A carb heat trick
 

I was talking with a client today, who is the chief pilot overseeing a flock of 172's which are used for survey. One of his pilots (a 1000+hr guy) had just limped away from destroying a 172, following a forced landing. Apparently the engine pretty well ended up in the passenger's seat beside him - lucky guy, other than I suspect he'll be looking for new employment.

There was some discussion that perhaps carb ice had been a factor in the engine failure which preceded the crash. While discussing carb ice topics, I mentioned a trick I was taught, and I was a little surprised that he had not been aware of this, so maybe it's time to present it again.

If you have decided that you need carb heat to deice the carb, you need lots, and you need it quickly, 'cause things are getting worse in the mean time. To get the most of the heat available from the engine, into the carb, after applying carb heat (and worrying more, 'cause now it's really stumbling), lean the mixture to achieve the most lean setting you can manage without making it run worse. You're not developing full power with carb ice anyway, so you're not going to damage the engine by overleaning it. The presence of carb ice will have enrichened it anyway, but leaning it to "normal" and thereafter to peak lean will result in more heat from the exhaust, which turns into the carb heat that you need.

As the carb heat begins to have an effect, consider flying at a reduced power setting (and leaning more if need be), as less volume of air going through the carb at the lower power setting needs less total heat to warm it and melt ice.

If you have an aircraft with a carb air temperature indicator, watch it as you do this, and the affect will be obvious. A CAT indicator is a great investment.

I have a couple of doubts.

Since peak EGT typically occurs very near to the mixture for best power, a mixture leaner than that might result in a lower EGT and therefore less heat to the carb. I like the idea of leaning to peak EGT but I'd want avoid leaning too much. (though the difference may be small in terms of heat energy transfer to the carb heat air)

At any position less than fully open, wouldn't the pressure and temperature of the air passing the throttle valve be even colder? Possibly contributing to faster icing?

I think perhaps sticking with the recommended procedure of applying full heat as soon as carb icing is suspected would do the most to help the situation. Then of course the mixture may be leaned as required to achieve a smooth(er) running engine. I think the key point is timely application of carb heat before the ice gets too thick.

Anyway that's my initial thoughts...

Oh, a question! Where exactly is your carb air temp sensor installed? Before the throttle valve or after?

westhawk

ST,

Great suggestion. I hadn't thought of that "trick" before.

Concerning carb icing, I had never experienced it prior to coming to Canada. The main reason for that was the RAF Chipmunks that I mostly flew in the UK had the carb heat wired in Hot.

My first experience was solo, flying a 172 from Calgary to Banff. There were some clouds ahead with virga underneath, so I thought I would do a carb ice check. When I pulled the carb heat control out, there was instantaneous rough-running that was so bad that my instinctive, inexperienced reaction was to push the control in! :eek:

So the moral of the story here is that students should get their instructors to show them the effects of carb icing on a suitable day, preferably within gliding range of a suitable landing area. ;)

I agree with your comments about the carburetor temperature gauge. I've just started flying a 182 that has one and it is very comforting to be able to glance at the gauge and make sure the needle is not on the yellow arc.

Call me a skeptic on this one.

I have never seen carb ice that did not clear simply by applying full carb heat. The engine may stumble for a bit but it will pick up quickly so by the time you finished faffing around with the mixture the ice will be gone anyway. I live in a temperate Coastal city that has conditions conducive to a very high probability of carb ice for at least 6 months of the year.

The signs of carb ice are clear if you are looking for them starting with a uncommanded reduction in power followed by rough running. There is simply no excuse to let the engine develop so much ice that it stops.

I have a bone to pick on the Carb heat usage issue, my friend recently totaled his plane and was incredibly lucky to walk away from the wreckage. Also, I recently read a CAA accident bulletin about a fatal crash in North Wales both incidents happened on final and both were attributed to carb ice.

