Use of Carb Heat

I'm starting to get geared up for my written test, and was finally reading
[thoroughly] the chapter on systems last night. I was reading about carb
heat and how it works and was surprised to learn that the temperature in the
venturi section could be as much as 50 degrees below the outside air
temperature, and that ideal conditions for carb heat were between 50 and 70
degrees with high relative humidity (stuff you guys all know).

Anyhoo, we went flying on Saturday and it was about 58 degrees out with
1800' broken and 2700' overcast in light mist (looked overcast all the way
to me, but it did lighten up after a while). That sounds like a good
scenario for carb icing to me. We were going to try a mini-cross country to
a local airport, but decided to do some pattern work since we wouldn't be
legal VFR without skimming the treetops, dodging towers, etc....

My question is, in these or similar conditions, would you consider using at
least partial carb heat to help prevent icing, or periodically turn the carb
heat on to clear out any ice that may have formed? Is there ever a time when
you would use carb heat as a preventive measure?


John K.
Student Pilot
Somewhere between solo, X-country, the ground, and space

On Tue, 30 Nov 2004 15:09:00 GMT, "John Kirksey"
wrote in g20rd.14599$%C6.279@trnddc02::


My question is, in these or similar conditions, would you consider using at
least partial carb heat to help prevent icing, or periodically turn the carb
heat on to clear out any ice that may have formed?

It depends on the recommendations in the POH for that aircraft type.
For Piper PA28-* full heat is recommended.

Is there ever a time when you would use carb heat as a preventive measure?

In a Cessna 150/2 operating in conditions that cause carb ice whenever
the carb heat is turned off, I use enough heat to keep it clear.

John Kirksey wrote:

My question is, in these or similar conditions, would you consider using at
least partial carb heat to help prevent icing, or periodically turn the carb
heat on to clear out any ice that may have formed?

I would never use partial carb heat under any circumstances. I've been taught to
either use full heat or none.

Is there ever a time when
you would use carb heat as a preventive measure?

Sure. A good example would be putting carb heat on during landing approaches
(standard procedure for many aircraft).

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

Use partial heat ONLY when you have instrumentation that tells you what is
going on in the venturi...there are several devices on the market, some of
them optical, others temperature-related. Without such instrumentation, it
is full heat or none. Go to the Textron-Lycoming web page for more info.

Bob Gardner

"John Kirksey" wrote in message
news:g20rd.14599$%C6.279@trnddc02...
I'm starting to get geared up for my written test, and was finally reading
[thoroughly] the chapter on systems last night. I was reading about carb
heat and how it works and was surprised to learn that the temperature in
the
venturi section could be as much as 50 degrees below the outside air
temperature, and that ideal conditions for carb heat were between 50 and
70
degrees with high relative humidity (stuff you guys all know).

Anyhoo, we went flying on Saturday and it was about 58 degrees out with
1800' broken and 2700' overcast in light mist (looked overcast all the way
to me, but it did lighten up after a while). That sounds like a good
scenario for carb icing to me. We were going to try a mini-cross country
to
a local airport, but decided to do some pattern work since we wouldn't be
legal VFR without skimming the treetops, dodging towers, etc....

My question is, in these or similar conditions, would you consider using
at
least partial carb heat to help prevent icing, or periodically turn the
carb
heat on to clear out any ice that may have formed? Is there ever a time
when
you would use carb heat as a preventive measure?


John K.
Student Pilot
Somewhere between solo, X-country, the ground, and space

"John Kirksey" wrote in message news:g20rd.14599$%C6.279@trnddc02...

My question is, in these or similar conditions, would you consider using at
least partial carb heat to help prevent icing, or periodically turn the carb
heat on to clear out any ice that may have formed? Is there ever a time when
you would use carb heat as a preventive measure?

Partial carb heat without a carb temp gauge is generally considered a
bad thing. You may heat up the carb enough to develop ice deeper down.
Some planes heat the air in the intake before taking it in. The metal
Pipers (Warriors, etc) are famous for this. They really don't have any
carb icing problems. On the other hand, the tube and fabric Aeroncas
and J-3s will get carb ice in the time it takes to turn off teh carb
heat and advance the throttle (I know from experience ). Its not a
bad idea to cycle the carb heat on and off in cruise if you are flying
a plane that is known to get carb ice.
All my IFR planes have always been injected so I'm not sure how I
would feel about regular IFR flying with a carb unless I had a carb
temp gauge.

-Robert, CFI