Carb Heat: What's the deal?

[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 problem

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.

[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.

[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]

Hi

Vee-tail-1

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