I understand that instructors and flight manuals alike call for a “check” for the need for carb heat upon approach. Here is my question why not put the carb heat on just prior to entering the pattern and power reduction AND LEAVE IT ON right up to short final and the runway is assured and where if you need full power for a go-around you have it and you will not be getting unfiltered air if landing on a dirt strip.

What’s the downside of leaving it on throughout the entire pattern till short final compared to the risk of picking up icing late on finals after you have "checked" for it back on the downwind and returned it to cold?

I thought that was standard practice? It's certainly how I was taught...

Same here. Carb heat on as part of the downwind checks, off on short final.

I wonder if this is actually true. A reduced power setting also means less power from the engine (duh) and thus less warmth produced. Which in turn reduces the amount of warm air available for carb heat - assuming carb heat is generated in the traditional way, with a cuff around the exhaust.

But there might be another reason not to do this. I'm not an expert, but here's my reasoning. Carb ice is formed due to a temperature drop in the carburetor. This temperature drop can be caused by two things: The pressure drop across the butterfly valve that regulates the MAP, and the evaporation effect of the fuel in the venturi.

The evaporation of fuel, and the resulting drop of temperature, scales more or less evenly with the MAP and thus the power applied. A higher MAP leads to more fuel evaporation, leads to more power from the engine, leads to more heat produced, makes more heat available for carb heat. So assuming you have carb heat applied, it would not matter all that much how much power you apply as these effects cancel each other out.

But the ice formed by the butterfly valve has a completely different, and in fact negative relationship. If you reduce power you do so by closing the butterfly valve. This leads to a lower MAP and thus a higher pressure drop across the valve. This may produce more carb ice, right at the time when the engine is producing less power and thus less heat.

Because of this last effect my gut feeling would be to open the throttle completely in case of carb ice, instead of restricting it. And in fact this seems to be confirmed in POHs and such, which call for full throttle in case of persistent carb ice.

Anyone can confirm I'm right?

Not as clear as it might seem
 

First a few words of caution, carb ice is not only a function of the engine and atmospheric conditions it is also a function of intake design my PA28 exhibits less inclination to carb ice than my DR400 but both have the same engine and carb.

I would strongly recommend that cab heat is applied just before the power is reduced to make the approach to the runway ( top of base leg or equivalent position) because at point the heat exchanger has the most heat in it and is capable of doing the most good in terms of ice removal and as the carb is now clear of ice the engine will continue to run and put some heat into the system. The real danger if you don't do this being that if the carb should ice during a glide approach you won't know it has iced until you try to go around when opening the throttle gives you no engine response.

EDIT

Backpacker I agree with you, the most likely time for a carb to ice is with the butterfly valve closed, this position gives a very large acceleration of the air ( so large PX & TX drop ) as it try's to get through the small gap, ad the fuel from the idle jet evaporating and you have the perfect conditions for carb ice.

A major ice encounter !
A few years back when in the cruise at FL090 in solid IMC OAT + 2 the DR400 engine started loosing power I applied carb heat and this slowed the power loss but it was clear that the aircraft was going to have to descend to increase the OAT being as the power was reducing the throttle was left in the cruise position because it would not move and as is the training mantra the mixture was set full rich...............l BIG MISTAKE !!!!! The rpm dropped and the EGT fell rapidly, my next action was to lean the mixture to give me max EGT this gave me the best RPM ( power ) and the most heat in the carb heat exchanger.
The mixture was set a long way lean of normal optimum for the ( rapidly decreasing ) altitude because the carb was so restricted that any more fuel would result in a massively over rich mixture and a decrease in the heat in the carb heat exchanger.
Eventual at about 3500 ft the engine power was restored and the settings retuned to the normal values................. A little later during the final approach to the airfield as we broke out of the overcast I was thinking that the Irish Sea looked very cold and uninviting !

After this incident the carb heat system was checked and a very well hidden crack found in the carb heat valve, the result of this crack was that the engine was only getting partial heat and so the systems ability to clear ice much impaired.

