Carb heat...again!

[Whirlybird]

And you thought all the possible carb heat questions had been asked! Well, if they have, I've forgotten the answers to these....

1) As many of you know, the C150 is particularly prone to carb icing, especially in the sort of warm, damp weather we've had recently. Ours has shown evidence of it before takeoff, straight after levelling off after take-off, during the cruise - almost any time except during the climb with full power applied, in fact. As a result, we use carb heat extremely frequently, and if flying near the coast, or over mountains where the options are limited in the event of engine failure, we fly with it on all the time. This meant that last week, dodging low cloud around the coast from Sleap to Caernarfon, we flew almost the whole route with full carb heat. Now, we all know you're not supposed to use partial carb heat. But in that sort of situation, why not? It would seem logical to apply full carb heat at around five minute intervals for about 30 seconds, which you tend to do in a C150 anyway (not that this is an absolute rule, but it seems to make sense), but then to push it in partially - to save fuel and have a bit more power. Could we do this? And if not, why not?

2) Having the carb heat on all the time like this uses more fuel. But how much more? Since our fuel gauges are of course hopelessly inaccurate, what percentage more fuel are we likely to be using if we have full carb heat applied except when climbing out from the airfield (or occasionally climbing for any other reason) during a cross country flight?

3) There are two schools of thought about use of carb heat in the circuit. One says that you apply it when you reduce power on base, and then leave it on. The second says that you put it off at around 200 ft on final, so that you have full power if needed for a sudden go-around. I always thought this was down to personal preferences, or often to instructor's preferences or what you were taught. I'm now wondering if it should be different in different aircraft. Certainly in the C150 I'm not pushing the carb heat knob in at 200ft; I'm quite sure it could get carb ice in that time. A friend who instructs on C152s says they always teach carb heat on and leave it on. I can't find anything in the POH about this, but does it vary for different aircraft? Or is it just down to common sense and personal preference?

Jeez...roll on fuel injected engines for all aircraft!

[Sunfish]

Have not had problems with carb icing in a C150 except on the ground with a relatively cold engine. No problems in the air.

Carb heat on for practice forced landings, all approaches and stalls. On approach, carb heat off at 300 feet. Temperatures down to about 10 C. Humidity not more than about 60 percent I think.

I don't know for sure, but I suspect the C150 has a relativley big engine which is strangled with a small carb to give 100 HP. This would mean that the pressure drop across the carb is going to be bigger (hence more icing potential) than for other engines.

Please feel free to correct me.

[LowNSlow]

All the small Continentals A-65/75/85/O-200 are more prone to carb icing than their Lycoming cousins. It's down to the location of the carb as I understand it. I thought it was acceptable to apply partial carb heat for cruise settings preferably accompanied by a carb ice / temperature gauge.

I used to apply carb heat every 10-15 minutes or so when I had a Continental engine in front. Seemed to keep the carb ice at bay. I don't know why your engine seems more prone Whirl. Maybe it's time to whisper sweet nothings at your pet engineer

[Sensible]

Right or wrong, when in the cruise, I check the rpm and only pull the carb heat when the rpm drops or I notice the engine note change. Of course in the circuit and when reducing engine power etc the carb heat comes on regardless.

Could it be that you are anticipating carb ice and pulling the carb heat rather than reacting to known carb icing? usually it's only in really damp air where carb ice is a real problem and carb heat is necessary every few minutes.

[justinmg]

The author of "The Killing Zone", which is an analysis of significant flight safety factors, advocates the use of constant carb heat when cruising in high risk conditions (high humidity / precipitation).
Experienced and diligent pilots are still troubled by carb icing. With the consequences being potentially so serious, Whirlybird raises the most important question. What are the penalties for engine and fuel??

I would imagine the power loss can be guesstimated being roughly equivalent to rpm lost.
I suppose the hourly fuel burn is about the same, but you just go a tad slower.
If using carb heat in the cruise gives you an overly rich mixture, does that meaning leaning would be more important?

I have heard a number of well respected pilots who think constant carb heat in the cruise is reasonable practice.

Awaiting further guidance,

Justin

[bookworm]

2) Having the carb heat on all the time like this uses more fuel. But how much more?

It doesn't necessarily use more fuel. If you lean the engine properly for the power setting with carb heat applied, the specific fuel consumption will be the same as (or very similar to) with carb heat cold.

[bar shaker]

Whilst fuel injection reduces the risk of carb (or throttle body) icing, it does not prevent it completely and virtually all injected engines also have throttle body heaters. This is because the pressure difference over a throttle butterfly will induce icing, albeit only in severe conditions.

