On Sep 8, 7:07*am, Tman x@x wrote:
Couple things I want to add to some of the (kind of) opinionated but
helpful replies in this thread.

My C152 POH, and this is a late -70s model, does not give a clear cut
protocol for use of carb heat. *It basically says use it when icing is
possible or suspected. *I think it's standard practice to pull it on
power reduction for approach, but as you can see from the replies here
there are a variety of standard practices. *If I read the POH literally,
I'd say turning the heat off when on short final is not contrary to what
it says, and is not really more in line with its suggestions than
leaving it full on till clear of the runway or go-around power is
needed. *Does one suspect carb ice in the last 30 seconds of approach?
Well there are some anecdotes in this thread that raise the possibility
of that, so maybe I won't do this practice anyways, but either way there
isn't a sentence in the POH that says to leave that carb heat on till
the wheels are on the ground.

By the way, the topic of leaning on downwind for approach under high
density altitude conditions was brought up in this thread. *The C152 POH
is almost silent on that also. *It talks about leaning in cruise,
leaning for max rpm in a full power static runup when over 3000 DA, and
to place the mixture rich or "for smooth operation" on approach. *I've
been taught the former, but another very experienced CFI told me to lean
it out "an inch" and push it in only if you need to go around. *Both
pieces of advice I'd say consistent with the POH text, and both different..

FYI, the engine stumble in the original post happens at about 2000 DA days.

Just to go off on this tangent, one CFI I has says absolutely avoid
operating your Cessna Lycoming less than 1,000 RPM -- not enough oil
pressure to lube up the beast, and in fact there is an examiner that
will demerit you if you taxi at less than 1000 RPM on your checkride.
Now another CFI is adamant about taxiing at minimum possible power -- to
save the brakes. *Both very respectable experienced CFI's, both with
reasons for their suggestion. *And both suggestions are "not contrary"
to the POH guideline. *So sometimes you gotta use your brain to
determine a good SOP, and solicit info on the pros / cons / risks to
make your own decision. *But I'm not bending any of the guidelines in
the thin POH.

And as far as taking it to the A&P... We've talked about this. *He says,
well grasshopper, you go the mixture full rich, it's hot, and you punch
in the throttle -- this is a carb engine, they sometimes stumble. *It's
also pretty accepted performance around here by some of the other more
experienced pilots -- there recommendation is in line with a few of the
posts here -- some combination of: don't pull the throttle to cutoff,
lean it out on downwind, push in the carb heat on short final, push the
throttle in more slowly, and/or don't worry about the momentary
hesitation / stumble , it'll get ya used to the power delay when you get
flying turbofans. *That last one a little tongue in cheek....
T

Dudley Henriques wrote:
gpsman wrote:
On Sep 6, 2:40 pm, a wrote:
Best practice without a lot of experimentation is to follow the POH,

I hope I never let my rookie ass become so confident I consider
deviating from the POH based on my own anecdotal experience. *Next
thing you know, you're an example of someone who made reasonable
conclusions except for a heretofore unknown exception.
This is excellent advice and I add my voice to what you have said.
General aviation engines particularly Lycoming and Continental, handle
carb ice just differently enough that every aircraft POH should be
checked to see what the engine manufacturer recommends pertaining to the

If the POH says to lean for smooth operation on downwind,
then do it if the carb heat makes it stumble. These carbs are designed
to provide a rich mixture at sea level, and when the DA is higher than
that the fuel:air mix can become too rich to run properly. WE operate
at a 3000' airstrip and the DA is at 5000' or more almost all summer.
You learn something about mixture, and you learn about carb ice on
clear days, too.
Lycoming's service manuals call for an idle setting of 650-750
RPM. I think they know a little bit about lubrication and so on, a lot
more than that CFI knows about engines. 1000 RPM is way too high and
just burns out brakes. The only place 1000 RPM should be used is when
the engine has been started from a cold-soaked state, and the oil is
still thick enough that it doesn't throw onto the cylinder walls too
well. Once it's warmed up some it's OK. If the oil pressure is falling
off badly at 650 RPM, the engine's bearings or oil pump or both are
shot and it shouldn't be flying. While we're on the subject of
dragging the brakes to control speed, if you go on to get a Commercial
ticket and do that to expensive airplanes, you might find yourself
looking for another job. Overheated brakes have been known to set fire
to tires and the whole airplane goes up in smoke.
As far as approaches go, we use the "plan-for-abort-unless-
everything's-good" training. Most landing accidents are a result of a
refusal to go around when things aren't lining up properly. Airspeed,
altitude, track---everything must be within reason or we just don't
land. SO, the carb heat is left on until we accelerate to get out of
there, and we don't slam the throttle open, either. Besides risking
stumble or outright failure, it's hard on the engine when cylinder
pressures rise so high at low RPM. Take two seconds to open the
throttle unless things are really urgent.

Dan