I was taught many years ago that you selected carb heat on as part of the downwind pre landing checks and left it on until within gliding range of the threshold. Only turn it off because its unfiltered air so as not to get dust into the engine.

In cruise select it as part of the regular scan check and if any of the signs as above. Leave it on for at least 30 seconds. If the engine runs rough you are burning water! Wait for it to smooth out before closing.

There should be plenty of heat in the exhaust system to melt the ice unless you are in a long slow descent when warming the engine every thousand feet is required for the engine's health as well as the carb heat!

Hmmm. The logic and physics sort of makes sense but do you really want to add another variable (mixture setting) to the equation when dealing with a non-normal event? As the onset and clearing of carb ice is a dynamic process, the mixture ratio will be altering anyway without any input from the engine controls. Why further complicate the issue with a procedure that’s not in the POH?

IMHO, many icing events could be avoided/mitigated by a) a thorough understanding of the causes, b) regular checking and c) applying the clearance procedure in the correct way, i.e. giving it time to work.

Running dog
 

Much of what you say is true but outside the training environment most people descend with some power in a cruise descend rather than a glide, this keeps the engine temp more stable and makes the practice of warming the engine unnecessary.

I also think that the "keeping dust out of the engine" thing that the training industry is so keen on is a myth unless you operate in a desert environment, the path for the air when carb heat is selected is such that in a normal northern European environment not a lot of rubbish will enter the inlet track.

I was taught to leave it on until on the ground (it's easy enough in a 152 to push it in at the same time as the throttle for the go-around or T&G).

Full wings
 

While your logic about changing to much at once is reasonable my guess is that if I had not leaned the mixture to get optimum heat in the engine my aircraft ( and may be me ) would be at the bottom of the Irish Sea.

I suppose the point I’m trying to make is that unless you have information that points pretty strongly towards an alternative course of action, following the manufacturer recommended procedure to begin with is generally a good idea. If it doesn’t work or produces sub-optimal results, then that is the time to experiment.

Boeing have this to say about their jets and I think it contains wisdom applicable to operating most other types of aircraft too:

“It should be noted that, in determining the safest course of action, troubleshooting, i.e. taking steps beyond published non-normal checklist steps, may cause further loss of system function or system failure. Troubleshooting should only be considered when completion of the published non-normal checklist results in an unacceptable situation."

We've all heard of FADEC engine controls. The acronym is useful for light aircraft periodic cruise checks:

F Fuel config
A Alternator/ suction
D DI sync
E Engine gauges
C Carb heat

@ST: I remember from once upon a time at checkride, when I was told by my old instructor to no longer use that "trick". It will do the job i.e. with an O-200 or O-300 in older Cessnas and if you are fully aware of what you are doing, but won't in many other cases - especially younger aircraft. If you encounter carb ice, you have to apply heat asap, and yes, a bit of leaning will help increase temperature (the same lean as it is used in cold winter to heat up morning cold engines), but there is a danger in too excessive carb ice melting - resulting in engine cut off once the ice melts. The instructors I know therefore decided it is not worth the bit of more heat at increased risk.

Isn't it about time carb heaters were standard. I had them fitted over 12 years ago which consisted of a water jacket (engine was water cooled) but I am sure even then electric heaters were available.
Still did the carb checks but it did work and I never experienced carb icing with them.
2015 and aircraft engines are still prone to icing? The one thing that results in ef and the one thing that can be cured.
Unless you have an injected engine when you cease worrying.

I often lean if a carb heat check indicates ice. I was taught to leave carb heat on until touchdown, but later was told to go to cold air on very short final. I went back to hot air until on the ground after the engine almost stopped when I opened the throttle to turn 180 and backtrack. O200 in Jodel DR1050 in Scotland is an excellent ice-making combination.

Carb heat should be used in accordance with the engine manufactures manual or when the engine shows signs of accumulating carburettor ice.

There are occasions when using the above method could actually induce carb ice if the atmospheric conditions are right, not one size fits all.

Not true, it is possible to get induction icing, hence the fitting of an alternate air source.