Lycoming Carb Heat

I once read a report about carb heat use published by the NTSB (which, of
course, I can't lay hands on now). In it, they pointed out the hazard of
having disparate methods of carb heat use depending on engine and airframe,
and suggested that full carb heat be applied any time the power was reduced
below cruise power...no matter who made the engine or airframe. They felt
that this standardization would have a positive effect on accident rates.

With regard to your transition into a Cessna model that I have not seen, is
the carb heat control still to the left of the throttle? Can it not be
pushed in with your thumb while the throttle is being pushed in by the base
of your palm?

Bob Gardner

"Mitty" wrote in message
...
Gents,

Though this is not strictly an IFR question, I am pretty sure that you
have opinions that I would like to get.

Subject is Lycoming O-360 engines. I have flown behind a number of them
in Pipers and the POH instruction on carb heat is always "as required."
Specifically there is no requirement for carb heat on the landing
checklist. This makes sense to me as the intake charge is routed through
the oil pan cum intake manifold and, with the throttle nearly closed hence
low flow velocity, should get adequately warmed up. At least that is my
rationale for why the POH does not call for heat.

I am about to get checked out in a Civil Air Patrol 172 that has an STC'd
O-360 installed in place of the original Lycoming O-320. The 172 POH
wants carb heat on approach. This makes no sense to me. If I have to do
a go around it is just one more workload item/one more thing to forget
and, from my Piper experience, it does not appear to be necessary. My
guess, without benefit of any actual facts, is that this POH requirement
comes from Cessna's Continental roots and has no engineering
justification. So it seems wise to ignore it.

Comments? (Please, let's not go to the FARS with this question. That is
not my interest.)

Bob Gardner wrote:
: I once read a report about carb heat use published by the NTSB (which, of
: course, I can't lay hands on now). In it, they pointed out the hazard of
: having disparate methods of carb heat use depending on engine and airframe,
: and suggested that full carb heat be applied any time the power was reduced
: below cruise power...no matter who made the engine or airframe. They felt
: that this standardization would have a positive effect on accident rates.

Perhaps, but I don't like to apply the carb heat unless I know it's needed.
In particular, there are situations where ice crystals would harmlessly flow through
the induction system. If carb heat (especially *partial* carb heat) is added, they
can melt enough to stick. Then you'd be stuck with an iced engine, below freezing,
and not enough heat to melt it.

Maybe I'm overly paranoid about it. I have had carb ice twice in Cherokees...
one with an O-320 in a long descent, and one in cruise (relatively low power in misty
weather at low altitude). Application was a non-issue once I saw the MP 2" below what
I set it. I'll agree that on an IMC missed or legitamate go-around the pucker factor
(and timing constraints) would be more of an issue.

I think that the Cessna left it in the POH from the O-300 Continental days as
a CYA when the switched to Lycoming. Never a good reason to change it, so left it was
it was required before.

-Cory


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

This is what Lycoming says in its "Key Reprints from the Lycoming Flyer"....
Note the sentence "It is also appropriate....in the traffic pattern."

Bob

"In conditions where carburetor ice is likely to form, the pilot may use
heat during cruise to prevent the formation of ice in the carburetor. It is
also appropriate to use full carburetor heat, if needed, to prevent icing
when operating at low power for instrument approaches, or for flight in the
traffic pattern. Unless the aircraft is equipped with a carburetor air
temperature (CAT) gage, and very few general aviation aircraft are, use of
full carburetor heat is recommended. An unknown amount of partial heat can
actually cause induction ice in the float type carburetor. This may occur
when moisture in crystal form in the incoming air that would ordinarily pass
through the induction system without any problem is melted by the partial
heat. This moisture then freezes when it comes in contact with the cold
metal of the throttle plate."

wrote in message
...
Bob Gardner wrote:
: I once read a report about carb heat use published by the NTSB (which,
of
: course, I can't lay hands on now). In it, they pointed out the hazard of
: having disparate methods of carb heat use depending on engine and
airframe,
: and suggested that full carb heat be applied any time the power was
reduced
: below cruise power...no matter who made the engine or airframe. They
felt
: that this standardization would have a positive effect on accident
rates.

Perhaps, but I don't like to apply the carb heat unless I know it's
needed.
In particular, there are situations where ice crystals would harmlessly
flow through
the induction system. If carb heat (especially *partial* carb heat) is
added, they
can melt enough to stick. Then you'd be stuck with an iced engine, below
freezing,
and not enough heat to melt it.

Maybe I'm overly paranoid about it. I have had carb ice twice in
Cherokees...
one with an O-320 in a long descent, and one in cruise (relatively low
power in misty
weather at low altitude). Application was a non-issue once I saw the MP
2" below what
I set it. I'll agree that on an IMC missed or legitamate go-around the
pucker factor
(and timing constraints) would be more of an issue.

I think that the Cessna left it in the POH from the O-300 Continental days
as
a CYA when the switched to Lycoming. Never a good reason to change it, so
left it was
it was required before.

-Cory


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

"Bob Gardner" wrote in message
...
With regard to your transition into a Cessna model that I have not seen,
is the carb heat control still to the left of the throttle? Can it not be
pushed in with your thumb while the throttle is being pushed in by the
base of your palm?

On the PA-28 I fly the carb heat is (rather idiotically) immediately to the
right of the mixture control, which made my instructor sweat one day when I
was looking out of the window, and leaned over and pulled the wrong one.
Fortunately I noticed when the engine stuttered, before I'd leaned it off
completely, and a quick shove restored power!

D.

David,

On the PA-28 I fly the carb heat is (rather idiotically) immediately to the
right of the mixture control, which made my instructor sweat one day when I
was looking out of the window, and leaned over and pulled the wrong one.
Fortunately I noticed when the engine stuttered, before I'd leaned it off
completely, and a quick shove restored power!

Reminds me of one of my touch-and-goes in a (then new to me) Pa 28, when
I intended to pull the carb heat up to "cold", but instead pulled the
friction lock up...

Enriching the mixture and opening the throttle became impossible then
and in the aftermath I'm glad it was a touch-an-go and not a go-aroud... :-/

Best regards

Kai

"Kai Glaesner" wrote in message
om...
On the PA-28 I fly the carb heat is (rather idiotically) immediately to
the right of the mixture control, which made my instructor sweat one day
when I was looking out of the window, and leaned over and pulled the
wrong one. Fortunately I noticed when the engine stuttered, before I'd
leaned it off completely, and a quick shove restored power!

Reminds me of one of my touch-and-goes in a (then new to me) Pa 28, when I
intended to pull the carb heat up to "cold", but instead pulled the
friction lock up...

It does make one wonder about "progress" in design. The PA-28 I mentioned
above is a late-1980s Warrior-II, and the carb heat is just to the right of
the mixture control (so you have to physically lean and reach past the
mixture control to get to the carb heat). The throttle friction is in the
vicinity too, thus making it susceptible to Kai's faux-pas.

Compare this with another PA-28 in our club which is 20 years older. The
throttle is a plunger-type, with the friction nut encasing the stem
(impossible to get wrong, albeit admittedly more fiddly than a lever), and
the carb heat is another plunger, placed sensibly next to the throttle (just
below and to the left, if memory serves) and a decent distance from the
mixture control.

Sadly, then, the designers of the later version made it easier to pull the
wrong thing, and more awkward to reach the carb heat. Oops.

D.