Testing carb heat

[TheRaven]

Just wanted some input here, before I pursue this further with my flight school. The flight school checklist for the CTLS calls for testing the carb heat during run-up, presumably resulting in a visible drop in rpm. Only thing is, there is barely a visible drop, say 10 rpm, which is within the normal fluctuation I see as I sit there anyway. When I mentioned this to my CFI, he didn't seem to think it was any cause for concern, and said the drop in rpms resulting from carb heat is usually very slight. However, this seemed a bit too slight to me, and I'd like to know if the carb heat is really working. So I consulted with the POH, and was surprised to find, unless I'm missing something, that the pre-takeoff checklist doesn't even call for testing carb heat. Any thoughts?

[Jim]

I never check it, I wasn't dinged on my PP checkride for not checking it, and I think most people flying behind a Rotax 4-stroke don't check it. It may be a checklist discipline thing in your school.

[TheRaven]

I never check it, I wasn't dinged on my PP checkride for not checking it, and I think most people flying behind a Rotax 4-stroke don't check it. It may be a checklist discipline thing in your school.

 

I guess my main concern is whether it's really working, seeing as how there's little visible drop in rpm. And if checking it is not in the POH, and no one in general bothers to check it, does that mean that carb icing is rarely a problem in the CTLS? Am I worrying for nothing?

[Ed Cesnalis]

The CTs don't have carb heat but instead utilize warm air from inside the cowling as intake air instead of the ram air when you pull 'carb heat.' Since there is no noticeable RPM drop checking carb heat seems pointless. You can check the operation when you have the top cowl off.

 

Rotax 912s are not prone to carb ice. Pilots do not routinely pull carb heat when they retard the throttle like they do behind a traditional aircraft engine. The danger is that you might realize carb ice on approach and not realize it or rectify it because carb heat is no longer routinely used.

 

It doesn't happen often but if it results in a power loss it is quite serious.

[TheRaven]

The CTs don't have carb heat but instead utilize warm air from inside the cowling as intake air instead of the ram air when you pull 'carb heat.' Since there is no noticeable RPM drop checking carb heat seems pointless. You can check the operation when you have the top cowl off.

 

Rotax 912s are not prone to carb ice. Pilots do not routinely pull carb heat when they retard the throttle like they do behind a traditional aircraft engine. The danger is that you might realize carb ice on approach and not realize it or rectify it because carb heat is no longer routinely used.

 

It doesn't happen often but if it results in a power loss it is quite serious.

 

Thanks for the explanation!

[TheRaven]

Are you planning to even buy or lease CTs in your flying future? If so, the 912i doesnt have a choke or carbs (and therefore carb heat) at all.

 

Carb heat is needed mainly when the air density drops and enough moisture in the air leads to forming ice on the butterfly valve. This will inhibit fuel/air to the engine and cause power loss or even shutdown.

 

Carb heat is mainly a diversion of hot air generated by a plate on the exhaust manifold. Adding hot air to the air flowing thru the venturi throat results in fewer molecules so engine rpm will drop when heat is applied (but very slightly). This is why you dont want to use carb heat on takeoff.

 

All of that is moot with fuel injected Rotax engines.

 

Unless I win the lottery (which I rarely play), I doubt that a lease or purchase will be in my future. But who knows! Thanks for your info, too.

[Tom Baker]

The CTs don't have carb heat but instead utilize warm air from inside the cowling as intake air instead of the ram air when you pull 'carb heat.' Since there is no noticeable RPM drop checking carb heat seems pointless. You can check the operation when you have the top cowl off.

 

Rotax 912s are not prone to carb ice. Pilots do not routinely pull carb heat when they retard the throttle like they do behind a traditional aircraft engine. The danger is that you might realize carb ice on approach and not realize it or rectify it because carb heat is no longer routinely used.

 

It doesn't happen often but if it results in a power loss it is quite serious.

 

CT your statement is true for your airplane, but not for the CTLS.

 

The CTLS uses a standared Rotax induction system that does have carb heat. The CTLS should have avout 40-50 rpm drop. I did have a problem with tha cable on my airplane coming loose and allowing the carb heat to stay on all the time. You might want to tell them if there isn't any drop it could be on all the time sucking hot un-filtered air, and that is not good for the engine.

[Ed Cesnalis]

Thanks for the correction Tom.

[TheRaven]

CT your statement is true for your airplane, but not for the CTLS.

 

The CTLS uses a standared Rotax induction system that does have carb heat. The CTLS should have avout 40-50 rpm drop. I did have a problem with tha cable on my airplane coming loose and allowing the carb heat to stay on all the time. You might want to tell them if there isn't any drop it could be on all the time sucking hot un-filtered air, and that is not good for the engine.

 

Uh-oh. Thanks, I'll mention this to the flight school. There's definitely no way I've ever been seeing a drop of 40-50 rpm.

[Ian]

Raven, I always check carb heat as part of the pre-flight and definitely expect to see around 40rpm - if I didn't I'd suspect something was wrong.