The solution for carb'ed engines is simplicity itself. Carb heaters are electric or water jackets that go around the carb barrel and heat the body of the carb so the ice cannot attch itself. Its the choking action of ice build up that reduces the power. By heating the carb body and not the air, the power loss is virtually immeasurable.

Age old carb air heaters are responsible for their own fair share of accidents and an update to carb body heaters is long overdue.

[stiknruda]

BS writes - virtually all injected engines also have throttle body heaters.

I assume that you are referring to road vehicle injection systems?

I've installed a couple of injector set-ups (one a standard Bendix, the other Airflow Performance) on Lycomings and have not been aware of any hot water plumbing or lecky connections!

I believe that induction icing in an injected aero-engine is unheard of - prove me wrong and I'll happily get the beers in.

Stik

[FlyingForFun]

Stik,

Many fuel-injected engines have an "alternate air" lever. I've only got a few hours on these engines, so I never really got to the bottom of this alternate air, but the way it was explained to me when I first got checked out on the C172 Millenium was "it's a bit like using carb heat, except that it's not heated". I took this to mean that inlet icing is still a possibility, and that the alternate air would overcome this at least in the short term. But from your post, I'd guess you don't agree with that? If that's not the case, what is the alternate air actually for?

FFF
-----------------

[smarthawke]

FFF

The 'alternate air' is really there to provide air in the case of a blocked filter (which may be due to impact ice for instance when flying in snow). Some aircraft have an automatic opening flap that just gets sucked open by the pressure drop, others have a pilot operated control.

Injected engines don't suffer from carb icing as its the cooling effect of the fuel through the carb venturi which allows the moisture in the air to freeze.

Continentals (as in the C150) have a much better carb heater system than Lycomings in a C152. The C150 has two silencers and a jacket around each. One provides hot air to the carb the other to the cabin for foot warming duties. The Lycoming set up is an open box which draws air over one exhaust down pipe.

The Continental system keeps a large amount of air in contact with a relatively large (compared to the Lycoming) area - the whole silencer.

Asa for how long you should leave carb heat on hot for - take an ice cube and play a hot air gun on it or poor boiling water over it - they're tough little things to destroy!

Read the AAIB reports and you will see how often aircraft 'land out' because of icing - that said, used often and for long enough, carb heat should prevent the onset of terminal ice.

As for waiting till the rpm drops. Remember that as the rpm drops, the exhaust cools and so does the temeperature of the air therefore reducing the effectiveness of the air to melt the ice - vicious circle time. When decending select 'hot' before reducing the rpm to ensure its clear of ice to start with.

Don't know why people insist on selecting 'cold' on final, if you have to go around push it in before opening the throttle - very easy on a C150, just stick your thumb out to collect the carb heat as you open the throttle!

I have yet to see an injector throttle body heater on a Lycoming or Continental - I stand to be corrected on that. Some Rotax engines have electric heaters on the carbs to get rid of carb icing.

Read the POH and abide by that is always a good place to start - there may be a specific comment regarding the use of partial heat - sometimes a bit of heat will bring the temperature to the right level to induce icing.

Whirly, isn't there some funny thing about R22s and use of carb heat?

Be aware, be very aware - or fly injected! Airflow Performance rocks.......

[Whirlybird]

LowNSlow,

It's not just our C150 that has the problem, it's C150s in general. I remember the first time I flew a C150. I assumed it would be like a C152, and was amazed and outraged when I got carb ice while taxiing and waiting to take-off; I'd never had that happen before. Someone showed me why, and it's to do with the difference between the engines of the two types of courwe, but not having a memory I can't remember now. But I don't think our C150 is worse than any of the others.

[bar shaker]

Stik

Yes I was, and that doesn't actually surprise me at all. Car manufacturers seem to take icing a bit more seriously than aircraft makers.

[Capt. Manuvar]

I'm currently flying in Canada and they have a different perspective to the use of carb ht in the circuit.
In cold temperatures, Carb ht is left on until full power is applied again(in a go-around or touch and go) then carb ht is set to cold. The engine will give a better response. It is also easier on the engine, I believe it's the same reason we wait for the engines to warm up before run-up or takeoff. In -50 deg canadian winters applying full pwr to a cold engine in not a good thing.
Safe Flying
Capt. M

[englishal]

I suppose there is always the risk of "forgetting" which is probably why POHs probably specify taking carb heat off on short final. The other worry I suppose, which is no doubt miniscule compared to carb ice, is the chance you could get a blockage close to the ground. There tends to be more bugs / dust floating around in the first 100' and if you do get something in the carb.....

Still, I'd be inclined to leave it in all the way down.....

[Send Clowns]

As I understand it, there are two penalties to using carb heat: maximum power is reduced, and in most cases the induction air is unfiltered. In most cases these are not relevant at the cruise. As far as I can work out if you lean correctly there should b no increase in fuel consumption, and while in the cruise at low level full power is not required. Therefore it is only above full-throttle height for required cruising power that there is any penalty for using constant carb heat in the cruise.

