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Leaning is Essential

We’ve all been taught to start leaning the engine for best performance at 3,000 MSL, but many pilots don’t realize that leaning is also very important at low altitudes, particularly at low RPM.  The carburetor for the Lycoming O-540 in most 182RGs runs the engine very rich, so rich that at RPM below 1,000 the engine lopes like a tractor.  One or more plugs can easily become fouled in the time to taxi from Gibbs parking to the runup area, unless the engine is leaned fairly agressively–mixture pulled out about an inch.  At that setting you’ll find the engine idles quite smoothly.

Try this experiment to see the difference proper leaning can achieve:

  • Set the mixture full rich at runup, 1,700 RPM
  • Note the mag check RPM drop–175 RPM on one mag is quite typical
  • With both mags, lean the mixture about an inch. You should see about a 200 RPM increase
  • Reduce RPM to 1,700 and repeat the mag check.  RPM drop will be less than 50, even approaching zero
  • Leave the mixture lean until cleared for takeoff, then advance to full rich after full throttle

On descent and approach to landing, you can leave the engine leaned–it doesn’t need the extra fuel anyway–until the last GUMPS check on short final, in the event a go-around is needed.  Then lean again as part of your after-landing checklist.

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