Carb Heat: What's the deal?

[piperboy84]

Under what conditions are light aircraft most susceptible to carb icing. I understand that in cold temperatures 10c and below and a close dew-point can lead to icing, however I recall reading something about icing occurring in far higher temperatures (20C etc.) due to the severe drop in the temperature of the carb venturi when the gasoline is converted from liquid to vapour . Is it a combination of outside temps, humidity and the venture temp drop? Is there a rule of thumb for “guestimating” when carb icing is most likely to occur?

[Torque Tonight]

http://www.caa.co.uk/docs/33/ga_srg_09webSSL14October.pdf

[Winhern]

Hopefully this link won't get blocked:
http://ibis.experimentals.de/images/carbicingfromcaassl14.gif

[Maoraigh1]

Engine and aircraft have a big effect. Pa28-161/Lycoming, C172/Continental? & Lycoming, and C152/Lycoming have given me far less icing than Jodel DR1050/Continental O200. Especially until the engine is hot. (after 10? minutes flying.) The O200 is an ice-maker. I've had the worst ice on warm, cloudy, summer conditions. (Warm for Scotland )

[RTN11]

It's all down to the engine.

In Lycoming engines the carb is usually located close to, if not attached to, the oil sump. Therefore a lot of heat is automatically transferred reducing the overall risk of ice. Students are still generally taught to liberally apply carb heat, and in a lot of schools to use it for every approach. This is completely unnecessary with a Lycoming engine, but for the average PPL it's probably good to get into a habit which will work on every aircraft, and there are engines out there that will simply stop if you reduce power without carb heat. For more on use of carb heat see this thread http://www.pprune.org/flying-instructors-examiners/427687-use-carb-heat.html

With older continental or rolls royce engines, the carb was positioned further infront of the engine leaving it much more exposed. I believe this was also the case on some older Gipsy engines found in Chipmunks and the like, but my experience on these types is fairly limited.

The main area of risk is from 5 - 20 degrees C, and the big danger is that it will happen in clear air if it is humid enough.

The only time I've had serious carb ice with a Lyco is taxiing on wet grass with a typical OAT of +8 to +15. Ultimately you have to know your engine and it's own dangers.

[Big Pistons Forever]

It's all down to the engine.

In Lycoming engines the carb is usually located close to, if not attached to, the oil sump. Therefore a lot of heat is automatically transferred reducing the overall risk of ice. Students are still generally taught to liberally apply carb heat, and in a lot of schools to use it for every approach. This is completely unnecessary with a Lycoming engine, but for the average PPL it's probably good to get into a habit which will work on every aircraft, and there are engines out there that will simply stop if you reduce power without carb heat. For more on use of carb heat see this thread http://www.pprune.org/flying-instructors-examiners/427687-use-carb-heat.html

With older continental or rolls royce engines, the carb was positioned further infront of the engine leaving it much more exposed. I believe this was also the case on some older Gipsy engines found in Chipmunks and the like, but my experience on these types is fairly limited.

The main area of risk is from 5 - 20 degrees C, and the big danger is that it will happen in clear air if it is humid enough.

The only time I've had serious carb ice with a Lyco is taxiing on wet grass with a typical OAT of +8 to +15. Ultimately you have to know your engine and it's own dangers.

While it is generally true that Continentals are more prone to icing than Lycomings, I think one has to be careful to avoid making absolute statements.
The design of the cowling and air intake are also a factor. In my experience the worst 2 airplanes for carb icing were the BN Islander and the Piper Apache , and both are powered by Lycoming engines.

Carb ice is where you find it. I fly a little Grumman AA1B with a Lycoming O 235. The West Coast of Canada spends pretty much all of the winter sitting right in the max carb ice danger zone yet in over 10 years of flying I never once experienced carb ice...until last year. I was cruising at 6500 feet on a beautiful clear October afternoon with an OAT of about + 8. The engine starting loosing RPM and was sightly rough. I at first assumed it was plug fouling, a common problem with low compression engines run on 100 Octane AVGAS, but a mag check didn't make show any difference. However going to full rich made the engine run even rougher. It was at that point the penny dropped :O and I selected full carb heat. After a few HARRUMPS from the engine it roared back to full power:ok:.

Any engine will give you warning that it is icing first with a loss of power and then rough running so pay attention and icing will never be a problem.

[piperboy84]

RTN11, do you apply/cycle the carb heat as part of your pre-landing check-list regardless of whether you think you are in the "icing zone"

[RTN11]

True, my statements shouldn't of been so absolute.

This is completely unnecessary with a Lycoming engineShould be "This is generally not required in a Lycoming"

I have mostly operated the O-320, but also the 235, pretty much every flight in either the "serious icing - descent power" or "serious icing - any power" bands, but only had carb icing twice in flight, and one of those was in cloud. On both occasions the engine gave plenty of warning with rough running, and a noticable drop in performance, and selection of carb heat solved it in seconds.

RTN11, do you apply/cycle the carb heat as part of your pre-landing check-list regardless of whether you think you are in the "icing zone" When flying for myself with in a PA28 with the O-320, before commencing the approach I check for carb ice. If none is found, and I am not in the serious icing any power band, I do not apply carb heat for the approach. This is in accordance with the POH.

Most schools I have taught at have the typical SOP to apply carb heat downwind, leave it hot for every approach and put to cold at 300'. This applies more to an older design of engine, or typically a continental and is not what the POH recommends for this aircraft type.

The reason for this SOP is that the schools think students are unable to think for themselves, and assess the real risk of carb ice, so they would rather have the carb heat hot for every approach. This does have some other affects, such as effectively enriching the mixture increasing the risk of spark plug fouling, something which I have experienced a lot more of on a Lycoming than I have ever encountered Carb Ice. As in the above post, the first thought was plug fouling, second was carb ice.

