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Carb Heat

Old 29th Nov 2021, 14:11
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Carb Heat

Elsewhere, I posed some questions about carb heat, to offer a new poster some research points. I'm not sure if the poster has taken up the challenge, but it's been a few days, so I owe some thoughts to the points I raised. Everyone is eagerly encouraged to offer their thoughts/experiences/exceptions to these points:

Some GA planes have a carb heat some don't - which are which and why?

Airplanes with fuel injected engines do not have venturi carburettors, so are naturally immune to carb ice formation.

What is the relationship to carb heat and alternate air?

Alternate induction air is a design requirement for certified planes. The carb heat doubles as the alternate air control. Alternate air is not filtered (by design requirement), and usually drawn from within the cowling.

Do planes which do not have carb heat still have alternate air?

Yes, it's still a design requirement.

Do planes have an alternate air control if not a carb heat control?

Some planes have an alternate air control, others have a spring loaded door, which pops open if the manifold pressure demands airflow and the main induction is blocked

What information is necessary for you to apply partial carb heat?

If you have a carb air temperature indicator, you can use it to apply partial carh heat, to just melt ice. Otherwise it is recommended to apply full carb heat, if you need to use it.

If you apply carb heat for a rough running engine, and the engine then runs worse, what should you do? Why? For how long?

When you apply carb heat, you're melting any ice which may have accumulated in the carburettor. As it melts, the water will enter the engine, and the engine may run worse. Keep the carb heat applied at least until the engine continues to run smoothly, though you may want to leave it applied for the rest of the flight in those conditions.

If you apply carb heat, and it's not enough, what two engine controls can you adjust to get a little more carb heat? Adjust how?

Leaning the mixture may increase the exhaust gas temperature - so hotter carb heat air, and closing the throttle a little will reduce the amount to [moisture containing] air entering the carb, so reduce the demand for heating.

When might you need to urgently apply carb heat, but have not encountered carb icing conditions?

If something else blocks your main induction air filter

Why should you never use carb heat while taxiing?

Carb heat is unfiltered air, so any dust, grit, etc. will be ingested by the engine.

If your plane is equipped with an indicator associated with carb heat, what is that indicator?

A carb air temperature indicator will show you the temperature at the venturi.

What does the yellow arc on that indicator mean? Where will the yellow arc be on the scale? Will there be any green or red lines on that indicator?

The yellow arc on the carb air temperature indicator is the icing range, so avoid continued operation of the engine in that range. Applying carb heat will heat out of the range (in most cases).

Will a piston engined airplane which does not have a carb heat control always have an electric fuel pump? Why?

If a piston engine does not have a carburettor, it is fuel injected. Fuel injection systems always require pumped fuel pressure.

Could an airplane which does have an electric fuel pump also have a carb heat control?

Yes, you can have fuel pumped carburettor equipped planes, so understand your systems!

Where does the heat resulting from the application of carb heat come from?

The heated air comes from a muff around the exhaust system/muffler

If you have had an engine failure, will applying carb heat help you get it started again? Why?

If your engine has stopped as a result of carb ice, you will no longer have exhaust heat to heat carb heat air, so getting it running by normal means is unlikely - start looking for a forced landing place!

What would be a practical warm outside air temperature where you would be unconcerned about carb icing?

I've seen charts up into the 20+ C temperature range, it's not just a cold day thing...

What other atmospheric factors will affect carb icing?

The moisture content of the air

Can you get carb icing on the ground?

Yes, so if in doubt, a brief application of carb heat prior to takeoff, and then assure expected full power is available before committing to take off.

If you're flying in very well below freezing air, is carb heat a risk?

Much less so, as moisture in very cold air will pass through the carburettor venturi frozen, so not accumulate there.




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Old 29th Nov 2021, 17:04
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The smaller-horsepowered Continental carburetted engines (A-40 to O-300) will suffer carb ice more often than similarly-powered Lycoming engines due to the differences in their construction, and the attachment of the carb to to the engine. So don't put the carb heat to cold at 300 ft AGL on final approach in a C150

I don't know enough about Franklin engines of similar vintage to be able to make a comment.

Russ
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Old 29th Nov 2021, 17:58
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Ha Ha.... Not quite 100% there DAR.
c.f. Para. #7.
Some aeroplanes do not have alternate air.
e.g. UK and USA at least, Rans S6 series.
Most in the UK are classed as 'microlights' so perhaps you can wriggle out there, but the lusty S6-116 carburetted model definitely "an Aeroplane" !
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Old 29th Nov 2021, 18:13
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Some aeroplanes do not have alternate air.
e.g. UK and USA at least, Rans S6 series.
Interesting, are they certified? Alternate air is a certification requirement. And a fine idea. I did hit very narrow band of freezing rain in the 150 once, and both the windshield and air filter iced over instantly, followed by the engine stopping completely. I pulled carb heat for the alternate air, and got the power right back, as I turned 180, out of what I had thought was just rain!
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Old 29th Nov 2021, 21:06
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"So don't put the carb heat to cold at 300 ft AGL on final approach in a C150."
I was persuaded to do that on our Jodel. Landed at Ashaig, Skye. Opened throttle to turn and backtrack. Engine spluttered. Attempting a go-around would have been an accident.
Went back to carb heat until landed.
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Old 29th Nov 2021, 22:36
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Good thread.

