carb heat and power reduction

matt152

Well-Known Member
Cessna 172: I was taught to put carb heat on prior to any power reduction below the green arc.

The school I am instructing at has standardized pulling the carb heat AFTER reducing power.

This made me think about why I was taught carb heat first. My understanding is that turning carb heat on prior to power reduction is preferred because of the large pressure drop in the carb when closing the throttle. This pressure drop contributes to an increased likelihood of carb ice. Also, you want to put carb heat on when the exhaust manifold is nice and hot. If you use carb heat after a power reduction, the exhaust manifold may be cooler, or you might forget to use it at all.

Question: Am I wrong? Does it make a difference? Should I just accept the way the school has standardized this procedure or try to change it?

Thanks
 

tgrayson

New Member
Question: Am I wrong? Does it make a difference? Should I just accept the way the school has standardized this procedure or try to change it?
An added reason is that during cruise, you may have already iced up the idle ports inside the throttle plate; closing the throttle could result in instant engine failure. I add carb heat for 10-15 seconds before reducing power in order to melt any pre-existing ice.

I see no advantage to the school's procedure.

BTW, I have a NACA (NASA) report mentioning the idle port as the reason low power settings are so susceptible to carb icing. Perhaps you might use this report to convince your management?
 

MidlifeFlyer

Well-Known Member
I'd be surprised if it makes any difference if the carb heat is pulled 2 seconds before throttle reduction or 2 seconds after.

For an operational standpoint, I think there's something to be said for moving the throttle as the first response to a need for more or less power and leaving the "clean up" for after you've flown the airplane.
 

matt152

Well-Known Member
An added reason is that during cruise, you may have already iced up the idle ports inside the throttle plate; closing the throttle could result in instant engine failure.
That's interesting. I had not heard that previously, but it makes sense.

Sure, I'd enjoy reading that report if you have it handy.
 

matt152

Well-Known Member
For an operational standpoint, I think there's something to be said for moving the throttle as the first response to a need for more or less power and leaving the "clean up" for after you've flown the airplane.
I would agree with this in cruise flight.

But if a large power reduction is needed, if the thottle was set to say 1700 rpm and then the the carb heat was turned on, it would require increasing the throttle to maintain 1700. Not a big deal, but I think it's easier to get a primary student to just pull the carb heat and set the throttle once.
 

amorris311

Well-Known Member
in my hours of sitting in carb engines i have come to think that high power settings are more suseptible to carb icing due to the amount of airflow in the engine. the float type carb is exactly like a wing and as we all know as air velocity increases the temperature decreases giving us a higher probability of carb icing. now i have only really flown in southern arizona and it is not really a problem but im sure in cool humid conditions you would see this happen. just my two cents. i get bored sometimes on cross countries. :)
 

dpgtime

Well-Known Member
Carb heat pulled before the green arc is what i learned.......also the engine should be leaned when carb heat is pulled do to the enriching of the mixture.
 

ozone

Well-Known Member
if everyone would just quit flying carb-outfitted airplanes this wouldn't even be a discussion. :). In all seriousness: all the planes i am learning on are fuel-injected. At some point should i worry about getting a few training hours in a carb-fitted plane?

My dream plane is a diesel powered diamond DA50 magnum.....but i first have to win the lottery.
 

MidlifeFlyer

Well-Known Member
if everyone would just quit flying carb-outfitted airplanes this wouldn't even be a discussion. :). In all seriousness: all the planes i am learning on are fuel-injected. At some point should i worry about getting a few training hours in a carb-fitted plane?
Only in the sense that any change or addition in procedures requires relearning to some degree, whether it be moving from fuel injection to carb, from a high wing without a boot pump to a low wing with one, from a fuel selector with a "both" setting to one where you have to switch, from...
 

ppragman

FLIPY FLAPS!
I would agree with this in cruise flight.

But if a large power reduction is needed, if the thottle was set to say 1700 rpm and then the the carb heat was turned on, it would require increasing the throttle to maintain 1700. Not a big deal, but I think it's easier to get a primary student to just pull the carb heat and set the throttle once.

Try not to do that. You want to avoid large changes in anything, power, pitch, altitude, airspeed, bank. Try to be gentle. Who says you can't start taking power out a little bit earlier? Gradual = good. If you have to make a large change, something is wrong.