[abgd]

Is it true to say that the aircraft that are not prone to carb icing don't make full use of their engine's potential capacity? For example, you could fly a c152 around all day with carb heat on, but you wouldn't get the best out of it when it came to go-arounds and climb performance.

Presumably aircraft that continuously heat the carburettor, by whatever means, are effectively doing this?

[PompeyPaul]

I was also told that having carb heat on also burns through fuel at a helluva rate. So much do I was relayed a story about pilots who only got half way because they had carb heat on.

[custardpsc]

abgd - in short - yes. Performance suffers because warm air is less dense so less weight of air is inducted per cycle and it can expand less and exert less pressure on teh piston. Also the reduced mass flow rate affects the mixture although this could in theory be sorted out by leaning.

The RAF used to lockwire the carb heat into hot on their chipmunks - not sure why actually, anyone?

[FlyingStone]

abgd - in short - yes. Performance suffers because warm air is less dense so less weight of air is inducted per cycle and it can expand less and exert less pressure on teh piston. Also the reduced mass flow rate affects the mixture although this could in theory be sorted out by leaning.

Not really - with mixture you can only set the fuel flow, which in terms affects fuel to air ratio, but you won't get 100% of rated power with carb heat to on, regardless of how you set mixture.

Rho = p/(R x T)

Carb heat reduces density of air just by increasing the temperature just as higher altitude reduces density of air by decreasing the pressure. Even with full throttle and mixture 50°F ROP for maximum power, the engine will not achieve its full rated power at altitudes higher than sea level (unless turbocharged), since the maximum engine output is more or less proportional to density of air entering the engine. The same goes for carb heat - once you set it to hot, the engine cannot achieve full rated power anymore.

[abgd]

I was also told that having carb heat on also burns through fuel at a helluva rate. So much do I was relayed a story about pilots who only got half way because they had carb heat on.

I thought that carb heat only used waste heat from the engine, so that whilst that is correct, if you lean for cruise there shouldn't be any effect?

[Memphis_bell]

Yes it's true that if you apply full or partial carb heat, you neglect the full potential of engine output. A good example is on final approach - you descend at an aplied rate (say a 3 degree descent) with full carb heat set HOT, but before touch down (approx 100 feet) carb heat MUST be set to COLD. The reason for this is simples = if you are forced to do a go around, full engine output will be required to ascend, hence the reason for turning carb heat to cold - because carb heat set to HOT reduces output.

Simples :-p

[Pilot DAR]

Yes, if you lean properly after the application of carb heat, your engine is about as efficient as corb heat cold, for that power produced - you just cannot produce full power any more. Leaning after carb heat application also assures that the most carb heat is available. Other than aircraft equipped with a carb air temp indicator, if you need carb heat at all, you need as much as you can get.

I have more than 3500 hours in O-200, C-90, and C85 powered aircraft. Though I occasionally have had carb ice, and used carb heat, I have never felt that these engines were unusually prone to carb ice. Carb heat should be used as instructed in the aircraft flight manual. That said, in my C 150, I check carb heat for every flight, and otherwise do not use it at all, unless I detect symptoms of carb ice (loss of power). This has worked fine for me that way for a long time - but I do have a carb air temperature indicator too!

[Memphis_bell]

I have to be honest and say that one must NEVER guestimate the possibility of Carb ice. I beleive it good airmanship to routinely apply carb heat in the face of symptoms or not. I have been surprised in the past of carb ice prescence when no symptoms of such ever resided.

[Big Pistons Forever]

Yes it's true that if you apply full or partial carb heat, you neglect the full potential of engine output. A good example is on final approach - you descend at an aplied rate (say a 3 degree descent) with full carb heat set HOT, but before touch down (approx 100 feet) carb heat MUST be set to COLD. The reason for this is simples = if you are forced to do a go around, full engine output will be required to ascend, hence the reason for turning carb heat to cold - because carb heat set to HOT reduces output.

Simples :-p

If it is so "Simples" how come No Grumman or Piper or Cessna POH requires you to select Carb heat off before touchdown ? What do you know that the flight test engineers at all the major light aircraft companies don't know ?

Since the optimum conditions for carb ice formation is high velocity air into the carb throat with a idle throttle setting, precisely the conditions on short final; what is stopping the formation of ice between the time you select carb heat off and then for what ever reason have to go around ?

And it should be noted that under the right conditions significant amounts of ice can develop in seconds:uhoh:

The fact is that the "requirement" to select carb heat off before touchdown at some arbitrary height is a unique to UK flying schools urban myth mindlessly passed down from Instructors who got it from their Instructors. Certainly it is not common practice in North America, where 70 % of the worlds light aircraft are operated.

The certification of light aircraft engines requires that the makers prove they can be run at full power with full carb heat on under any normal circumstances and so there is no reason to prevent you, in the event of a go around, from simply applying full throttle and then selecting carb heat off. If conditions warrant carb heat on final then I firmly believe it should stay on until you have landed and vacated the runway.

[piperboy84]

PISTONS, do you do this?(keep the carb heat on till vacated) And how are you gauging "if conditions warrant carb heat" are you basing it on preflight weather reports and/or personal observations or pre landing OAT and visual cloud conditions or is there other factors you take into consideration when deciding to hold the heat on all the way down?

[Big Pistons Forever]

PISTONS, do you do this?(keep the carb heat on till vacated) And how are you gauging "if conditions warrant carb heat" are you basing it on preflight weather reports and/or personal observations or pre landing OAT and visual cloud conditions or is there other factors you take into consideration when deciding to hold the heat on all the way down?

Well two of the aircraft I regularly fly have carb temp gauges which makes it pretty easy. If the carb temp is above 10 deg C then the carb heat stays off. I fit is below that I apply enough carb heat to get the temp up and out of the icing range.

