Lycoming Carb Heat

[Mitty]

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.)

[Dave Butler]

Mitty wrote:

> 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.

Your Piper experience differs from mine. Facing a long slow ILS through wet
clouds in a Piper, I'd set full carb heat, a minute or two before reducing
power. Ever taxi off the runway and have your engine quit?

[paul kgyy]

The key is the "as required". That puts the monkey on your back.
Lycomings do incur carb ice, though perhaps less often.

[xyzzy]

Mitty wrote:

> 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.)

Bottom line, Pipers need carb heat a lot less than Cessnas because of a
different air induction system design.

Cessna 172's need carb heat pretty much all the time below a certain RPM
(it was 1800 in the last one I flew) because they don't route their
induction air as close to the manifold so it doesn't get heated as well.

On the runup with a Piper when you check carb heat you get a lot smaller
RPM drop than a Cessna does. This is because the Piper's air is already
pretty warm.

[Roy Smith]

In article <1126723296.783817@sj-nntpcache-3>, Dave Butler > wrote:
>Mitty wrote:
>
>> 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.
>
>Your Piper experience differs from mine. Facing a long slow ILS through wet
>clouds in a Piper, I'd set full carb heat, a minute or two before reducing
>power. Ever taxi off the runway and have your engine quit?

I have had it quit on short final (in a PA-28-181). Almost exactly
the situation you described -- ILS on a cool day with very small
temp/dp spread, low vis, low scattered layer. Over the airport
boundary, I pulled the throttle back to idle to land and things got
quiet. I was on the ground almost before I had a chance to realize
what went wrong. Hung out for a while on the runway while the ice
melted then started up and taxied off. Had a mechanic look at it, he
found no problems, so we assumed carb ice.

From that day on, I used carb heat in Pipers on instrument approaches.

I've also gotten carb ice in an Archer at cruise power in clouds.

[Mitty]

On 9/14/2005 2:15 PM, xyzzy wrote the following:
<snip>
>
> Bottom line, Pipers need carb heat a lot less than Cessnas because of a
> different air induction system design.
>
> Cessna 172's need carb heat pretty much all the time below a certain RPM
> (it was 1800 in the last one I flew) because they don't route their
> induction air as close to the manifold so it doesn't get heated as well.
>
> On the runup with a Piper when you check carb heat you get a lot smaller
> RPM drop than a Cessna does. This is because the Piper's air is already
> pretty warm.
>
I think you've just described the difference between Continental and Lycoming
engines, no? This is a Cessna with a Lycoming, where a single casting functions
both as the intake manifold and the oil sump.

[xyzzy]

Mitty wrote:

>
>
> On 9/14/2005 2:15 PM, xyzzy wrote the following:
> <snip>
>
>>
>> Bottom line, Pipers need carb heat a lot less than Cessnas because of
>> a different air induction system design.
>>
>> Cessna 172's need carb heat pretty much all the time below a certain
>> RPM (it was 1800 in the last one I flew) because they don't route
>> their induction air as close to the manifold so it doesn't get heated
>> as well.
>>
>> On the runup with a Piper when you check carb heat you get a lot
>> smaller RPM drop than a Cessna does. This is because the Piper's air
>> is already pretty warm.
>>
> I think you've just described the difference between Continental and
> Lycoming engines, no? This is a Cessna with a Lycoming, where a single
> casting functions both as the intake manifold and the oil sump.

No I haven't. The Cessna I flew (a 1975 M) had a Lyc, and so does the
Piper Warrior I fly now.

Cessnas need more carb heat for the same engine. Not all of the
induction system is designed by the engine manufacturer.

--
"You can support the troops but not the president"
--Representative Tom Delay (R-TX), during the Kosovo war.

[Bob Noel]

In article >, Mitty >
wrote:

[snip]
> 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,

I first learned in a 172 in 1987, and haven't flown a cessna
since around 1990 (just cherokees). I *still* remember the
pre-landing checklist, the emergency checklist, and the go-around
checklist for the 172. Putting in the carb heat is really quite simple -
but maybe I have an advantage because I first learned on the 172.

--
Bob Noel
no one likes an educated mule

[Bob Gardner]

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.)

[Michael]

> 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."

Carb ice happens when it happens. You need carb heat when you need it.

If the OAT is 30F and the dewpoint -5F, no airplane needs carb heat -
there's simply not enough moisture in the air to build any carb ice.
When flying an approach with an OAT of 65F and the dew point the same,
through thick, fluffy cloud every carbureted airplane needs carb heat -
as soon as you throttle back for the descent, you will be building up
carb ice even if you're running a Lycoming-powered Piper.

