# Key Takeaways - Fuel mixture must be leaned to: - produce optimal engine performance - prevent spark plug fouling from lead deposits - Avoid excess/unplanned fuel burn # Details Engines burn fuel best at a particular ratio of weight of fuel to weight of air. But at high altitudes, we have less dense air. The mixture control allows us to reduce the weight of fuel to keep the proper ratio. ## Why to Lean From [[Engine Operation for Pilots]]: > - Improved engine efficiency. > - Greater fuel economy (i.e., minimum specific fuel consumption) and longer range. > - Smoother engine operation - saves engine accessories and engine mounts. > - Longer spark plug life, less fouling. > - Reduced maintenance costs. > - Reduced operating costs. > - More desirable engine temperatures while operating at cruise altitudes. ## When to Lean Your [[POH]] is the official source for this answer, although generally you'll lean a normally aspirated engine whenever the engine is developing <75% power (usually above 5000' MSL). Many trainers recommend leaning in cruise above 3000' MSL. You want to enrich the mixture entering the pattern (part of the [[GUMPS]] check) to prepare for a go-around. ## How to Lean From [[Engine Operation for Pilots]]: > ### Tachometer Method > (for use with fixed or variable pitch propellers) > > Set the controls for the desired cruise power setting as shown in the POH. Then gradually lean the mixture from full rich until the tachometer reading peaks. In smooth air, you should also notice a slight increase in aircraft speed. At peak RPM, the engine is operating within the maximum power range. For best economy operation, the mixture is first leaned from full rich to maximum power, then the leaning process is slowly continued until the engine starts to run rough. Then, enrich the mixture sufficiently to obtain a smooth firing engine. Obviously, some engine power and airspeed is sacrificed when operating at best economy. What you gain, however, is increased endurance. > > ### Engine "Rough" Method > (for use with fixed or variable pitch propellers and engines equipped with float-type carburetors only) > > With this method, first set the throttle to the appropriate power setting, (75 percent power or less). Lean the engine gradually until the engine starts to run rough; then enrichen the mixture slightly until the engine is again running smoothly. You will then be operating near the "best economy" mixture setting. > > ### Fuel Flow Indicator Method > (for use with any type propeller) > > The [[POH]] for aircraft equipped with fuel flow gauges contains appropriate fuel flow settings or, alternatively, the fuel flow gauge may be marked for correct flow at each power setting. You need only lean the mixture to the published or marked fuel flow values to achieve the correct mixture. > > ### Exhaust Gas Temperature (EGT) Method > (for use with any type propeller) > > Peak [[EGT]] occurs essentially at the rich edge of the best economy mixture range. Operation at peak EGT not only provides essentially minimum specific fuel consumption but also 95 to 96 percent of the engine's maximum power capabilities for a given engine speed and manifold pressure. In addition, engine operation is very smooth at peak EGT. In comparison, a very noticeable power loss or roughness will occur when the engine is operated at the lean side of the best economy range. # Additional Resources - [[Engine Operation for Pilots]] #concept