Recently I was flight testing a Murphy aircraft with a Van's style air box / filter. The air filter is in a plenum, when carb heat is applied the intake to the plenum is blocked and the heated air intake opens. This means that if the filter ices up, there is no alternate air intake, the only option is to melt the ice. Checking the certification requirements that might be applicable to a similarly equipped certified aircraft it requires the carb heat to be able of generating a 100F rise. I've never seen a system that can deliver this much heat but we have a carb t probe on the way and we'll see what it can do.

If you are flying an amateur built aircraft, best check how the carb heat / alternate air works.
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Old 29th Nov 2021, 23:13
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Applying carb heat will heat out of the range (in most cases).
Flying our PA28-180 today, fitted with a carb temp gauge, showed -10deg C, near the lower end of the yellow arc. Applying carb heat only raised the temp to the top end of the arc, not out of it. Still, got about a 50RPM drop in revs, followed by the classic return to the original RPM 'no rise, no ice'.
You might have mentioned NOT using carb heat in very cold conditions, when doing so can put the inlet temp INTO the carb ice range, from a nice too-cold position.
We regularly get carb ice in the PA28 when starting from cold over grass, good demo to the student of what it looks like. Personally, I've never seen significant carb ice in a PA28-180, over decades. Now, in my C150, wow, a different story!
I think there are very few days in the UK when the conditions are outside the critical part of the graph.

Next thread - 'Use of Pitot Heat'.

TOO
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Old 29th Nov 2021, 23:51
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It's worth mentioning: that the venturi can reduce the temperature by up to 25 degrees centigrade, also that carb. heat enriches the mixture and therefore leaning is important when carb. ht. is to be applied for long periods.

Opened throttle to turn and backtrack. Engine spluttered.
The argument as to whether the carb heat should be maintained until touch down or not is an old chestnut. As many failures can be caused by the carb. ht. being left on too long and causing a rich cut instead. This can be particularly so during touch and go and a go around, and especially on hot days. Remember that when the throttle is increased an accelerator pump gives an extra squirt to minimize take up lag. I think that 300ft is too soon to go to cold air but below 200 ft is a good compromise. I suspect that 300 ft is rollover to do with multi-engine 'committal' heights than carb icing considerations.
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Old 30th Nov 2021, 00:26
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Originally Posted by Fl1ingfrog View Post
As many failures can be caused by the carb. ht. being left on too long and causing a rich cut instead.
We can put that one to bed. Carburetor heat induced rich cuts are an OPT (Old Pilot's Tale). Doesn't happen.
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Old 30th Nov 2021, 00:27
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Originally Posted by TheOddOne View Post
near the lower end of the yellow arc. Applying carb heat only raised the temp to the top end of the arc, not out of it.
Does your gauge have actual temperature markings? I'd be very interested to know where the yellow arc for this aircraft lies.
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Old 30th Nov 2021, 09:06
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One tip that I was given long ago was to leave the carb heat on for a minute or two (or even longer) during the runup so that the carburettor body would be warmer. The idea was that no ice would then be able to form during the subsequent minutes of the take off. I can't claim the idea or confirm that it works... any thoughts on this?
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Old 30th Nov 2021, 10:10
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Originally Posted by Jhieminga View Post
One tip that I was given long ago was to leave the carb heat on for a minute or two (or even longer) during the runup so that the carburettor body would be warmer. The idea was that no ice would then be able to form during the subsequent minutes of the take off. I can't claim the idea or confirm that it works... any thoughts on this?
Carburators are (were) made of pot metal so very conducive to temp changes.
Start with a hot carburetor body and within seconds after applying power youd have a cold carb again. I would put that one to bed.