Most situations fall somewhere between the extremes and call for a
pilot to monitor the engine and decide when carb heat should be used.
Some airplanes call for carb heat at specific times because most pilots
are not terribly good at determining when they need carb heat. Pilots
manage to ignore all the warning signs of carb ice right to the point
where the engine quits with disturbing regularity.

The induction system on Cessnas is not really the same as it is on
Pipers. The installation makes a big difference. Before Piper started
making Lycoming O-320 powered Cherokees, Piper made Lycoming O-320
powered TriPacers. I used to own one, and it iced every chance it got.
The current owner installed an STC'd modifications to the oil cooler,
and it doesn't ice up the way it used to. So what I'm trying to tell
you is this - there's more to carb icing potential than the engine, or
even the airframe it's on. The particular modifications installed can
even make a significant difference.

I would suggest that the procedural use of carb heat on approach to
landing (VFR ir IFR) is wise in any case, and ignoring the POH
requirement for it would be unwise at best unless you're confident you
can reliably detect the formation of carb ice in the descent based on
engine sound and instrument indications. My experience is that most
pilots can't. If you forget to turn off the carb heat on go-around,
performance will be somewhat anemic - but it takes only a second to
realize it is anemic and turn the carb heat off. But if you develop
carb ice on the descent to landing, the go-around can get REALLY
exciting.

Michael

[[email protected]]

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]

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 *
> ************************************************** ***********************
>

[Ash Wyllie]

Mitty opined

>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.)

I flys a 172 with an O-360 engine and d not use carb heat on approach, I have
never had a problem, on approach.

The 2 times that I have had carb ice problems have been in cruise and high
humidity. Go figure.


-ash
Cthulhu in 2005!
Why wait for nature?

[David Cartwright]

"Mitty" > wrote in message
...
> 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.

I'm not familiar with the 172, but in the PA-28s I fly I was taught always
to have carb heat on when below 2,000rpm, which would certainly be the case
on an approach - so much so that checking/adjusting the carb heat setting is
now a ritual that I subconsciously carry out whenever changing the power
setting.

At 300 feet or so, though, you'd switch it to "cold", mainly to give you
maximum power in the event of a go-around, but also in the knowledge that
having the carb heat on means you're getting some unfiltered air into the
engine, and the air can be quite mucky at, or just above, ground level.

D.

[David Cartwright]

"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.

[Kai Glaesner]

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

[Dan Luke]

"Roy Smith" wrote:
>>
>>Your Piper experience differs from mine. Facing a long slow ILS through wet
>>clouds in a Piper, I'd set full carb heat, a minute or two before reducing
>>power. Ever taxi off the runway and have your engine quit?
>
> I have had it quit on short final (in a PA-28-181). Almost exactly
> the situation you described -- ILS on a cool day with very small
> temp/dp spread, low vis, low scattered layer.

My CFII always used carb heat on a C-172 O-320 at anything less than full
power in wet conditions, and I've continued the practice with my 172RG O-360.

I have had one carb ice incident in 700 hours of operating an O-360, and that
was after a long taxi out on a wet morning. The engine would not throttle up
for the mag check, and half minute of carb heat cleared it.

Has using carb heat during damp approaches really kept me out of trouble? I
don't know; maybe it was my lucky key chain.
--
Dan
C-172RG at BFM

[Victor J. Osborne, Jr.]

I too have had carb ice in the clouds, cold OAT. Noticed low power (full to
maintain alt.) Carb heat solved it. Never had a problem on final but then
I didn't have that many IA's in the cold/clouds.

--

Thx, {|;-)

Victor J. (Jim) Osborne, Jr.

"Roy Smith" > wrote in message
...
>
> I have had it quit on short final (in a PA-28-181). Almost exactly
> the situation you described -- ILS on a cool day with very small
> temp/dp spread, low vis, low scattered layer. Over the airport
> boundary, I pulled the throttle back to idle to land and things got
> quiet. I was on the ground almost before I had a chance to realize
> what went wrong. Hung out for a while on the runway while the ice
> melted then started up and taxied off. Had a mechanic look at it, he
> found no problems, so we assumed carb ice.
>
> From that day on, I used carb heat in Pipers on instrument approaches.
>
> I've also gotten carb ice in an Archer at cruise power in clouds.
>

[David Cartwright]

"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.

[Ron Natalie]

paul kgyy wrote:
> The key is the "as required". That puts the monkey on your back.
> Lycomings do incur carb ice, though perhaps less often.
>
Tell me about it. I used to have a GO-435 lycoming. The engine
always ran hot and the conventional wisdom was that the PS-5C
pressure carb was near immune to carb ice. Well, we had it
happen. Margy flew back from the next field over at 20" of
MP (about ten minutes) and it wasn't that cold of a day even.
The thing iced up about the time we hit the pattern.

Of course, the replacment engien can't get carb ice (no carb).