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?)

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.

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.

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.

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.

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

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.

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.

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

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

Sometimes it's not too hard!

Quite.

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.

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.

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.

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.

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.

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.

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.

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.

Hi
 

Vee-tail-1

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

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.

I found this explained carb ice very well.


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:

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!)

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

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

Confused ?...form 20 ?

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.

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

A twenty pound note.....:)

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!

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?

Thing:

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

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

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

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!

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.

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.

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.

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.