an actual loss of some engine parameter (RPM or Man. Press.). Manifold Pressure guages will depict a loss of induction system efficiency much sooner than the tachometer will. That's why I believe it to be more useful than any other guage in detecting actual carb ice formation.
The actual amounts of moisture in the atmosphere also has very little to do with whether or not carb ice will form (as long as there actually IS moisture present). West Texas desert aircraft suffer from carb ice just as frequently as Gulf Coast aircraft do. The only things necessary for the ice is a cold carb, and humidity in the induciton air,...any amounts will do under the right conditions.
You are correct to say that some frozen ice crystals will be sucked on into the combustion chambers and on through the engine. Those are completely harmless, of course, and there is no indication whatsoever of that activity. But the ice that sticks to the inside throat of the carb and induction plumbing will block the flow of intake air, and causes a loss of induction air-pressure and guess what? That is measured by a particular type of guage in airplanes. It's called a Manifold Pressure guage. Not a carburetor tempurature guage.</HTML>
Re: carb ice
<HTML>Just to give a bit of adjustment to my previous post,... It was not my intent to lecture Matt. He has an excellent message that he posted, and unfortunately this new forum did not accept the "grin" I sent along with my sarcasm. Apparently this format does not accept that particular punctuation, and I was actually attempting to gently tease, ...not to appear as critical as my msg came accross.
So,....forgive me Matt. Thank you.
Next, I'd like to attempt to explain a little aviation history to some of my fellow Cessna enthusiasts, which might help explain the entire "Carb Temp Guage" issue.
Carburetor Temperature Guages were born out of the era of Turbo-Compound radial engines. The pilots of those days all understood carb ice, and almost ALL of them relied upon Manifold Pressure guages to give the earliest warnings of carb ice accumulation. The problem arose when carb-equipped engines became super-charged for high-altitude use. This first presented a problem due to the fact that the more traditional methods of carb ice detections no longer worked.
Prior to superchargers there was little or no loss of RPM, due to the use of Contant-Speed propellers. When the engine slowed down due to carb ice, the props simply flattened pitch and recovered the lost RPM, so,....no indication. Until the Manifold Pressure failed sufficiently that the engine either quit, or began to run rough, (unless the pilot or flight engineer happened to notice the Manifold Pressure losses). So,...the manifold pressure guage was THE best indicator of the formation of carb ice.
At least until airplanes began to be supercharged. Then the manifold pressure would fall a little, and the supercharger waste-gate would automatically close off and provide boost to the induction system, which in turn would mask the loss of efficiency because the MP guage showed the supercharger-maintained manifold pressure. A little more carb ice would build up, the supercharger would make up for it again, and so on, and so on, until....bang. It quit. Hmmmmmn.
So the aeronautical engineer types came up with the idea of providing a flight crew with a temp guage to warn them of the presence of conditions conducive to the formation of carb ice. When those carb/induction temps got into that range which allowed ice to form, they simply turned on the induction heat (carb heat) and kept the carb warm enough to avoid the ice. There was little or no penalty for this because the loss of ram-recovery was a non issue in a supercharged engine, ....the supercharger simply added a little more boost to make up for it. And the increased richness of the fuel/air mixture? No problem. The sophisticated carbs on those engines had an auto-lean feature, which automatically re-leaned the fuel/air mixture to the correct proportions, and the flight just simply droned on.
So, in those aircraft, with those sophisticated systems, ....a carb temp guage made sense. (It also helped that many of those aircraft were large transport-category aircraft with dedicated flight-engineer positions which contantly guarded against engine vagaries.)
But in a simple, normally aspirated, carbureted, engine in a light plane? NO.
Flying along with partial heat to keep a simple carburetor within a narrow temperature range to avoid possible carb ice is a loss of induction efficiency and fuel loss that light planes do not afford very well. I still believe the best method of avoiding dangerous levels of carb ice is to keep accurate tabs on manifold pressure. When it falls off for no apparent reason, ...then apply full carb heat to remove possible ice. When the engine performance recovers, remove the carb heat and keep an eye on the Manifold Pressure. If this occurs over, and over again,....then change altitude sufficiently to make a substantial change in temperature to mess up the atmospheric conditions which have given you the ice.
Also, always recognize that in some especially serious conditions, it may be possible to have carb ice form continuously. If you get in that situation, (or if changing altitude isn't possible or desireable for other reasons), then pull full carb heat on, leave it on, and re-lean the engine for the new conditions.
Hope this helps. Happy Aviatin'.</HTML>
Re: carb ice
<HTML>Interesting point about physical characteristics. My coweling is obviously not the original - just a smidgen large and doesn't fit exactly right in fron - yet no mechanic (I've had considerable work done in two years) has mentioned this except one who said there was something "funny" about the coweling. It has been suggested that I have someone machine it down to size before painting.</HTML>
Re: carb ice
I know in my case,the engine mounts are original(30 years old),and there is a certain amount of difference in looking at the Air Filter box,kinda like the engine is cocked(more air space on the RH side than the left.Ive noticed other 150s at OLV,and theyre relatively square.I suspect that this is caused by sagging mounts.I also wonder(and im sure George will have a suggestion on this) if the ram-air slipping between the Air Filter,and Cowling would have any adverse effects(such as enhancing the possibility of Carb-Ice) One would thing that this "gap" should be sealed in some way with of course something like cowl-seal.I have looked in my parts manual,and can see no seal for this area....I DO however realize that Carb-Ice is an internal problem(in the venturi),I just am at a loss to exactly WHY my aircraft is so much more prone to Carb-Ice than others....Tailwinds!</HTML>
Re: carb ice
<HTML>Well I have read through all the carb ice messages,and I am impressed with all the differing logic,however in the long run I have to agree with George Horn.MANIFOLD PRESSURE GAUGE,that is the best indicater of ice forming.It is amazing how many pilots do not know,or unaware of this.I was flying D.C.3s up and down the west coast of Africa in the late sixtys.We were at 9,000 heading from Port Harcourt to Lagos, Nigeria. I got out of my seat to go back to use the toilet facility(honey bucket)when I got back up front, as I was getting back in my seat,I noticed the manifold pressure was low,and no change in altitude.I told the first officer,"we are getting carb ice",and I pulled on the carb.heat.BLIP BLAB BURBLE,and walla manifold pressure back to normal.He looked at me and asked "how did you know we had carb ice"?Just another little ditty before I get boring.Sometime we would get ice formed around the carb scoop on top of the cowling.I think I frightened one first officer out of two years living.he saw the ice first on his side and said(high pitch voice)how will we get rid of that,"don't worry kid",as I pulled the mixture levers back,and a great big bang,and yellow flame out the scoop.no more ice.no more tales from willy, blue skys.</HTML>
Re: carb ice
<HTML>I recently installed the Air Plains STC 180 HP conversion kit on my 1998 172R. The 172Rs have fuel injected engines. The Lycoming IO 360 L2A is derated by the factory to produce 160HP as I understand it, and the conversion kit consists of a new McCauley prop designed for the 172Q (one inch longer and producing 180 HP at 2700 RPM) and a new induction airbox with a manual "alternate air" switch for icing, but not a new engine. The IO 360 L2A has a history of over rich fuel mixture problems in the 172, so I wonder if the manual alternate air switch will help or hurt control of the mixture. I'd be interested in the comments of some of you folks who are miles ahead of me in understanding this stuff.</HTML>
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