View Full Version : When's it too cold for Carb Heat?

The Flashing Blade
23rd Mar 2006, 12:47
As it's been hovering around the zero celsius mark these last few mornings I'm curious to know at what temperature it becomes better not to use carb heat?
Since the carburettor cools the air further would it be safer to not add carb heat than use it and raise the temperature above freezing only to refreeze any moisture inside the carb throttle body?

23rd Mar 2006, 13:24
:eek: Use the carb heat! Air temperature isn't the whole story...

Talk to your instructor next time you fly about this, but in the meantime, read CAA Safety Sense Leaflet 14 on engine icing. It should explain all. It's in LASORS - you can either borrow or buy a hardcopy or else download it from the CAA website:


23rd Mar 2006, 13:41
Generally, instructions given to pilots are to apply carburettor heat as required to keep CAT gauge needle out of the yellow arc. But notice how on many such gauges the arc stops short of the bottom of the gauge - this is deliberate because ice will not form on the butterfly in extremes of cold.

In the UK, it generally never gets that cold, but thats not to say elsewhere it won't. By adding heat in these circumstances you may well raise the temperature into the risk area of carburettor icing.

Follow the instructions in the POH, and if you fly a Robinson read Safety Notice 25

23rd Mar 2006, 15:59
Interesting article on carb heat research in this quarter's GASCO safety bulletin. Private company at Oxford and CAA conducting research into types of carb ice (butterfly and venturi) and how to prevent them. There is evidence that Carb Ice can cause "rich cut" on the engine - essentially the ice builds up in the venturi and cools everything down as well as causing venturi effect to speed up. So - too much fuel in the mixture because there's less air to carry it and the engine cuts.

They are also trying to see if they can certify a butterfly heater.......so sometime in the next 50 years that might get past the bollox of red tape we have in Euro certification at present.

And the other thing they say in the article (sorry - no link) is that the warmer months of the year are more likely for carb ice in the UK......but it still happens in the colder months. It needs humidity - so the very cold air of the past month would probably not have produce carb ice. But they warn about flying too close to clouds where carb icing conditions are more likely.

Impress to inflate
23rd Mar 2006, 16:15
It was stressed to me by my old met teacher that water can remain in the atmospher in a liquid form down to temps down as low as -40C. This is on a sliding scale, i.e at -1c there is alot of super cooled water droplets in the atmospher and the colder it gets the less there is if that makes any sence. So use common dog and put the carb heat on, you know it makes sence.

The Flashing Blade
23rd Mar 2006, 17:19
The safety sense leaflets are good but make no mention of when not to use carb heat if it is severely cold. The R22 POH and safety notices don't even come close to answering this question. Can't find anything in the wagtendonk and coyle books either.

Surely there is some temperture where with carb heat the air temperature is raised into to danger arc?


23rd Mar 2006, 18:23
FB: don't get carried away with "temperature" or you might end up being carried away in bits from your 22. The physics of the carb are mightily complex because of the inter-relationship between air temperature, humidity, speed of airflow, temperature of the fuel, air pressure. There are 2 places where ice can form - the butterfly and the venturi. And the ice forms in those places for different reasons.

Icing on the butterfly is easily dispersed with a little heat, but venturi icing can be a major blockage. As I said above, the acretion of ice in the venturi means that the airflow is actually speeded-up - which you would think is a good thing. But the volume of the air is decreased, and so the mixture gets too rich and cuts the engine.

Butterfly icing is a little simpler - it freezes the throttle control.

I can't quite understand your interest in this from a flying point of view. Are you one of those whackos who experiments with leaning at altitude whilst flying your heli ?:oh:

Be better if you conducted some carb icing experiments on an old BSA Bantam or some other low-flying carbed beast.

23rd Mar 2006, 19:46
My two bobs worth:

have a look here: http://brumbyhelicopters.com.au/carbheat.htm


23rd Mar 2006, 19:49
That question is actually a very legitimate one.
As it was pointed out, if really REALLY cold, carb heat could put you right in the "icing zone". A rule I have seen and heard regarding Lycoming O- engines is no carb heat below -10C.
I dont know much about the whirly birds but most fixed wing dont have a carb temp gage, so it's either full hot or full cold. And during part throttle operation (descent), where the icing is more likely, there is just no way to know how hot the hot air really is after a few minutes (colder exhaust).
And below -10C, there should not be a lot moisture in the air anyway.
I stick to that rule and never had a problem so far... (and BTW, running at peak EGT during descent is also a recommended practice where I come from...)