However, the only aircraft in which I have ever suffered carb icing in the cruise is the Cherokee 140, and then only in poor visibility, so on a clear day, keep cold and just check during FREDA checks.

As for the final approach, I personally think it is unimportant. Icing below 300' on approach is neither likely nor catastrophic (should be within glide range by the time it could stop the engine), and on the go around you should be checking carb heat anyway.

[pondlife]

Refering to Whirlybird's original questions (what are you doing flying fixed wings anyway?) : -

1. Yep, Continental engined C150s are more prone to carb icing than Lycoming engined 152s. This is mainly due to the different positioning of the carb.
It's generally not reckoned to be a good idea to use partial carb heat because this can potentially take the air temperature into a range where carb icing is more likely rather than less. Carb icing is most likely when the temperature drop in the carb takes the (humid) air to zero degrees C or very slightly below. If it is well above zero then it will not form ice and if it is well below zero then the ice is less likely to stick to the carb venturi. Full carb heat is nearly guaranteed to make the temperature well above zero in all conditions but partial heat can, in some circumstances, warm the air to the point that the ice is more likely to stick.

2. Having the carb heat on limits the amount of power the engine can produce and in most small aeroplanes (including the C150) allows unfiltered air into the engine. It doesn't, in itself, cause more fuel to be burnt unless you are not leaning properly.
That the air is unfiltered is unlikely to matter when you're high up and away from the dust and debris at ground level

3. I think that this is a common sense type problem (and no - you're not likely to find guidance in a POH). You'll also find that some pilots and instructors are adamant that there's only one right answer, which is strange because the one right answer varies from one instructor to another. I think that you should assess the risk for yourself and decide wether you are more likely to come a cropper from an engine failure on final approach or from a potential slight delay in having full power available on the go around. Personally, with a Continental engined C150 I would consider the possibility of an engine failure at 200 feet to be far more serious than a half second delay in having full power available to go around. When I'm flying a Lycoming engine though my views differ a bit.
You don't, of course, have to turn off the carb heat before applying full throttle to go around. If you do the carb heat afterward then you still got a lot of power during that half second.

[Whirlybird]

pondlife,

Thankyou; that all makes a lot of sense. Your comment about partial carb heat making icing more likely rings a bell...I have heard it before, but had forgotten...really wish I had a memory.
I quite agree about leaving carb heat on right to the ground on final; maybe you could glide that last 200ft, but if the reason for putting in the carb heat is that you might need to go-around.... I'm sure you all get my point.

As to why I'm flying these strange machines with non-whirly wings - they're cheap, and I get to do what I want, not what my students need. And it's an interesting change having to land and then slow down rather than the other way round.

Oh, and please, no comments from anyone saying as an instructor I should know about carb heat; in the R22 I do, but it has a carb temp gauge and you can use partial carb heat so it's all rather different. When it comes to f/w, I'm an ordinary PPL(A) who doesn't fly enough or know enough or get enough practice.

[bookworm]

It's generally not reckoned to be a good idea to use partial carb heat because this can potentially take the air temperature into a range where carb icing is more likely rather than less. Carb icing is most likely when the temperature drop in the carb takes the (humid) air to zero degrees C or very slightly below. If it is well above zero then it will not form ice and if it is well below zero then the ice is less likely to stick to the carb venturi. Full carb heat is nearly guaranteed to make the temperature well above zero in all conditions but partial heat can, in some circumstances, warm the air to the point that the ice is more likely to stick.

I've seen this asserted previously but I don't think the physics makes sense. Temperatures that are too low for ice are too low because there is not sufficient water vapour in the air for carb icing to be significant (in other words, the dewpoint is necessarily too low).

Heating the air, partially or fully, does not add water vapour. The only circumstance in which this might be an issue is when flying in glaciated cloud, where the air contains ice particles. If the ice particles are melted, they may refreeze in the induction system, hence it's better to let them go through as ice. But barring flight in glaciated cloud, I can't see how heating the air can do much harm.

[BraceBrace]

Whirleybird,

If the world of pilots would know everything there is to know about carburator heat, the why's and the when's, then nobody would be talking so much about it, you wouldn't find safety leaflets about it, and at the end of the story... crashes due to iced up carburators would be almost non existent.

I've found this to be a very interesting topic, since I'm not flying a C150, but I'm about to fly one for the same reasons as yours (it's cheap!). Better safe than sorry!

[Brooklands]

In relation to the discussion about going around with the carb-heat set hot, didn't the RAF permanently wire-lock the carb-heat on all their Chipmunks to hot?

Brooklands