If I am flying an airplane that does not have a carb temp gauge then environmental conditions dictate whether or not I consider carb heat. If, for example, it is dry and 25 C the carb heat isn't going on. :rolleyes:

If it is in the well in the icing range then I monitor the engine for signs of icing and always select carb heat on in the prelanding check and look for any signs of the heat clearing any ice. If I do think there was some icing present then the carb heat goes back on and stays on. In conditions particularly prone to icing like wet air with temps between zero and 10 deg I keep the heat full on during the approach as a prophylaxis against icing.

[abgd]

What are the feelings here about keeping carb-heat on prior to takeoff? My examiner, who clearly knew what he was about, encouraged me to use it whilst taxiing from the run-up area to take-off.

My concern:

1) You might forget it to reset it - not a problem in a light aircraft on a long runway, but a potentially major problem in a heavy aircraft on a short runway.

2) FOD - I just don't have the experience to know to what extent it's an issue on your average runway. Presumably a major consideration bush-flying in the Sahara, but is it worse on tarmac or grass? Is it likely to cause gradual erosion of the interior of the engine, or is ingesting a stone likely to cause a catastrophic failure e.g. by hitting a valve?

3) Presumably if you set full power against the brakes, prior to takeoff... if you have carb icing you'll detect it and can deal with the problem before take-off. If you don't have carb icing, you're not likely to get more after you let go of the brakes.

[gasax]

With both the C90 and O-200 engine aircraft I had taxiing on damp grass almost required carb heat - otherwise the engines would go all 'lumpy' and die. Carb ice at near idle.

I've had a similar 'conversation' with a friend whilst flying my Terrier (Gypsy powered) - when he commented I should have removed the carb heat on short finals. So I did as we taxied back to the hangar - and luckily(?) it then went all lumpy - until I re-applied carb heat.

Granted the humidity on the ground close to grass will always be higher but it does illustrate the point quite well. But my friends O-320 never suffered from these issues.....

[Tarq57]

Only times I've experienced carb ice was in a 172, taxiing out in cool or cold, and very humid conditions. (Dew on the ground, no wind.)

Carb heat remedied it in a few seconds.

Carb icing is more likely at a reduced throttle setting, because there is a greater resulting pressure difference between upstream and downstream of the butterfly.

Carb icing is slightly more likely if, in addition to the above, the mixture is left fully rich when it needn't be, because more gasoline is forced to evaporate, thus producing a greater cooling effect. It is not suggested that other than full rich is used on the ground when taxiing, nor during takeoff, when the enrichment also provides an important engine cooling function.

Dust/grit is the biggest enemy of piston engines. It can be too fine to see, but still get in and scratch things. Even melt on things, if silica based. avoid using carb heat on the ground any more than absolutely necessary when on a dusty surface.

Fortunately, around the parts I've usually flown, weather conditions conducive to dust blowing around are usually not those that lend themselves to carb ice.

[A and C]

Intake design also is a factor, I have had very little carb ice in a PA28 be it 140, 160 or 180 HP but. DR400's with the same engines fitted are very prone to carb ice.

Saying that having the carb attached to the engine sump will prevent most of the carb ice is tricking yourself into a false sense of security that is likely to end in tears.

[Maoraigh1]

I tried going to Cold on very short final - until I had severe carb ice on opening the throttle to turn after landing. I'm back to Hot until on the ground. In some conditions, I start the take-off run with Hot selected, and go to cold on the run. I've had carb ice stop the engine on a Konsin deiced runway on several occasions. (DR1050/O200 in Scotland.)
I seldom or never have touched carb heat in W. Colorado-Utah-Arizona.

[abgd]

Well, I have on occasion forgetten to set carb-heat to cold just prior to touchdown, so I'm pleased to hear it's not such a terrible sin.

[RTN11]

http://www.lycoming.com/support/publications/service-instructions/pdfs/SI1148C.pdf

In the case that full power needs to be applied under these conditions, as for an aborted landing, return the carburetor heat to “Full Cold” after full power application.

Lycoming, at least, are pretty clear on their use of carb heat. If you use it on the approach, it should only be selected cold after full power has been selected for a go around. Other than that, it can be selected cold once on the ground.

The whole putting it cold at 3-500' is a UK flying school nonesense and can simply act as a distraction at a rather critical phase of flight. The only exeption I would include is landing on rough grass when you wouldn't want unfiltered air going into the carb, so would select cold as soon as practical.

[Memphis_bell]

Big_Pistons_Forever

You have said some really important errors in BASIC aircraft operation and also in airmanship. Now you seem like a pretty opnionated person, so i will leave you to discover and note your shortfall.

[Memphis_bell]

ABGD

The best advice i can offer is to consult official sources. Dont listen to bods on here (including me)

If in doubt, there is NO doubt. And if in doubt, chicken out !

Ice is a problem - get rid of it! Carb heat induces a loss of power (recall carb heat check on run up) so lose it !..especially on final !

Inbox me dude, i'm from around your area :-p

[Big Pistons Forever]

ABGD

The best advice i can offer is to consult official sources.



Could you be more specific as to which "official sources" advocate turning off the carb heat on final ? I am asking because I am not aware of any aircraft manufacturer supporting this practice. Is the CAA the ones who are pushing this ?

[abgd]

I tend to listen to lots of people and also check with the official sources.

My view is that conditions and aircraft vary, so there can be no one true checklist to cover all situations. You get the basic protocol nailed, then occasionally adjust it on the basis of principles and experience. Hence my interest in what people here have to say. It would never have occurred to me that you could get carb icing as quickly as some here have experienced, for example, as the only carb icing I've ever encountered has been relatively insidious.