Carb heat associated rich cut? Definitely.
Ive had two engine stoppages as a result of a too agressive application of power with carb heat on.
After that I started teaching the 3-second rule, 1-2-3 count from idle to full power or back to idle.
Carb heat OFF 1-2-3
Carb heat ON 1-2-3
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Old 30th Nov 2021, 10:23
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We can put that one to bed. Carburetor heat induced rich cuts are an OPT (Old Pilot's Tale). Doesn't happen.
Unfortunately it isn't and it does. Complacency though, "doesn't happen" should be guarded against, it leads to too many avoidable and tragic accidents: "there old pilots and bold pilots but there are no old bold pilots springs to mind". The Lycoming 0-235 engine installed in the C152 is very prone to running very rich being just one example. Rich cuts as with carburetor icing leaves little or no trace and therefore makes it very difficult to establish the cause after the event.
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Old 30th Nov 2021, 11:56
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Originally Posted by B2N2 View Post
Carburators are (were) made of pot metal so very conducive to temp changes.
Start with a hot carburetor body and within seconds after applying power youd have a cold carb again. I would put that one to bed.
My back-of-the-envelope calculations told me so too, but it's one of those things that you do as told by the instructor/examiner at the time but you don't really start thinking about it until later. As mentioned before, the temp increase from carb heat isn't all that much, so it is not as if you are heating up that carb with a blowtorch. Once you switch the carb heat off and go to full power, you've got the original -20C air going through it again and at a higher mass flow. As you say, it's not going to stay warm for long. If someone has a carb temp indicator and wants to try this on a cold day, I would be interested in the numbers.
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Old 30th Nov 2021, 14:00
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The carburetor heat systems are very varied in design and effect. The techniques applied are also very varied. I agree with the point made by B2N2 that the benefit from carb ht once set to cold is short lived.

The purpose of the power checks and vital actions at the hold is to ensure that the aircraft is fit for flight. Any alterations once the checks are complete can introduce a problem, so should be done with caution. I've lost count on the occasions that an additional application of carb. ht. is made and then forgotten so lining up with heat still applied. I am insistent therefore that the hand must not be removed from the control until it is put back to cold when this additional check is done.

How many pilots know the minimum static RPM for their aeroplane with the throttle fully open. It is a much more valuable check when done against the brakes immediately prior to the T.O. roll.

Last edited by Pilot DAR; 30th Nov 2021 at 14:27. Reason: spelling error
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Old 30th Nov 2021, 14:31
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The minimum static RPM for many fixed pitch propeller planes is stated on the type certificate data sheet as a limitation (meaning the plane is not fit for flight, if it cannot produce that RPM).

I am not a proponent of full power against the brakes, unless absolutely necessary, and personally, aside from maintenance power checks, I have never found it necessary. Propeller damage can be serious with full power runs over a loose surface. I like to do my flying power checks as a quick confirmation of the tachometer reading early in the takeoff roll - with the nose being held light.
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Old 30th Nov 2021, 15:05
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I like to do my flying power checks as a quick confirmation of the tachometer reading early in the takeoff roll - with the nose being held light.
Knowing the RPMs is the surest check that the power required is being achieved and I agree that you can do this on the roll.
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Old 30th Nov 2021, 18:41
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Originally Posted by Jhieminga View Post
One tip that I was given long ago was to leave the carb heat on for a minute or two (or even longer) during the runup so that the carburettor body would be warmer. The idea was that no ice would then be able to form during the subsequent minutes of the take off. I can't claim the idea or confirm that it works... any thoughts on this?
I had a look at some recent data and here's the longest application of carb heat I could find. It's in the air obviously as running carb heat on the ground longer than absolutely necessary is bad for the engine. You can see how quickly the carb t drops after the carb heat goes off. Half in the first few seconds, the rest in less than a minute.



Originally Posted by B2N2 View Post
Carb heat associated rich cut? Definitely.
Ive had two engine stoppages as a result of a too agressive application of power with carb heat on.
After that I started teaching the 3-second rule, 1-2-3 count from idle to full power or back to idle.
Carb heat OFF 1-2-3
Carb heat ON 1-2-3
I did not realize you were talking about a power application. It sounds to me like the culprit here is the aggressive application of power rather than the carb heat. Does the engine not have the same trouble if you slam the throttle open with the carb heat off?
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Old 30th Nov 2021, 19:47
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Yes, the fuel provided from the accelerator pump will be the same. The fuel supplied by the pump bypasses the carburetor so it adds to that supplied by the carb. In fact, by utilizing the accelerator pump you can prime the engine for starting and this is often the preferred way. In some types it is the only way of priming the engine to start. Amazing to me is that the hand primers fitted to the 150/172 series may only provide fuel to just one or two cylinders. In this case I always find that using the accelerator pump by pumping the throttle is preferable.
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Old 30th Nov 2021, 20:03
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"Y​​ou might have mentioned NOT using carb heat in very cold conditions, when doing so can put the inlet temp INTO the carb ice range, from a nice too-cold position."
Can someone explain this? "Too cold for carb ice" means the air at that temperature cannot contain sufficient water vapour to cause carb ice at the venturi temperature drop. Raising the temperature without a source of water will only reduce humidity further.
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