Carb Heat IMC?

[Boilermonkey]

Is there any need to use carb heat in IMC (visible moisture) if running at 65%-75% power (MPs above 20?).  I'm guessing not, but I had read somewhere that people were using it in IMC, and I'm a bit paranoid as a friend had an off airport incident because he did not use carb heat while practicing slow flight w/o carb heat.  His mistake, but it has me thinking.  The M20C POH only states use during approach and landing...or if you experience carb ice symptoms.  What do you do?

[gsxrpilot]

Lots of IMC in the few years I flew a C. Never once used carb heat. The C just doesn't seem to be all the susceptible to carb icing. I'm sure it can happen, but it's pretty unusual. Might have something to do with the position of the carb on the Lycoming engine as well as the tight cowling of the Mooneys.

[Hank]

Yes, our planes are pretty carb ice resistant. On the other hand, I really like having a Carb Temp gage, it eliminates the question of whether I need carb heat or not. Without it, you need to use all of it if you decide to use it at all.

I also descend Power On, eliminating another common source of carb ice in other planes.

[RLCarter]

Carbureted Lycomings aren’t as susceptible to Carb Ice as the Conti’s are. I’ve had Carb Ice several times with the Conti’s in severe clear (2x at night), it will get your attention. If visible moisture is present (VMC or IMC), the Carb Heat comes on. Why chance ingesting ice if you don’t have to.

[carusoam]

BM...

The search is your friend.

Along with everyone else here...

Carb Ice is pretty dangerous if you get it.

And POHs from the 60s are pretty dangerous to follow religiously.

+1 for having a carb temp indicator. Low cost and easy to install.

Without carb temp,  full carb heat on is the only recommended solution.

carb heat robs power.

use caution when fully loaded stuck below clouds.  Carb icing is more probable and having less power because the carb heat is on is a tough place to be.

Glad you are here,

-a-

[carusoam]

RLC,

what plane were flying with a carb'd Continental? (Non Mooney right?)

Best regards,

-a-

[RLCarter]

2 minutes ago, carusoam said:

RLC,

what plane were flying with a carb'd Continental? (Non Mooney right?)

Best regards,

-a-

Cessna 150 & 172, have 40 or so hours in M20C but never had an issue but in visible moisture the carb heat was on

[AlexLev]

I was flying in IMC today training for my IFR rating and in the clouds noticed what may have been some roughness and put carb heat on, despite the carb temp gauge not being in the yellow.

It may have improved, but then later when I put it on a second time, the engine seemed to run rough (cough every once in a while) with carb heat on. My instructor speculated it was because the unfiltered air may have some water that was disrupting the engine. Is it common to get a coughing engine with carb heat on in the rain? I know I have less power with carb heat on, but the cough is a little startling in IMC especially (makes me want to take carb heat off immediately)...

[RLCarter]

17 minutes ago, AlexLev said:

I was flying in IMC today training for my IFR rating and in the clouds noticed what may have been some roughness and put carb heat on, despite the carb temp gauge not being in the yellow.

It may have improved, but then later when I put it on a second time, the engine seemed to run rough (cough every once in a while) with carb heat on. My instructor speculated it was because the unfiltered air may have some water that was disrupting the engine. Is it common to get a coughing engine with carb heat on in the rain? I know I have less power with carb heat on, but the cough is a little startling in IMC especially (makes me want to take carb heat off immediately)...

Un-filtered air should have been closed in the rain as per the POH

[Boilermonkey]

 

9 hours ago, AlexLev said:

I was flying in IMC today training for my IFR rating and in the clouds noticed what may have been some roughness and put carb heat on, despite the carb temp gauge not being in the yellow.

It may have improved, but then later when I put it on a second time, the engine seemed to run rough (cough every once in a while) with carb heat on. My instructor speculated it was because the unfiltered air may have some water that was disrupting the engine. Is it common to get a coughing engine with carb heat on in the rain? I know I have less power with carb heat on, but the cough is a little startling in IMC especially (makes me want to take carb heat off immediately)...

 

I read that flying with carb heat on at high power levels can be somewhat dangerous as it may cause detonation....so there's pros/cons to everything we do.  After all the discussion and the lack of POH guidance I'm inclined to:

1) Use carb heat on approach and landing as required in the POH.

2) Use carb heat on decent if power settings are low.

3) Use carb heat in IMC occasionally to check for ice presence and as a safety measure, but not leave it on for prolonged periods.

4) Look at installing a carb temp gauge.

 

[gsxrpilot]

1 minute ago, Boilermonkey said:

 

I read that flying with carb heat on at high power levels can be somewhat dangerous as it may cause detonation....so there's pros/cons to everything we do.  After all the discussion and the lack of POH guidance I'm inclined to:

1) Use carb heat on approach and landing as required in the POH.

2) Use carb heat on decent if power settings are low.

3) Use carb heat in IMC occasionally to check for ice presence and as a safety measure, but not leave it on for prolonged periods.