[Pilot DAR]

but before touch down (approx 100 feet) carb heat MUST be set to COLD. The reason for this is simples = if you are forced to do a go around, full engine output will be required to ascend,

If the carb heat must be set to cold before landing, the aircraft flight manual will state this. The aircraft flight manual is the only "official source" of procedural information about the operation of the aircraft systems. It would not be an error to operate an aircraft in accordance with the procedures stated in the flight manual - in fact, it is expected with good airmanship! The selection of flight manuals I have here generally say "carb heat to cold after landing", though one flight manual I have says: "The approach technique is as follows....and carburettor heat off [cold] unless carburettor icing conditions prevail". I have never seen an instruction in a flight manual to reselect carb heat to cold on short final approach.

If, the pilot decides to overshoot, the carb heat can be moved to the cold position quickly - in many aircraft, without removing one's hand from the throttle. Certainly easily in any aircraft, and without detracting from the ability of the pilot to safely control the aircraft on climbout. A pilot who's workload is too high when having to select carb heat cold at the time of the decision to overshoot, is perhaps flying beyond their skill. It gets a lot more busy when you're flying a multi, and having to reconfigure flaps, cowl flaps, trim, and get the gear up after positive rate - carb heat is nothing compared to all of that!

The aircraft will safely climb with the carb heat hot, if all other things as they should be. Certainly at least enough climb to begin the overshoot, and get the aircraft reconfigured. If you think about it, the application of carb heat generally reduces the RPM (for fixed pitch prop engines) about 100 or so. Can you takeoff and climb away a C 150 at 2400RPM instead of 2500+? Of course you can. It's going to take more space, but is safely possible in most normal dimension runways. For the C 150, takeoff and climb away is possible at engine speeds as slow as 2100 RPM - though it's going to take a lot more runway and clearway. If you're going into a runway from which you cannot safely overshoot carb heat hot, you have already committed yourself to a landing from which a safe overshoot is not possible - and there are some, so it happens.

[Big Pistons Forever]

Big_Pistons_Forever

You have said some really important errors in BASIC aircraft operation and also in airmanship. Now you seem like a pretty opnionated person, so i will leave you to discover and note your shortfall.

That would be because you are incapable of articulating where I have made any important errors. Go ahead and prove me wrong :rolleyes:

BTW I note you have listed your age as 28. I started my commercial flying career when I was 27, I am now 52 :E

[Miken100]

I always find these threads useful as they make me think about my own current knowledge/skill and what should I brush up on.... :)

What makes me despair however is the constant 'dick-measuring' which adds absolutely nothing to the debate... :(

As for the Trolls and armchair pilots on PPRuNe! :mad:

[manix-cs]

I'm delighted that I don't have to worry about selecting and deselecting carb heat.

I fly a Rotax 4 stroke engined aircraft. The carb bodies are permanently heated by the engine coolant but there is no power loss because the air entering the carbs is at ambient temperature.

[thing]

I was always taught to select carb heat downwind and leave it on until you land. I can't see the problem on a go round, you just shove the carb heat and throttle forward at the same time. You might have to take another half second to do it on a 28. I suppose a minor advantage as well is that if your idle setting is slightly high you would have less residual thrust when on the landing roll.

[piperboy84]

As the OP of this thread I have found it extremely interesting and educational reading the different viewpoints on carb heat use, I have deduced the following:
1. Re-read and understand what the AFM says, and just as importantly what it DOES NOT say in regards to carb heat.
2. Carry with me the “carb icing danger zone chart” and understand how to relate its calculations to the combination of actual weather conditions before/ during flight and prior to the finals
3. When considering when to engage and disengaging car heat be cognizant of not just the elements (temperature, humidity etc.) but also field conditions such as thin grass with loose dirt, fleshly cut grass and wet grass both on TO and landing that can bypass air filtration when crab heat is on and cause additional problems. But remembering that if faced with a choice, unfiltered air is better than an iced carb
4. When in doubt whatsoever there is very little downside in any flight configuration to putting the carb heat on to verify there is no icing.
5. If landing in the conditions susceptible to carb icing keeping carb heat on all the way down till reaching ground effect, then only closing the carb heat due to either having to go around or conditions on the ground favouring filtered air such is the case on my airstrip which swings from wet to dusty with a thin cover of grass
6. Expect the unexpected and expect it happening quick and at times of high workload. And carb icing can happen pretty much at the entire normal temperature range for the area I fly in.

Thanks folks

[Piper.Classique]

Good summary, Piperboy.

Though you can probably select car heat according to the comfort level you require when driving to the airfield :D

[BackPacker]

And to add further... Crab heat is probably best left to the better restaurants.;)

[Memphis_bell]

Big_Pistons_Forever

Unfortunately i am very poor at articulating your errors, so i will leave you to measure how big your pistons are.........although i'm guessing there pretty big :-) Regarding my age....i'm actually a 75 year old ballet dancer..which confirms ???

[Pilot DAR]

Good try at a reasonable thread Piperboy. I'm happy for you that you got some useful responses, before the thread drifted into silliness. I would take Big Piston's advice verbatim any day. You can see that there are varied ways in which carb heat is used by pilots. and some passion about it too.

Keep your eyes open for an opportunity to fly an aircraft equipped with a carb air temperature indicator - watching that while selecting carb heat, and then adjusting the power and mixture to optimize the heat, is an education. What you will see is that the techniques which are taught, are often only a part of the story. The flight manual techniques err to the simple and conservative, as one would expect. Pilots who feel confident that they have assured their security from carb ice simply by selecting carb heat, and then ignoring the other engine controls could be in for disappointment - I have been. Like any control of the aircraft, carb heat should be applied as a part of good, informed pilot decision making.

I applaud your effort to become more informed. Good attitude! Perhaps another will begin to act their age - well, at least one of them.....

[Piper.Classique]

I second pilot DAR. Remember some people who post here have far more opinions than their experience can justify. Big Pistons is NOT one of them, neither is pilot DAR. I leave you to guess who is (or is that are?)

