How to use carburrettor heat?

[GoneWest]

Just read a flight safety item in an American magazine...which taught me something I never knew about carb heat.

<<About 8% of the verified engine failures were due to carburetor ice, often when the pilot ignored the need for carb. heat, but sometimes when the carb heat was used properly.

Aircraft certification requirements state that a carb-equipped engine must be able to take 30-degree incoming air and deliver it to the carburetor at 120 degrees, which is hot enough to melt carb ice and prevent more from forming. However, the certification requirements allow the manufacturer to use 75 percent power to heat the air>>

So, when pilots drag on the carb heat because they have decided to close the throttle and enter a glide (way below 75 percent power) - it may all be a waste of time and not producing enough heat to do any good.

Could be scary in the wrong (or "right") conditions.

Any comments from pilots or instructors? Any examples?...or anybody disagree?

[Flying_Steph]

People have already lost their engine in hot & sunny Arizona during an idle descent... most probable cause of the failure: icing !
:D

It might look a bit "overkill" to use the carb. heat when the OAT is 42°C, but it's better to be safe than sorry.
;)

[Polar_stereographic]

Good airmanship in piston engine aircrafts in my books includes planning the decent in such a manner that power is NEVER reduced to idle at any time during the decent.

In a fixed pitch single, I never reduce RPM below 1800. Saves shock cooling, ensures the engine is kept up to temperature (and producing heat for carburretor), prevents plug fouling, does not frighten any unfortunate passengers to mention but a few benefits. In most cases, it simply means starting the decent a bit earlier.

Enjoy the flying.

PS

[fireflybob]

Subject to any unique engine characteristics, carb heat should be selected before closing the throttle because by doing so you are utlising more "heat" from the engine as it is then running at significant power setting.

On a glide descent the engine should be "cleared" every 1,000 ft or so, to purge the plugs of any lead deposits and also check for normal engine operation. Before the engine is "cleared" carb heat should be selected off and then selected on again before closing the throttle.

If you stick to this procedure you should stay out of trouble on a glide descent.

[Keith.Williams.]

The crux of this quesion appears to be "when the engine is running at low power in a descent how can it produce enough heat to prevent carb icing?"

The short answer is that it cannot.

The thing to rememeber is that most carb heat systems draw their heat from the exhaust manifold. In a long descent the exhaust manifold will cool down, eventually making carb heat ineffective. This can be prevented by increasing power periodically to warm up the exhaust manifold.

[Vandelay]

At what temperature would you consider not turning on the carb heat for low power settings?

Also, this is a really novice question, but I don't have much experience in non-fuel injected singles... if carb ice forms due to the fast airflow, then wouldn't high power settings make it more likely for icing? Why is carb ice more likely to form at low power settings/below the green arc?

Thanks

[Keith.Williams.]

The standard manifold air temperature gauge has the following four colour bands:

Red. This smeans the air is too hot and there is a danger of detonation. Carb heat must be turned off.

Green. This means that air temperature is within safe limits. The air temperature is such that neither icing nor detonation are likely. If the carb heat is on you must leave it on or you might go into the yellow band. If it is off you must leave it off or you might go into the red band.

Yellow. This means that icing is probable. The carb heat must be turned on to get back into the green band. If you are in the yellow band and heat is already on it might be beneficial to turn it off to go into the white (uncoloured) band.

White or uncoloured. This means that the air is too cold for carb icing. If the heat is off leave it off or you might go into the yellow band. If it is already on then the OAT is pretty damned low and you should probably be reassessing your reasons for flying today!!!

In a float type carb system the air temperature in the inlet manifold is reduced for two reasons:

Firstly the choke tube is a venturi. It is intended to reduce static pressure to draw in the fuel, but it also reduces temperature (static pressure and temperature tend to go hand in hand).

Secondly in order to evaporate, the fuel draws heat from the surrounding air (latent heat of evaporation. This cools the air.

Both of these factors cause the air temperature to reduce and if it gets low enough and there is sufficinet moisture in it, then ice will form.

In fuel injected systems the fuel is injected into the inlet ports within the (very hot) cylinder head. This elimates the problem of cooling due to evaporation but not cooling due to acceleration of the air in the manifold.

The magnitude of the pressure and temperature drop caused by the choke tube is proportional to the degree to which the air accelerates. At low power settings the gaps around the throttle valve are very small so the acceleration around it is greater. This high acceleration causes a big drop in static pressure and temperatre.

The danger of icing is therefore greatest at low power settings.

This whole subject was publicised by the CAA some time ago in AIC 133/92 (Pink 68). Most of the JAR questions and FTO notes on the subject are based on this document.

[slim_slag]

Flying_steph


People have already lost their engine in hot & sunny Arizona during an idle descent... most probable cause of the failure: icing !

It might look a bit "overkill" to use the carb. heat when the OAT is 42°C, but it's better to be safe than sorry.


I've only come across unexpected carb ice (or what I thought was carb ice) in AZ when flying low through the moisture laden air of the Colorado over by Blythe. 42°C is a bit on the high side for carb ice, I'd suspect something else first like over lean mixture or fouled plugs. I guess its more theoretically possible if you use Mogas, but not totally impossible.

I agree with you that you should always have carb heat in your mind because you certainly can get complacent in AZ. I remember coming home to the UK and being checked out at Shoreham after having spent several months flying in AZ. I nonchalently did the AZ standard 'carb heat on, yes 150 drop, carb heat off' pre takeoff check, and promptly and correctly had my wrist slapped by the instructor. I was made to put carb heat on again and watch the tach for the next minute. Yep, all of us have something to learn in every session of dual.

Alas they still teach not to use carb heat in AZ. I suspect they don't want the students to forget to turn if off and so get dusty unfiltered air into those expensive engines. Hopefully these students get their wrists slapped by the instructors when they head to damper climates.