23rd Mar 2006, 21:39
All good advice so far. Supercooled droplets in temps below freezing are almost certainly going to be visible. i.e. cloud and therefore airframe icing conditions exist, so you definitely should not be there in a Robbie.

In clear air at sub zero temps carb icing is unlikely, even with no carb heat. However, if you fly into a mass of warmer, damp air with your cold carb you are asking for carb icing. When I flew Robbies, on this basis I would always try to stay on the right side of the yellow arc. Headsethair is correct about the rich cut which will give very little warning before the engine stops. Trevor Thoms air law & met book has a good section on icing.

Better still, fly fuel injected.

23rd Mar 2006, 21:49
Gaseous - appropriate name for this discussion - it's NOT really all sound advice so far. PC-6 admits he/she is a fixed wing person. FW - no carb heat guage, whereas helis do. And whilst FWs have EGT guages, helis don't (generally)
It's only safe to say - look at the guage regularly, apply full carb heat as instructed by your POH and don't muck about trying to second-guess the designers. Of course I don't give a damn for your safety - just don't do your experiments over my patch!
It's all very well playing games in a FW - you get a second chance if the engine cuts at altitude. But you won't get that in a heli - the only thing that will get the ice out will be the impact with the ground as you get the auto wrong.

23rd Mar 2006, 22:30
PC6s advice comes from Lycoming and as he says, is fine for fixed wing. It is not however, appropriate for helicopters fitted with a gauge. No one said it was. I should have emphasised that.

Incidently, one is not necessarily a 'whacko' to lean a helicopter. It is standard operating procedure detailed in the pilots operating handbook for some piston helicopters.

Edit. I have found Lycomings article on this. 3 passages are relevant. (Flyer Reprints 1991-Related factors when using carb heat - page 47) I think these confirm mongoose237s post below but see my comments on SN25 below.

1) In known or suspected icing conditions follow the POH
2) At a temperature of 20deg F or below any moisture in the air is frozen and heat should not be used.
3) partial heat melts these crystals and they form carburettor ice when they come into contact with cold metal....

In short. I interpret this as: Keep the needle out of the yellow arc. If its above, keep it above. If its below, keep it below. This agrees with Section 4 of my old Astro POH says simply keep the needle out of the yellow.


SN 25 says the induction system should be warmed to above the yellow before flight and kept there using carb heat.

Does the SN override the POH? I think it might.
Clear as mud.

23rd Mar 2006, 22:31
I think this is a valuable discussion point. There is nothing to indicate that The Flashing Blade is intending on experimenting, merely a desire to know "what temperature it becomes better not to use carb heat?"

A fair point that deserves an answer.

Lets use Robinson as an example. The R44s s/n 201 and prior may well be marked with a yellow arc from -15C to +5C. The later models all had a yellow arc from -19C to +3C.

Note how they could have marked them up "+3C and below" but they didn't. This comes back to how readily different types of ice form, and adhere to metal surfaces, at different temperatures.

Now lets switch to page 4-12 of the R44 Raven POH - "it is recommended that the control knob be unlatched (to activate carb heat assist) whenever the OAT is between 80F (27C) and 25F (-4C) and the difference between dew point and OAT is less than 20F (11C)"

As a pilot, it is quite simple. Apply carb heat as required to keep CAT gauge out of the yellow arc, and apply full carb heat when there is visible moisture. If your aircraft is fitted with a gauge that reads incorrectly at low power settings, apply full carb heat whenever the manifold pressure is below 18".

So if you glance down at your CAT gauge and it is reading -23C, then adding carb heat would be in contravention to the POH as you would not be using carb heat to keep the CAT gauge out the yellow arc!

If you are flying around in the extreme cold, think Canada and Alaska, then yes the thought of it being too cold may well be valid. But in the UK it is unlikely!

We should be encouraging understanding in pilots, not just telling them, "that is what we do so don't question it"

Fly safe, carb ice is a killer

Brian Abraham
24th Mar 2006, 01:07
They are also trying to see if they can certify a butterfly heater

If I recall correctly the Rolls Royce Merlin of Spitfire, Lancaster, Hurricane, Mustang, Mosquito etc fame had such a set up. Ran engine oil through the butterfly. Sounds like the wheel is being reinvented 65+ years down the track doesnt it? Good idea though.

24th Mar 2006, 07:00
Forget the theory. When doing my FI course a few years back, in mid-January, I was shutting down the R22, and when I closed the throttle the engine stopped due to carb ice. The temperature was about -2C, with apparently very dry air.

I reckon you can get carb icing in ANY weather conditions that the UK is likely to throw at you!!!