[Stephen Furner]

I must admit I had not realised there was such a large variation in how carb heat is used. This discussion has certainly had me back checking to see what my POH has to say. There was nothing in there I could see about its use in landing to say whether it should be on or off.

My impression from what’s been said here is that different local practices around the use of carb heat have arisen in response to the typical needs of the aircraft and climate pilots are operating in.

Interestingly, it also seems to influence which engine parameters are being monitored. I have not so far seen carb temp monitors in aircraft where I fly but I have seen many exhaust gas temp monitors .

I think this is probably because the daytime temperatures that the average private pilot flies in around my part of the world are rarely below zero. The risk of applying carb heat and moving it into the icing range is consequently low. Whereas fuel prices are very high and the risk of wasting fuel or damaging an engine through improper leaning to reduce fuel burn is also high.

For myself carb heat comes off at 300ft on final. I fly an old Cessna 172 that has 40 degree flaps and only 145 hp max output from the engine. I need every ounce of power and minimum pilot workload if it is necessary to go around at the last minute with a full load and all the flaps out.

[vee-tail-1]

How sad that after all these years carb heat is still a problem. Lycosaurus engines have been around for two centuries! Surely by now the PPL training schools should have perfected carb heat instruction so as to make engine induction icing a non event. Faffing around with carb heat, using up valuable thinking space when other things need your attention is potentially disastrous.
So all air cooled carburetted aero engines WILL ice up sometime, some more than others.
A pilot needs to avoid ice build-up before take-off by selecting carb heat on after engine start, and off just before applying take-off power.
On again for the downwind check, and off when applying take-off power for a go around.
In flight the standard engine ice check should be performed every ten minutes on your FREDA check. (E = engine: check Ts & Ps & Ice)
Hands up those who have forgotten the standard in-flight engine ice check?
1. Set cruise power, and ensure engine rpm is stable.
2. Apply carb heat and observe engine rpm: If there is an immediate drop in rpm and no other indications, select cold and look for an immediate return to cruise rpm. The engine is free of ice ... continue for another ten minutes.
3. Apply carb heat and observe engine rpm: If there is no drop, or a rise in rpm, with or without other indications, you have an iced engine. Leave carb heat on until rpm stabilises and condition 2. above returns.
This check should become habit ... you should be able to do it automatically ... it should not use up valuable thinking space which you need for non routine decisions & emergencies.
One day all engines will be diesel or fuel injected, but meanwhile we still need to learn engine intake ice management by rote ... otherwise new young PPLs will continue to die needlessly in completely avoidable icing incidents.

[thing]

2. Apply carb heat and observe engine rpm: If there is an immediate drop in rpm and no other indications, select cold and look for an immediate return to cruise rpm. The engine is free of ice ... continue for another ten minutes.

Now then there will be people who will disagree with that, our CFI for one. No good asking me to explain, I don't understand complicated stuff. He reckons that the carb heat should be out for a minimum of thirty seconds on the FREDA check, in fact increase the RPM a little first before you apply carb heat. He's not a newbie CFI either, he's ancient and has squillions of hours on squillions of types.

[vee-tail-1]

3. Apply carb heat and observe engine rpm: If there is no drop, or a rise in rpm, with or without other indications, you have an iced engine. Leave carb heat on until rpm stabilises and condition 2. above returns.

thing. For sure it might take more than 30 secs to clear the ice, but having done so check 2 takes only a few seconds. Thereafter until out of suspected icing conditions do check 2 more frequently, perhaps every five minutes or so.
You might also consider changing altitude to get out of icing conditions, or leave carb heat on and accept the reduced engine performance & efficiency.
The point being that it takes only a couple of secs to check for carb ice formation, but much longer to clear the ice. So better not to let that ice form at all.

[biscuit74]

Can I just say that when going to carb heat to check for icing, leave it that way for some time - twenty to thirty seconds is a good minimum time.

Think about it please - you are trying to melt any ice. That takes time and energy, so applying carb heat 'for a few seconds' is fairly pointless. Tells you nowt.

And if you have got icing, expect the engine to start running even more roughly as the ice melts. Engines tend not to enjoy gulping water. (No, this is not how water injection power increase works !)

So, when the engine sounds like it is going to die on you after pulling carb heat on, don't immediately go to back to cold power - keep carb heat on while turning towards a suitable field, just in case.

This affects any carburettor equipped aeroplane in the wrong circumstances.
Also, I recommend you don't wait till you start a water crossing before checking carb heat. Guess how I know!

biscuit

[Maoraigh1]

The Canadian official site has a download with the usual carb ice diagram, but the caution that this diagram only applies with avgas, and carb icing can occur 20C higher with mogas.

[abgd]

Well, despite the kerfuffle I've found this thread interesting and informative. I'm grateful, as always, to those of you who've answered both the original poster and my own questions.

[thing]

There's always a kerfuffle, it's sorting the wheat from the chaff that's the important bit!:)

[piperboy84]

Agreed, the important part is “separating the wheat from the chaff” The good thing is once you get several knowledgeable view points and actually apply them while flying it makes for a more enjoyable, confident and safe flight experience. Even on things that may appear to be elementary or minor to the more experienced pilots on this site such as the carb heat, can, when not fully understood (as was the case with me even with 450 VFR hours) make what should be a routine and enjoyable flight a slightly stressful experience due to the nagging feeling that you may not be doing things correctly, not to mention tying up time worrying about it when something else that could seriously interfere with safe operation is going unnoticed. I firmly believe that marrying the “official resources” of the AFM with past/current instruction and gathering opinions on here help me to zero in on best practices that suit my specific flying profile and mission. I can assure you that the initial CFI for my PPL on the long tar runways of sunny Southern California did not spent much time gearing my training to flying a tail dragger out of a wet and windy dirt strip in Scotland, not that that was his job, but as your flying horizons open your need for drawing on other pilots experiences and opinions make a difference. That’s why I am on this forum

[abgd]

Agreed, the important part is “separating the wheat from the chaff”

Sometimes it's not too hard!

