In short: At high density altitude a normally aspirated engine can’t make 75 percent power, so the extra fuel from full-rich and the enrichment valve just robs performance. Rod Machado explains how to lean for takeoff, using maximum RPM with a fixed-pitch prop and several methods with a constant-speed prop, always checking your POH.
Video: “Leaning the Mixture During Takeoff” by Rod Machado, from his YouTube channel.
What you’ll learn
- What a high density altitude takeoff accident in a Stinson shows about a full-rich mixture
- Why engines use extra fuel for cooling at high power, and when that’s no longer needed
- The general rule of thumb: lean when the engine can’t develop 75 percent power
- How to lean for maximum RPM in a fixed-pitch airplane
- Four ways to set the mixture with a constant-speed propeller
- Why detonation is uncommon at lower power settings, and why the POH still has the final word
A full-rich mixture where it doesn’t belong
Rod begins with footage of a four-seat, 165-horsepower Stinson trying to depart Bruce Meadows in Idaho at a density altitude around 9,100 feet. The cockpit photo shows the mixture full rich. The airplane ends up in the trees, and while everyone walks away, it’s a vivid lesson in lost power.
The reason comes down to heat. Engines are designed to run at sea level without much mixture management. At 75 percent power or more they run hot and depend on extra fuel for cooling, which is why a fuel enrichment system adds roughly 15 percent more fuel at full throttle. Rod’s generalization: below about 4,000 feet density altitude the engine can make that power, but higher up it can’t, so the extra fuel only hurts performance. Check your performance charts to know where you stand.
Fixed-pitch propeller technique
In a fixed-pitch airplane, do a full-throttle runup and lean for maximum RPM, then enrich slightly so you’re on the rich side of peak. Reduce power carefully so the engine doesn’t quit, take off with that setting, and fine-tune for maximum RPM in the climb, since the windmilling prop will produce a little more power than the static runup did.
Constant-speed propeller options
With a constant-speed prop, RPM won’t tell you much, so Rod offers four choices. You can lean by sound during a full-throttle runup. With a multi-cylinder engine monitor, lean to peak EGT on the first cylinder to peak, then enrich by about 75 to 125 degrees F for best power. With a fuel flow gauge, set the takeoff flow your POH recommends for the conditions. Or, if the runway allows and you can handle the distraction, adjust during the takeoff roll, useful when gravel near the runup area could damage the prop. In every case, follow the manufacturer’s guidance.
Go deeper with Rod Machado
Rod covers engine operation, mixture control and aircraft performance in detail in his Private/Commercial Pilot Handbook. Learn more in Rod Machado’s Private/Commercial Pilot Handbook.
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