4) Look at installing a carb temp gauge.

 

I'll disagree with you. Carb heat is rarely if ever needed in a C. 

Use carb heat if you suspect ice in the carb because of engine roughness and the conditions are right. Other than that, I'd only check it during run-up to verify proper operation.

[Alan Fox]

9 hours ago, RLCarter said:

Un-filtered air should have been closed in the rain as per the POH

Carb Heat is unfiltered air...Has nothing to do with ram air..... Unless you have an air filter on your muffler...

 

 

Edited May 5, 2017 by Alan Fox

[carusoam]

1) Running with carb heat on has a way of messing with the mixture towards the rich side.  Less air/same FF...(?) not sure how this increases the chance of detonation.  Test: check EGT, add carb heat, check EGT again.  Did it go up or down?

2) accidently leaving carb heat on during a go-around would be bad if max power is needed.

3) checking the health of the alt air / carb heat system is a good idea.  Some of these things are falling apart after 50 years.  Mine broke the control wire a couple of times.  The butterfly valve parts were pretty well worn...

4) Having carb temp is just intelligent.  It was rare coming from the factory, as were seatbelt shoulder harnesses... Having FF and engine monitor data is pretty cool too...

5) Unexplained loss of MP while flying level is a subtle hint that Ice is constricting airflow in the carb.  Adding carb heat, the MP returns... Now you know it is most likely carb ice.  MP returns with carb heat on, then fades with carb heat off...repeat... (could be ice or something covering the air filter too, check the OAT and windshield for corroborating signs of ice)

6) Carb ice is not a common occurrence.  Possible, just not common.  I had it once in a decade of M20C flying. Memorable.

Pp thoughts only, not a CFI....

Best regards,

-a-

Edited May 5, 2017 by carusoam

[orionflt]

the carbs in the C are very resistant to icing especially if you are WOT or close to it, use carb heat when you need it, and a carb temp gauge is very inexpensive for the benefits if you are going to be flying a lot of IMC. I ran across a story several years ago written by a gentleman who was flying the plane I currently own, he was flying IMC in weather and lost the engine because of too much water being ingested. nice to say it was a good outcome. here is a link to the story.

Brian

http://www.airspacemag.com/daily-planet/it-was-a-dark-and-stormy-night-1-66741228/

[Alan Fox]

15 minutes ago, gsxrpilot said:

I'll disagree with you. Carb heat is rarely if ever needed in a C. 

Use carb heat if you suspect ice in the carb because of engine roughness and the conditions are right. Other than that, I'd only check it during run-up to verify proper operation.

Its never needed , until its needed , and hopefully before it gets critical.......After 22 years of flying , I finally got carb ice ...  Took a while to recognize it .... After playing with mix and mags it finally dawned on me it was ice...... Hit the heat , cleared up in about 6 seconds .... Now whenever I fly a carbbed plane , I cycle the carb heat , every ten minutes or so.....And just to be clear it was not a Mooney , and it was a clear day....

[Mooneymite]

1 hour ago, Alan Fox said:

Its never needed , until its needed....

EXACTLY!  Anyone who says they've never had carb ice should always add the word, "YET".  Fly enough and you will....I can guarantee it.  

A stumbling engine is a tough place to try to get enough heat to melt carb ice out of.  When you need it, you actually needed it five minutes before.

I don't think Mooney put it in the POH just to add to the word count.  

[tigers2007]

Okay how about this - carb heat in visible moisture at just below freezing temps? Im just too paranoid I guess after a severe carb icing incident in an O-200 powered C150.
I was dodging some squalls / mist whilst flying VFR under a 2700MSL or so cloud base and my carb temp was like in the 20's. I turned on the carb heat to 40F and leaned it out.


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[Mooneymite]

Structural icing is normally found in visible moisture and below 5 degrees C.

Carburetor icing is a different animal.  It is based on the evaporative cooling in a carburetor and temp/dew point spread and can form in clear conditions and fairly high temperatures.

Here's my favorite chart for carb ice prediction:

 

[RLCarter]

Carburetor Ice

Three categories of carburetor ice are: 

• Impact ice - Formed by impact of moist air at temperatures between 15 and 32 degrees F on airscoops, throttle plates, heat valves, etc. Usually forms when visible moisture such as rain, snow, sleet, or clouds are present. Most rapid accumulation can be anticipated at 25 degrees F. 
• Fuel ice - Forms at and downstream of the point where fuel is introduced, and occurs when the moisture content of the air freezes as a result of the cooling caused by vaporization. It generally occurs between 40 and 80 degrees F, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50 percent. 
• Throttle ice - Forms at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the throttle valve. Since the temperature drop is usually around 5 degrees F, the best temperatures for forming throttle ice would be 32 to 37 degrees F although a combination of fuel and throttle ice could occur at higher ambient temperatures. 