[peterh337]

How sad that after all these years carb heat is still a problemQuite.

When I was looking for a plane to buy in 2002, I decided to simply not even look at anything that had a carb.

Why the industry puts up with this, I don't know, when perfectly good fuel injected engines exist and have done for decades. I suppose it is a marketing decision to offer FI only on the bigger ones, because a fuel servo (e.g. the Bendix RSA5AD1) costs a few hundred $ more than a carb.

[vee-tail-1]

Can I just say that when going to carb heat to check for icing, leave it that way for some time - twenty to thirty seconds is a good minimum time.


Sigh! ....

biscuit74 the idea is to CHECK for icing ... it takes a few seconds: Pull out the carb heat control - observe an instant rpm drop with NO other indications - push in the carb heat control - observe an instant rpm return to cruise setting. Now you KNOW there is no carb icing and can get on with flying the aeroplane, not faffing about with head in cockpit.

[Big Pistons Forever]

I think it is important to note that the first indication of carb ice in cruise flight should be the pilot noticing an uncommanded reduction in RPM (or MP in an aircraft with a constant speed prop). At that point the first action should be to suspect carb ice and apply full carb heat.

There is nothing wrong with the practice is to check for carb ice by applying carb heat at some regular interval, but if you were to find carb ice during one of these "checks", I think one should be asking themselves why they did not notice any earlier signs of ice developing.

Finally the RPM drop when carb heat is initially applied has nothing to do with carb ice it is simply the fact that the hot air has enrichened the mixture to such an extent that the engine will lose some power due to a overly rich mixture. This drop only proves the carb heat control is actually working. Only if there is a rise in RPM (MP) after the heat has been applied can we deduce that there was carb ice and it has now cleared.

How fast carb heat will take effect depends on the efficiency of the carb heat installation. Some airplanes will deliver massive amounts of instant heat others are less effective. Personally I would say that a few second application of heat may not be effective and the fact the RPM returned to the same value as before the heat was briefly applied may only be due to the fact that the resumption of cold air has returned the mixture to a normal value and therefore allowed the engine power to pick up, but without clearing ice that was still there.

I teach my students that if they apply carb heat they should leave it on for at least 10 seconds. In every aircraft I have flown this would be sufficient to provide unequivocal evidence of icing.

Finally carb ice is most likely to develop at low power settings so it is most dangerous in the landing and takeoff phases and therefore extra attention should be used in those phases of flight.

A related item IMO is importance of knowing the static RPM (fixed pitched prop) or field baro MP (constant speed prop). Sitting with the engine idling while you are waiting to takeoff is a prime time for carb ice to develop and I often give the engine a shot of heat as I manoever into position for takeoff on days conducive to icing, But your last line of defense is to assure that the engine is making full power is to check that you are getting static RPM as soon as you have applied full throttle.

[vee-tail-1]

This is getting a little like the discussion on AF447 ... lots of detail, which somewhat obscures the main problem:
All carb equipped aero engines can develop intake icing, and this will cause the engine to stop if nothing is done to remove the ice.
The desired action of a pilot is to recognise the condition, apply carb heat, and get out of the icing area if possible.
Agreed the first indication in cruise is a gradual drop in rpm. Thereafter in my experience engines react differently. If you are lucky the engine runs rough, gives some mighty bangs, and generally frightens the s***t out of you. You apply carb heat, wonder why you missed the onset of icing, and resolve to keep a better watch on the engine performance.
The other sort of engine just stops and you now have an irrecoverable situation leading to a forced landing.
It is because the latter situation continues to kill new PPLs that I am making such a meal of this thread. The regular carb heat check that I have been suggesting really works ... it gives pre warning of icing and enables the pilot to take timely action. Too many have died due carb icing, and it's time that particular danger was put to bed by proper instruction.

[Jan Olieslagers]

it's time that particular danger was put to bed by proper instruction

Instruction is a good thing, and is the only solution to difficulties that are unavoidable, such as gravity, sudden and/or unpredicted changes in weather, and the abrupt disappearance of all known visual references when flying genuinely visually.

This particular danger of carb icing otoh could have been done away with by the proper application of technology - no need to add another complication to recreational pilots. Only the stupidest of administrations allow new aircraft types in the air that are so very much prone to this very real danger. There's plenty of alternatives available, injection best of all and the superbest in this respect is a diesel; but even if a carb is chosen, for reasons of economy likely, it can be mounted in a clever way like on a Rotax. I'll allow icing isn't absolutely impossible on Rotax carbs but the risk is far far smaller.

EASA could make itself a lot more useful by disallowing the old brigade of carburetted Lycosaurs, but that wouldn't be politically correct i suppose.

[abgd]

I found myself wondering what carb icing actually 'looks' like... Wouldn't it be fun to put a webcam in a carburettor. And of course on Youtube, somebody already has.

Carb Icing on my VMax - YouTube

It's a motorbike engine, and my impression is that it uses a piston rather than the butterfly valve in my textbooks. But still interesting, and kudos to the chap who made it!

[abgd]

We were talking about this in my flying school... They were against buying anything _without_ carb heat on the grounds that whilst this would be safer for students, sooner or later most pilots will end up flying something that requires it.

I'd have thought the ideal for training would be something that iced up gracefully - gave good symptoms of icing, but was unlikely to stop outright.

[Jan Olieslagers]

Re the video:
-) yes , a great idea , thanks for sharing!
-) seeing the piston's movements, I guess this is a so-called constant-vacuum carb, like the Bing 55's commonly found on 4-stroke Rotaxen - they have a piston, controlled by venturi pressure, but they also have a butterfly valve, controlled by the throttle. I can't imagine any pilot in a more or less normal state of mind to operate the throttle in the rhythms we can see this piston practicing.