 

However - having flown a range of Cessnas and Pipers and having had an engine stop on me in a C150 with suspected carb icing, I can honestly tell you that I've never seen the symptoms of carb icing on the Rotax.

[Roger Lee]

Hi Raven,

 

Carb heat as someone stated is only from the inside cowl air temp on the CTSW. Depending on what that is at any given moment can and will affect the drop in the rpm you see. A 10-40 rpm is very normal and at times you may see nothing. During the winter with a lot of cold air could be slightly different from summer rpms. With the engine off you can stand outside the plane and pull the carb heat knob briskly and you will hear the door open and close.

From what you posted I would not worry at this time it all seems normal.

 

The CTLS gets its heat from air around the heated muffler. This is also affected by OAT temp.

 

Carb ice in the Bing 64 is not as easy to form as some of the other carbureted engines because of the variable throttle. It can happen, but a little less likely with the Bing 64..

[Ed Cesnalis]

Roger,

 

Can you explain the difference between the CTSW and CTLS 'carb heat' setups? I'm now confused by Tom's correction.

[Tom Baker]

Hi Raven,

 

Carb heat as someone stated is only from the inside cowl air temp. Depending on what that is at any given moment can and will affect the drop in the rpm you see for the CTLS. A 10-40 rpm is very normal and at times you may see nothing. During the winter with a lot of cold air could be slightly different from summer rpms. With the engine off you can stand outside the plane and pull the carb heat knob briskly and you will hear the door open and close.

From what you posted I would not worry at this time it all seems normal.

 

Roger, The CTLS uses the Rotax airbox with 2 inlets. One is connected to the airfilter the other to the heat muff. When you pull carb heat it opens the hot air valve and closes the filtered air valve the 2 valves are linked together. There is no opening to the engine compartment. All of this is hidden under and behind the airbox and really hard to see.

[Roger Lee]

Hi Ed,

 

The SW gets it's warm air from the air inside the cowl through a little trap door on the bottom of the airbox.

The LS gets it's air from off the warm air that circulates around the muffler, but goes into the airbox just like the SW. They are both affected by OAT because of different heat and cold during different times of the year.

[Roger Lee]

Hi Tom,

Sorry,

I corrected that. I'm a little discombobulated right now. Death in the family and trying to make everything work. Continuity of the thought process isn't quite normal today.

[FastEddieB]

First, I can't imagine NOT checking carb heat on runup. You do want to make sure it's functioning, as it normally also serves as an alternate air source.

 

Second, the rpm drop on the ROTAX in my Sky Arrow is barely perceptible.

 

Third, less dense air (from being hotter) with the same fuel will result in a richer mixture, hence the normal rpm drop.

 

Fourth, sometimes the air follows a more circuitous and/or restrictive route when carb heat is applied, yet another source of rpm drop.

 

Anecdote: one time in my Tiger I had a HUGE rpm drop when I applied carb heat on runup. Taxied back in. I forget the details, but a baffle had come adrift in the muffler somehow and blocked the carb heat shroud. Hard for me to picture now, but I had to get my muffler rebuilt.

 

Advice: ALWAYS check carb heat during your runup. If it's not on your checklist, think about adding it.

 

I do have a video that includes a carb heat check during runup - I'll post to it later.

[Roger Lee]

Many Rotax engine over the last 23 years do not have carb heat. (Yes, a few have, but mostly certified 912S) This came into the fore front since the SLSA hit the market.

[paul m]

Is theRaven training in a CT? I didn't get that he was.

 

 

http://ctflier.com/index.php?/topic/1796-incredibly-frustrated/

[Tom Baker]

Hi Tom,

Sorry,

I corrected that. I'm a little discombobulated right now. Death in the family and trying to make everything work. Continuity of the thought process isn't quite normal today.

 

Roger, sorry to hear that.

[Roger Lee]

Hi Tom,

 

My Mom (88) passed yesterday. Trying to keep family organised for the task that lay ahead.

[sandpiper]

Roger - sorry to hear that. Our thoughts and prayers are with you.

 

Raven - On my SW the carb heat RPM drop on run up is hard to detect. If it really concerns you, try this. During a full power climb, such as to pattern altitude, pull the carb heat before you reduce power. You should both hear and see the RPM drop. It's not much, but definitely noticeable. If you hear and see no difference, then you may have a problem with carb heat not working properly. My guess is you will see/hear the difference and nothing is wrong.

[FastEddieB]

Roger,

 

So sorry.

 

Condolence from Karen and myself.

[Doug G.]

My thoughts and prayers are with you Roger.

[Ed Cesnalis]

Raven just wanted to know if testing carb heat was reasonable during run up (given he is a student).

 

Raven wanted to know if the carb heat was working due to the negligible RPM drop. Your advice doesn't apply.

 

Without an RPM drop another technique is required, I like Sandpiper's the best.

[TheRaven]

Thanks all. Just to clarify, I'm flying a 2009 CTLS with a Rotax 912ULS. I also like Sandpiper's idea, and will give it a try.

 

And my condolences, Roger.