In general, carburetor ice will form in temperatures between 32 and 50 degrees F when the relative humidity is 50 percent or more. If visible moisture is present, it will form at temperatures between 15 and 32 degrees F. A carburetor air temperature (CAT) gauge is extremely helpful to keep the temperatures within the carburetor in the proper range. Partial carburetor heat is not recommended if a CAT gauge is not installed. Partial throttle (cruise or letdown) is the most critical time for carburetor ice. The recommended practice is to apply carburetor heat before reducing power and to use partial power during letdown to prevent icing and overcooling the engine.

To prevent carb ice:

• Use carb heat ground check 
• Use heat in the icing range 
• Use heat on approach and descent 

Warning signs of carb ice include:

• Loss of rpm (fixed pitch) 
• Drop in manifold pressure (constant speed); rough running 

Pilot response to warning signs should be:

• Apply full carb heat immediately (may run rough initially for short time while ice melts) 

In the chart below, the curves encompass conditions known to be favorable for carburetor icing. The severity of this problem varies with different types, but these curves are a guide for the typical light aircraft. Light icing over a prolonged period may become serious. When you receive a weather briefing, note the temperature and dewpoint and consult this chart. 

 

[Boilermonkey]

That's why I asked the question.  If you look at the carb ice charts and guidelines....it says use carb heat in visible moisture between 32-50 deg.  So, if you are in IMC (aka visible moisture) in that temp range, would use use carb heat as a precaution?

...you better look out for other icing too...

[M20F]

4 hours ago, Mooneymite said:

Structural icing is normally found in visible moisture and below 5 degrees C.

Carburetor icing is a different animal.  It is based on the evaporative cooling in a carburetor and temp/dew point spread and can form in clear conditions and fairly high temperatures.

Here's my favorite chart for carb ice prediction:

 

This is the correct answer a lot of people never see carb ice because they do not fly in conditions that will create it.  

Carb ice is a fickle thing. 

[Hank]

33 minutes ago, M20F said:

This is the correct answer a lot of people never see carb ice because they do not fly in conditions that will create it.  

Carb ice is a fickle thing. 

You don't fly when weather is as shown in the example on Gus's chart--54°F with 50% humidity? Around here, that's a cool spring day, with great flying. According to the chart, it's also good conditions for carb ice, which I have yet to experience . . . .

[gsxrpilot]

You fly your airplane, I'll fly mine.  I don't fly a carb'd airplane anymore, but when I did, I never used the carb heat. I flew many times in classic conditions for carb ice. But I never once experienced it. Out of the many thousands of hours of M20C time on this forum, there have been maybe two instances reported of carb icing. Yet almost everyone who's ever flown a C150, 172, etc. has experienced carb ice. Therefore my point is that carb icing is exceedingly rare in the M20C. I'd check the function of the carb heat during every run-up. And certainly know where the knob is and how to pull it. But in a C, the only time you'll ever need it is if you're having difficulty getting smooth LOP ops. Sometimes a shot of carb heat will be just the ticket to smooth things out. But don't leave it in, just a shot of it.

[MBDiagMan]

This has made me realize something that I have observed, but not thought about very much until reading this thread.

My Cessna has a carb temp guage and so does my Mooney.  I have a habit of glancing at it on downwind.  I have seen the Cessna temp in the yellow sometimes, but have yet to see the Mooney indicate the need for carb heat.  I had even wondered if maybe the guage wasn't reliable, but after reading this thread I realize that probably the Mooney is just not prone to carb icing.

[cliffy]

OK time for a lesson in engine design-

If you look at a Cessna 150 Continental carb you'll see that it is mounted onto an open 4 way spider that distributes the fuel/air mixture to four tubes (or runners) that then turn to enter the cylinder head. Because the carb is mounted away from the engine no heat from the engine case helps to heat the carb so it gets cold very easy. Carb ice is a very real and often encountered condition in 150s. Or any Continental carbed engine. If we remember, fuel mixes with air and vaporizes releasing heat, making the induction mixture just downstream of the throttle butterfly drop below freezing and then freezing any moisture in the mix into ice that collects in the throat of the carb. Get enough ice and the engine dies from no airflow through the carb due to the ice. Carb heat brings the induction mixture above freezing and melts the ice - hence the "roughness" for a few seconds as the engine tries to consume the now water.

Now look at the way the carb is mounted on your 180 hp Lycoming. It mounts right to the bottom of the oil sump. The "spider" is contained inside the oil sump. What's in the oil sump? HOT oil. Heat that transfers to the body of the carb thereby not letting the carb body get as cold as the Continental installation does. Hence, not as susceptible to carb icing. It can happen, just not too susceptible to it. 

With any controllable prop carb'd airplane in cruise, one can often just watch the manifold pressure gauge and note any decrease in setting. That is the ice closing off the throat of the carb just like pulling the throttle control back. If you see that in cruise, pull the carb heat to get rid of the ice but be prepared for a little roughness as it digests the now water. With Lycomings it usually doesn't happen. It is always best to follow the manufacture's recommendations on the use of carb heat.