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Date: Tue, 22 Dec 1998 20:21:38 -0600
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Subject: Re: Lean mixture during run-up
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Hilton Goldstein wrote:

> What does mixture rich have to do with climbing?

Lycoming (and I believe Continental) permit operation at
peak EGT *at less than 75% power*.  This is "best economy"
leaning, and is actually the assumption made in most of the
performance charts in the POH (and one reason people who use
them, but who are leaning for "best power", get into trouble.

If one is operating at 'best economy', one must enrichen the
mixture before advancing the throttle past 75% (as in a climb).

Lycoming also recommends no leaning at all in climb below
5000 ft DA, but leaning is permissible at any altitude below
75% power.  So even if one is leaned for best power, but 
below 5000 ft DA, one must enrichen the mixture before
advancing the throttle.

> If I'm cruising along at 5000' and ATC tells me to climb and maintain > 7000', I advance the throttle (which may *not* be full power as 
> recommended by the POH) and since the mixture is already set for that > altitude, I don't need to adjust until the DA changes.

1) if you have leaned for "best economy", as permitted below 75%
   power, you must enrichen the mixture before advancing the throttle
2) if you have leaned for "best power", the optimum mixture for
   climb may in fact be a little different than the optimum mixture
   for cruise. This is because you don't just lean for a density
   altitude, you lean for a mixture--and the optimum mixture will
   change slightly when the throttle is advanced to climb. (I
   assume in this one is leaning for optimum rpm/airspeed and
   disregarding a single-cylinder EGT because the leanest
cylinder        in cruise may not be the leanest cylinder in climb).

Snowbird


