Lycoming Carb Heat

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.)

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?

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

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.

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.

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.

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.

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

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.)

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