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.