I know every flight instructor says "all right, once you're abeam your touch down point, you should pull the power to 1500RPM. Then add you're first notch of flaps. Ok, now you're on base, you can add you're second notch of flaps, ok now you might have to put a little power in because we're heavy/its hot/etc., ok now you can...." No. Just be really gentle, and fly more power on approaches. The only time you really need to make a rapid power adjustment is to practice power off 180s. Then, I would say, pull your carb heat first, because that's going to give you heat while you know the engine is still hot (not that it would really have a chance to cool off, but I've seen some pretty fast carb ice on cool, damp fall days). Plus, the effect of carb heat is going to be most noticeable at high power settings, so you'll be sure that its working (sometimes the cables break, or a host of other problems) and it would be good to know if you might not be able to get rid of it before you pull the power, rather than after you pull the power when the addition of carb heat might not be as apparent.

Just my .02
 

Fly_Unity

Well-Known Member
I always pulled Carb Heat before power reduction but a mechanic once told me that putting carb heat on at high power settings (full power) can be hard on the engine, so now I usually pull power to around 2000 RPM, add carb heat, then down to 1500 RPM or whatever is needed.
 

tgrayson

New Member
mechanic once told me that putting carb heat on at high power settings (full power) can be hard on the engine
I wouldn't rely on a mechanic for much about operating procedures. For the brief time you're talking about, the extra heat isn't going to have much of an impact on the engine, particularly since you aren't talking about full power in the downwind leg and the airspeed is relatively high. The procedure you describe seems needlessly complicated.

Another area that people are overly concerned about is during go-arounds. Lycoming says in the O-360 Engine Operators Guide:

In the case that full power need be applied under these conditions, as for an aborted landing, the carburetor heat should be returned to "Full Cold" after full power application.
I see people delay applying full throttle, reaching first for the carb heat control, which results in an unnecessary delay in the maneuver.
 

ppragman

FLIPY FLAPS!
I wouldn't rely on a mechanic for much about operating procedures. For the brief time you're talking about, the extra heat isn't going to have much of an impact on the engine, particularly since you aren't talking about full power in the downwind leg and the airspeed is relatively high. The procedure you describe seems needlessly complicated.

Another area that people are overly concerned about is during go-arounds. Lycoming says in the O-360 Engine Operators Guide:
In the case that full power need be applied under these conditions, as for an aborted landing, the carburetor heat should be returned to "Full Cold" after full power application.
I see people delay applying full throttle, reaching first for the carb heat control, which results in an unnecessary delay in the maneuver.
:yeahthat:

When I do a go around in a 172 or something, I simultaneously apply power and move carb heat to cold (with my thumb I push the lever in), however, in a balked landing power 1st, carb heat second.
 

MidlifeFlyer

Well-Known Member
I see people delay applying full throttle, reaching first for the carb heat control, which results in an unnecessary delay in the maneuver.
That may be a good reason for power first during reductions - consistency in procedure. The guy who's delaying may well be thinking about which one he's supposed to do first. "Power first, then carb heat" universally applied solves the problem.
 

Fly_Unity

Well-Known Member
I wouldn't rely on a mechanic for much about operating procedures.

He also has over 20,000 hours flying the bush with about 10,000 hours dual given.

Im not sure either way, but it does seem to me that with full RPM that carb heat does put quite a "sudden" heat change on the engine.
 

tgrayson

New Member
He also has over 20,000 hours flying the bush with about 10,000 hours dual given.
Not really any better. People are either knowledgeable on a subject or they are not, and I've never seen much connection between that and flight time, unless it's an inverse one ;). Now if you told me he was a mechanical engineer that worked at Lycoming or Continental, then I'd sit up and take notice.

Answer me this:

Why can I stick my hand in a 400 degree oven to stir something without having it shrivel up and turn black?

The reasons are this:

  1. The mass of the air in the oven is low and the mass of my hand is large, much larger than the air.
  2. Air is a poor conductor of heat.
  3. The time my hand is exposed to the heat is short.
All of those things means that the heat actually absorbed by my hand during a short stint in the oven is very small, and likely due more due to the radiation of the coils than the air itself.

The same thing applies to the carb heat. The mass of the hot air flowing into the engine is very low compared to the mass of the engine. Since air is a poor conductor of heat, and the length of time you actually have the carb heat on is very short (5-10 seconds), the quantity of heat transferred to the engine is very small.

Where you're more likely to have a problem, and this is probably what your friend was really concerned about, is during full power climbs at low airspeeds, where you may already be close to detonation limits of the airplane due to the high power and lack of airflow through the engine.

And lastly, according to 14 CFR 23.1093, the carb heat system only needs to produce a temperature rise of 95 degrees F, which is about 1/4 the temperature of the air in my oven.
 
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