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There is an excellent question/answer on leaning here. However what I have not yet understood is why one would NOT lean for takeoff below 3000 feet? Why does the Lycoming Service Directive 1497A/B still say to use Full Rich on takeoff?

Is it not right that a leaned engine is producing its highest power output and would you not want the highest power output on takeoff?

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    $\begingroup$ The air is dense below 3000. You won't gain much by leaning that low. It's much better to trade a few HP for better cooling. Note that the SB specifies DENSITY altitude, not just indicated. A hot day in Phoenix will push you well above 3000' DA despite being only 1500' MSL. In this case, leaning before takeoff is appropriate. $\endgroup$ – acpilot Dec 18 '18 at 19:22
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You are correct - you do want highest power. But you also want an engine that doesn't overheat and destroy itself.

Your initial climb-out is at an airspeed considerably less than cruising speed so you have less air flowing through the cylinder cooling fins. To compensate for this, flying full rich keep the combustion gasses cooler during the climb-out.

When you reach cruising altitude and level off, you may lean it out in accordance with the engine manufacturer's recommendations.

This "full rich on takeoff" procedure is not limited to Lycoming engines, either. TCM Continental and even the older Pratt and Whitney radials also go full rich on takeoff. It is close to a universal procedure for piston powered aircraft engines.

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    $\begingroup$ Another great plus about this is that it means that you can lean for more power should you ever discover that you don't have quite enough power to complete the takeoff safely. If you screw up calculating the weight, density, or distance, if you realize it soon enough, you can lean the engine and live to talk about it. $\endgroup$ – David Schwartz Dec 18 '18 at 20:00
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Full rich for max cooling when making high power. At higher altitude airports, air is thinner so you lean for the max power you can get at that altitude to be able to make it out.

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