[Memphis_bell]

Vee-tail-1

You have had some really constructive input into this discussion.....definately confirming some of my understandings. A++++

[India Four Two]

So, when the engine sounds like it is going to die on you after pulling carb heat on, don't immediately go to back to cold power - keep carb heat on while turning towards a suitable field, just in case.

biscuit74,

This is the first time (that I can remember) that I have seen this aspect of carb icing mentioned.

I had been flying for about six years (~500 hours) before I experienced carb icing. I was flying a 172 in the summer time, about 4000' AGL near the Canadian Rockies, with cloud base not much higher (i.e. about 4000' lower than normal), with virga coming out of most of the clouds. Passing near one of these clouds, I thought it would be prudent to check for icing. I pulled on the carb heat and got the symptoms you described. The point of this long-winded preamble is that it scared me so much that I instinctively put the carb heat back to cold! :eek:

It was only after thinking about it for a minute that I realized what was happening and put the carb heat on again and let the engine cough and splutter as the ice melted.

I wish one of my instructors had mentioned this issue during my training. Not one ever did.

[SEP Flyer]

I found this explained carb ice very well.

Carburator Icing - YouTube

I fly a C152 and a Rotax powered C42. I prefer the Rotax engine for a number of reasons, one is not having the worry of carb ice. Biggest worry when going back to the 152 is forgetting to use the carb heat!:eek:

[thing]

I wish one of my instructors had mentioned this issue during my training. Not one ever did. I think you can become a club instructor after 300 hrs, passing the CPL exams (but not the flying side) and doing the FI course? Someone will no doubt put me straight on that but it's not surprising that some instructors don't know. Apart from having the CPL exams under their belt there's no reason they should know more than I do and I know Jack. (In fact someone asked me at the club the other day if I fancied instructing, I'd like to learn how to fly first!)

[Memphis_bell]

I'm currently sitting my ATPL exams...and i can tell you, there not easy ! Haha

[thing]

Don't forget the form 20 for the examiner then. :)

[Memphis_bell]

Confused ?...form 20 ?

[n5296s]

I'm currently sitting my ATPL exams...and i can tell you, there not easy
Luckily, correct use of English is not a requirement.

@peter... I have the best of both worlds, a carburetted engine whose carb is tucked up at the back of the engine where it never gets cold (and a carb temp gauge too just in case). I've flown injected aircraft and hot starts can be a nightmare. Never a problem with a carb.

I've flown with quite a few instructors (in the US and for that matter the UK too) and never come across this idea of turning off carb heat on short final. Sounds nuts to me, a distraction just when you don't need it (especially for low time pilots but also for everyone else too). On everything I've flown, carb heat can be reset in the same movement as applying throttle for a go-around, and indeed that's what I've always been taught to do.

[Big Pistons Forever]

I'm currently sitting my ATPL exams...and i can tell you, there not easy ! Haha

Unfortunately the studying required to pass the CAA or EASA ATPL exams has been carefully planned to avoid any possibility of learning practical useful information, especially for the operation of light aircraft.

It does however seem to do a splendid job at encouraging a usually undeserved "God of the Skies" mentality in many of the pilots who undertake the ludicrously long and complicated process required to prepare for the exams :E

[thing]

Confused ?...form 20 ?

A twenty pound note.....:)

[Pull what]

I think it is important to note that the first indication of carb ice in cruise flight should be the pilot noticing an uncommanded reduction in RPM (or MP in an aircraft with a constant speed prop). At that point the first action should be to suspect carb ice and apply full carb heat.
and select full power

A lot of instructors when teaching PFLs start the initial actions by waffling about speed for height, selecting a field planning, a circuit, making a call and by the time they get round to finding out why the engine might have stopped, guess what the engine is much colder. Is that going to help if it stopped due to carb icing?

Engine roughness partial power loss or failure on SE a/c with a carburettor should be immediately:

CARB AIR HOT, SELECT FULL POWER

Dont wait till the engine has cooled down!

[piperboy84]

Select Full Power ?

Do you mean full power the instant you realize there is icing or after the heat has been engaged and the icing cleared? Also if icing occurs while in cruise configuration and heat is engaged then full power either simultaneously or after ice clearing would the mixture not have to be enriched prior to full power, I recall reading about potentially harmful effects on the engine of going to full power while running lean, any thoughts?

[Memphis_bell]

Thing:

FINALLY, some humor ! I thought it was a form 50 !! Haha :-)

[Big Pistons Forever]

[B]CARB AIR HOT, SELECT FULL POWER

Dont wait till the engine has cooled down!

If you are at cruise flight power and you get carb ice, selecting carb heat without adding power will still get more then enough heat into the engine to melt the ice.

If you let the engine actually stop due to carb ice then full throttle won't do you a lot of good. However I heartily agree with your point about taking early action to try to rectify a faltering engine. The best way to deal with an engine failure is not to do the perfect forced approach it is to restore engine power. In the case of a carb ice induced engine failure there is a point of no return where you can't get the engine back with carb heat. If the first thing you do when the engine starts fading is spend a lot of time on some elaborate flight school forced appraoch ritual by the time you get to the carb heat you may have created your own totally preventable engine failure.

Bottom line: If you are flying along and you see a loss of power or the engine starts to run rough, Immediately apply full carb heat

[Memphis_bell]

This is such a MASSIVELY over-engineered debate. Geeeeeees !

[n5296s]

I think this Memphis Bell guy has some fairly serious issues... here's a PM which I just got (and I guess I'm not the only one):

Memphis_bell

Join Date: Jan 2012
Location: Hertfordshire
Age: 28
Posts: 34

hI
What a boring individual.

