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Engine cuts with carb heat


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Any experts on carb heat out there?

I recently got my '65 C model back from annual.  During a recent run up check, I applied full carb heat and it resulted in the engine cutting off.  I immediately turned carb heat off and the engine recovered normally. 

 

I opened up the cowling and inspected all the ducting to the heat exchanger and to the carb air inlet.  All clear. 

I checked the position of the carb heat flapper door in the air inlet (pic attached) and it closes all the way with carb heat applied.  Feels like the mechanic may have greased that cable and now I seem to get a little more throw.  As such, the door closes pretty tight.

 

My question is, is the flapper door suppose to remain open a little even with carb heat fully on?  The engine will only cut out as I apply the last quarter inch or so of carb heat.

 

thanks

 

Steve

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Ok, thanks for all the good ideas.  But here's a couple more pics and observations.  But previously my carb heat check during engine run up was always a 50-75 RPM drop, I have no carb temp gauge.

the second butterfly valve mentioned several times is connected to the small screen at the nose below the prop.  Isn't this an alternate air intake incase the air filter becomes blocked??

 

The other pic shows the carb heat cable fully retracted (carb heat on and flapper door in the full down position) and the second butterfly valve is closed too.  Looks like when the flapper door is closed, it allows heated air from the heat exchanger to enter the air intake from behind the closed door. To me, it only looks like the second butterfly valve is for alternate air (scat tube shown disconnected and you can see the valve closed with carb heat on).

Steve

 

 

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post-9464-0-24286800-1382499360_thumb.jp

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David, If the carb temp gauge shows a temp above freezing, then you don't have carb ice...unless the gauge is wrong! :o

 

My dad had a C for a while and would use partial carb heat to keep carb temp above whatever number he felt comfortable with. 37df seems to ring a bell.

 

Srozenzw,

 

You are looking at your system with the engine off. I suggest you check to see if there's enough play in any of the components to choke off the engine under vacuum.  Is it possible a hose is collapsing/kinking or a door/butterfly is being pulled closed by airflow while under vacuum?

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There's a secondary butterfly valve that vents the hot air. Do you know if that's working correctly?

 

I agree you need to look at the scat. This valve will not stop the engine. I had my butterfly valve for the dump fail and it didn't stop the engine, it just didn't give me as much of a drop. That's how I knew something was wrong. I would look at all the SCAT lines and check for damage or possibly a bird nest or something inside. This is the most likely answer. Hopefully it's just a kink! Goodluck.

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My '65 C had a challenge with that second butterfly. The small wire that operated it bent/broke...

Just another vote for getting it looked at to make sure it is working properly.

Carb temp is definitely a good idea. I only had ice once, in the summer. The solution of full carb heat was painfully slow....

Having the carb heat gauge gives better options....

-a-

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N78880 ( 65 "C" Model ) had a carb temp probe on the UGB-16 engine monitor and when I first had the monitor installed I was trully amazed at what the carb did, the temp swings it made during different phases of flight. One thing i was surprised at is how little OAT had to do with carb temp, so remember this, summertime or wintertime has little bearing on whether or not you can get carburetor ice. Its all about if the carb is cold and how much moisture is in the air and that doesnt take much either. I dont own 880 anymore but if I were to own a C model again I definately would invest in a way to monitor carb temperature.

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P

And I think your point is ... Just look at the gauge ... It will tell you if you need it...

Pretty much. Or....parish the thought...you could amend your check list to say "verify carb temp" instead of "carb heat on". If you have an instrument that tells you your carb temp is 52 degrees, it just seems silly to pull the carb heat. Rote behavior has its place in GA, but it really shouldn't take the place of analyzing the situation and acting accordingly!

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For those that currently have a way of monitoring carb temps... Do you tie it into your engine monitor or have a discrete gauge?

Also, when I enquired about adding it when I had my EDM 700 installed, they said my carb wasn't ported to accept a probe. Sounds like I need to get a second opinion.

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It's funny, a new airplane is a new airplane...bigger..smaller...heavier...faster...slower...whatever, you're busy when you have to think about everything you have to do...no muscle memory of how things should feel or where they are. It's challenging.  I am currently embarassing myself in my father-inlaw's Decathlon. I've got 2 hours of dual and 20 landings, 15 of which were unassisted all of which were ugly.  I have no swagger around his hangar at all. I listen, keep my mouth shut and maintain a low profile. He wants me to do a little X wind work before turning me loose.  I am nervous as hell about flying his airplane (been married 2 months) whether he's in it with me or not. 

