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Date: Sat, 19 Dec 1998 16:29:42 -0600
From: Snowbird <snbird@ibm.net>
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Subject: Re: Carb Heat Comment
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Dan Dillon wrote:

> On flights prior to this particular flight that I am referring to, I
> noticed (with my instructor) that this particular plane didn't have
> much of a carb heat rpm drop.  I was told that it was ok, and it
> seemed like a logical conclusion which was confirmed with a mechanic.
> On at least ten flights in this plane, I was convinced that I could
> see a slight rpm drop on appliccation of carb heat.

> At this point in my story, we come to <preflight for xc>. I saw that
> carb heat hose was broken off.  I had been flying without carb heat
> for over a month in this plane- I couldn't believe I had found this.

I like this story.  I too have had an encounter with a plane which
had very little rpm drop on application of carb heat (under conditions
where other planes of the same make and model have a noticable
100+ rpm carb heat.  I too was told by a CFI and a mechanic not to
worry about it, "some planes just don't show much drop".  Since 
I didn't fly that plane again, I didn't :)

But in retrospect, it seems to me that can't be the whole story.
The mechanism of rpm drop is that warm air is less dense; how can
one plane have an rpm drop, and another plane not under similar
conditions, *if the carb heat in both cases is producing warmer
less dense air*?

I think something was wrong with the carb heat in that plane, just
like in your "don't worry about it" no rpm drop plane.

One of the things I love about my plane is that I can open the
entire top half of the cowling on *every* preflight, and really
check things like the ignition harness, the primer lines, the
throttle, carb heat, and mixture attachments, the hoses, etc.

One of the things which distresses me, as a plane owner, is how
different the standards of different mechanics seem to be.  One
mechanic will say "don't worry about" things which another mechanic
will immediately disassemble and fix.

Snowbird

PS you may not have been "convincing yourself" you could see an
rpm drop; the air inside the cowling might have been sufficiently
warmer than the normal intake air so as to produce a very slight
drop.


