Re: [COUPERS-TECH] On carb ice

[Hartmut Beil]

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Well friends, I too heard that pulling the carb heat on run-up shows you that it is working but also removes any ice build up that might have occurred already while taxiing. Therefore one has to watch the rpm drop closely and leave the carb heat out for say 10 seconds . If within these ten seconds the rpm is picking up again while carb heat still applied, then you had already carburetor icing and one would be best advised to leave that heat on even on takeoff, or at climb out.

 

Whether our engine makes 2000 rpm or  2500 makes no difference for the carburetor. The carb just hangs under the engine and gets a full blast from the outside air through the third cowling "hole" that is actually there to provide cooling air for the oil sump I think. This placement makes the continental engine develop carb ice earlier than Lycomings under the same conditions. Lycoming carbs are also mounted to the oil pan as I understand. That warms the carb significantly. Our carburetor basically hangs just free under the engine.

When applying carb heat, a portion of the cooling air will be directed around the exhaust pipes and then supplied to the carburetor.

Question is if the exhaust pipes are still hot enough at 2000 rpm. I would think so. As long as the engine develops power, the pipes will be heated as a side effect. It is a different picture though when you glide for some time and the engine does not need to develop power. Even if the windmilling prop makes 2000rpm, the pipes might be cold . Now that situation is critical. Like I said I managed to get into that devils circle ones. It took quiet some time for the engine to come back to live again. (Or was it just me, counting every second in sweat?)

Of course when reducing power, the laws of physic are also benefiting the development of ice, so it is a good idea to pull carb heat ones before entering the pattern. I would think.

 

Always good landings

 

Hartmut

 

----- Original Message -----

From: Sydney Cohen

To: DONALD BOWEN

Cc: COUPERS TECH

Sent: Wednesday, May 24, 2006 9:25 PM

Subject: Re: [COUPERS-TECH] On carb ice

----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]----

Don, the reason we are supposed to pull carb heat on our runup is to check to see if it is working.  If the rpm didn't drop and then return, it would indicate that the carb heat cable has disconnected itself from the air box.  In an earlier post you said you didn't pull carb heat when landing.  This is not a good practice.  Pulling carb heat anytime the rpm's are at or below 2,000 is a very good idea, since the engine doesn't produce as much heat when you are at low rpms, making carb icing much more possible.

Syd

DONALD BOWEN wrote:

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Hartmut's comments were excellent, and Paul Anton's chart is useful, too. Yes, carb icing can and does occur here in Florida, despite the fact that I have  experienced it only once in nine years of flying. That happened on a cold day (for Florida) in December, flying barely VFR in  mist just below an overcast layer at 2000'. The LYCOMING engine lost 150 rpm. Application of carb heat corrected the problem. I am aware that carb ice can build up ,  on a warm, humid  day, even while taxiing. I use a pre-takeoff check following my mag checks. I run the engine up to full, static rpm (2250 or so for my C-90). My brakes are good, so I don't move forward. I pull the carb heat out and watch the rpm drop and stabilize. I then push the carb heat control back in (off), and see if the rpm returns to where it was. I am advised that this check, if it is OK,  prevents takeoff attempt with ice in the carburator?

Another handy item for avoiding carb ice, even here in the sunny South, is a Carb Air Temp gage which can be purchased for less than $75. It displays temp right at the carb, not OAT. There can be a great difference in those readings, but the carb temp is the critical one . I believe the icing range is between plus 10 degrees C, and minus 10 degrees C. It is an easy way to check icing temps while airborne. Please do not misinterpret my input as any claim for expertise on my part. Frankly, I don't understand all I know about this subject, and readily admit so.

DON  

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