 

So look on the bright side, you just have to get used to a new plane. Trying to get used to a new plane with a 15000hr+ AA Captain/Check airman in the back that happens to be your wife's father is far more stressful...I promise!

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It's funny, a new airplane is a new airplane...bigger..smaller...heavier...faster...slower...whatever, you're busy when you have to think about everything you have to do...no muscle memory of how things should feel or where they are. It's challenging. I am currently embarassing myself in my father-inlaws Decathlon. I've got 2 hours of dual and 20 landings, 15 of which were unassisted all of which were ugly. I have no swagger around his hangar at all. I listen, keep my mouth shut and maintain a low profile. He wants me to do a little X wind work before turning me loose. I am nervous as hell about flying his airplane (been married 2 months) whether he's in it with me or not.

So look on the bright side, you just have to get used to a new plane. Trying to get used to a new plane with a 15000hr+ AA Captain/Check airman in the back that happens to be your wife's father is far more stressful...I promise!

This is absolutely true. The bulk of my hours are in Decathlons and I feel they're absolutely pussycats to land. Everything comes easier with time and experience. For me, I'm still embarrassing myself in my Mooney. Maybe someday I'll know I've landed when I feel the tires rolling like the good old days in the Decathlon.

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Until, now I just turned on carb heat because the book said so ... No matter what that gauge said. I am realizing that I probably didn't (really) need it ... but now as winter approaches I probably will need it.

9901IcingChart_lg.jpg

Carb icing can be a factor in the summer too. 

 

http://www.avweb.com/news/eyeofex/182189-1.html?redirected=1

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Chris, you raise an interesting point (though I'm not sure if it's the one you meant to make). Which is a Carb temp gauge is a great tool, but one needs to understand what conditions are conducive to carb ice. If you're cruising along at 10,000ft and the CTG says -5df , that does not mean you should pull enough carb heat to get it above 40df! Freezing venturi temps are unavoidable in winter in some locals, but freezing temps does not necessarily mean ice. That's a cool graph, thanks for posting!

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With temp and dew point one can calculate RA. With RA and temp one can calculate dew point. The graph actually shows the RA at a given dew point and temp so you don't have to calculate... More importantly it shows the risky zones. I don't have that stuff memorized, about the best I could do off the cuff is to say that the risk of ice increases with RH. If I flew a carbed AC, I'd tape that graph to my kneeboard.

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The important thing to know...

Once it get's iced up, the engine dies and very little heat or air flow is available to melt the ice...

The owners manual does not spend a lot of time on this. In '65 not every plane got a carb temp guage. So the only safe way to operate in the traffic pattern was full on.

(1) having a guage is good,

(2) without a guage, full on is the logical choice.

(3) partial carb heat without a guage can lead to increased icing with little time or data to make a proper decision.

Why carb ice happens...

Pressure drop caused by closed throttle position generates a temperature drop. PV=nRT rules in this case. When moisture is present and air temp is dropped below freezing, icing can occur.

I suppose jet operations and training don't mention carb ice. Neither does fuel injected mooney training.....

Hope this helps our friends new to carburetors...

Let me know if I missed something.

-a-

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All I see that you missed is that the pressure drops going through the carb at all throttle settings. The center of the carb is a venturi, they work by reducing the pressure in the throat. PV=nRT works, as Pressure goes down so does Temperature. Fortunately it often takes high(er) humidity to cause ice buildup, but here in the Ohio Valley there is almost as much humidity as being down South.

 

[if you want to play with the equation, you have to convert Temp into absolute [ºC + 273 or ºF + 455], and I don't recall right now a good approximation for pressure drop, but it will vary with throttle position anyway. You have to do it three times:  once in front of the carb, once in the narrow part of the venturi, and again at the carb exit. Nothing worth knowing is easy . . . ]

 

If you have the carb temp gage, partial carb heat works. If you do NOT have a carb temp gage, do NOT use partial carb heat! Ice makes her run rough and you lose manifold pressure--applying carb heat will begin melting the ice, running water through the system, and it will get worse before it gets better. [Never experienced it myself, but I've read and talked to people who have.]

 

Have fun out there, but be careful! We are anticipating our first possible snowfall tonight. Time to brush up on winter flying. :(

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  • 2 weeks later...

Another thought here as to possible cause for the stumble or even shut down when applying carb heat at run-up. After eliminating the usual things like butterfly, leaky exhaust, check the float bowl on that carb. If it has worked loose it can cause the engine to quit because of the additional air being sucked in and making the mixture too lean. When the float bowl was loose on mine I would see excessive RPM drop at run-up.

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