Jose,

If you read the POH and look at the power setting charts, it says
something along the lines of "with lean mixture".

Lean anytime you are in level flight, otherwise you cannot get the fuel
burn numbers in the POH. That includes if you are 500 feet MSL or
15,000 feet MSL.

At power settings above 65% lean to about 75 to 100 degrees F rich of
peak (50 degrees ROP is about the worst possible mixture setting - see
John Deakin's engine operation articles on AVweb www.avweb.com). At
65% power or less, you can lean any way you want, you cannot generate
enough heat or pressure to damage the engine through detonation. You
are not running a fuel injected engine, so you may or may not be able
to operate lean of peak without it running rough, although you can try
it without hurting anything. Sometimes running carb heat will stir
things up enough that you can run smoothly LOP.

The stuff in the POH about leaning below 5,000 or 3,000 or whatever,
has to do with leaning in the climb. It has lead to the aviation
falsehood that one should not lean in cruise below some magic altitude.


With avgas at $5 a gallon, it's utterly foolish not to lean in cruise
flight.

All the best,
Rick


Jose wrote:
There was a discussion in my club about leaning the engine (of our
cherokees) below 5000 feet in cruise. This was prompted by the
observation in the manual that the engine should be leaned above 5000
feet at all times in cruise, and below 5000 feet at the pilot's
discrescion. So, how should the pilot discrede? The old timers seemed
to agree that:

1: at low power (say 65% or less) you can't hurt the engine by leaning
to peak EGT, but...

2: at high power (above that, including the 75% many like to fly at to
go fast) one should only lean if it's cold enough, and that the best
thing to do is run full rich if you're in doubt.

This runs counter to my understanding and practice. I lean (50 degrees
ROP) in cruise at all altitudes, including the ones where I can pick the
leaves off the trees, and I run 70-75% power. Running full rich is just
dumping a third of the fuel out the tailpipe.

Any opinions on Usenet about this?

Jose
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