Blocked anyway. Hope he doesn't get his ATP - it's a bit frightening that he could be in the right seat of a 737 one day!

[piperboy84]

This is such a MASSIVELY over-engineered debate. Geeeeeees !

Maybe, Maybe not, however one thing is for sure I (and many others no doubt) know a whole bunch more on the subject than we did when it started.

[abgd]

I've enjoyed it too... Learned a lot + the more you discuss something, the better it gets rubbed into your mind.

Another part of my reasoning is that I see something homebuilt in my future. The POH may not be quite as definitive as it is on a PA-28 and the more I understand about the fundamentals of how everything all works the better.

[abgd]

Memphis Bell(e?)

I'm afraid I couldn't see the flaws in Big Piston's post. Based on his explanations and experience, I'm inclined to take his advice over yours, so perhaps you could post an explanation of what he got wrong for my benefit - if not for his.

[Pilot DAR]

If you are at cruise flight power and you get carb ice, selecting carb heat without adding power will still get more then enough heat into the engine to melt the ice.

Big Pistons, please allow me to supplement that thought a little. My experience has been that some types have marginal carb heat capability. This is based on my doing quite a bit of testing. One of my early certification projects was to modify, test and approve a Continental IO-520 to be an "O"-520. This meant the installation of a carb, and development of a carb heat system for it. It was surprisingly difficult to achieve the required heat rise for certification.

I did find that different power settings gave differing card heat rise with the application of carb heat. A greater effect could be had by leaning to peak once the power was reset (and often selecting cabin heat off, to direct all of the hot air to the carb).

A carb air temperature indicator is vital for making these determinations. If you have chosen carb heat because you really mean it, consider peak leaning the engine once it is applied. Choose the power setting you like, and start to lean 'till the engine falters (more), and rich it up just a little. Carb heat hot enrichens the mixture, so leaning it gets you back to the optimum mixture, which will give you the most heat, which you need....

[Big Pistons Forever]

Big Pistons, please allow me to supplement that thought a little. My experience has been that some types have marginal carb heat capability. This is based on my doing quite a bit of testing. One of my early certification projects was to modify, test and approve a Continental IO-520 to be an "O"-520. This meant the installation of a carb, and development of a carb heat system for it. It was surprisingly difficult to achieve the required heat rise for certification.

I did find that different power settings gave differing card heat rise with the application of carb heat. A greater effect could be had by leaning to peak once the power was reset (and often selecting cabin heat off, to direct all of the hot air to the carb).

A carb air temperature indicator is vital for making these determinations. If you have chosen carb heat because you really mean it, consider peak leaning the engine once it is applied. Choose the power setting you like, and start to lean 'till the engine falters (more), and rich it up just a little. Carb heat hot enrichens the mixture, so leaning it gets you back to the optimum mixture, which will give you the most heat, which you need....

Thanks for the amplification and your point is well taken. My experience is that in your average C 152 or C 172, cruise power will give you plenty of heat. However deteriorated scat tubing or a beat up leaking carb air box will definitely reduce the amount of hot air going to the carb. Certainly the maximum amount of hot air will be at full throttle, leaned mixture and cabin heat off.

However even without a carb temp gauge the actual effectiveness of the carb heat will be evident during the run up check. You should see a definite drop of at least 150 RPM drop when carb heat is selected on.

[thing]

You should see a definite drop of at least 150 RPM drop when carb heat is selected on. That's interesting, our 28 drops about 30 rpm when carb heat is selected. I've checked the linkage to the box, all seems in order. I haven't checked inside the box. General consensus is that 'it just does that.' I have had carb icing in that particular a/c and applying carb heat cleared it.

[Pull what]

If you are at cruise flight power and you get carb ice, selecting carb heat without adding power will still get more then enough heat into the engine to melt the ice.

I am sure you are right but carb icing isnt just going to occur in just one spot in the sky especially if you are level. There are two stages to managing the threat.
1 Is to remove the ice
2 Is to stop it forming again

Selecting full power makes it less likely to form again. Someone mentioned a problem if you are leaned-well carb heat enriches the mixture so that will not be a problem and in fact continuing with step two is in fact to lean the mixture if you need to run with carb air hot to stop further icing.

I note our C152 manual says also carb air hot & select full power as indeed does the CAA safety sense leaflet, or it certainly encourages the use of full power.

[Big Pistons Forever]

Pul what

I do not think you have convinced me but I am sure not going to say you are wrong. There is no doubt that carb heat on with full power will produce the most amount of hot air and if any doubt this is the safest action.

My only point of debate is whether this should be the universal automatic practice anytime carb ice is suspected. I personally do not think so but this issue like many others will have more than one "right" answer

[Big Pistons Forever]

I just got the May addition of Pilot on my i-pad. I always make a point of reading
the "Safety Matters" column which IMO is an excellent learning opportunity for every pilot, no matter what their experience.

The second accident profile was a Pa 28 that was forced landed after an engine failure. Since the engine was subsequently tested with no faults found, the flight was on December 27 and thus with a temp and humidity doubtlessly smack in the middle of the severe carb icing range, and the engine reportedly sputtered and refused to accelerate after a low powered descent, it is pretty hard not to conclude that carb ice was the most probable cause for the failure. And carb ice is totally preventable by the pilot.

If the engine does get a big dose of carb ice the engine will initially sputter as soon as the heat takes effect because of the thawed ice water going through the engine. The carb heat must be left on sufficiently long to clear the ice which may require up to 20 to 30 seconds of heat.

The good news was a successful forced landing was carried out with no injuries although the aircraft was heavily damaged. The pilot reported that he attributed his success to "frequent forced landing practice" which leads me to wonder if he had used some of that practice time to learn and practice how to keep the engine from failing in the first place the requirement to use all that forced landing practice for real, might have been avoided......