At what density altitude must pilots be prepared to lean the mixture?

Leaning the mixture is an important technique for pilots to ensure optimal engine performance as altitude increases. At higher elevations, the air becomes less dense, meaning there is less oxygen available for combustion in the engine. As a result, the fuel-air mixture that is used by the engine needs to be adjusted.

At 5,000 feet density altitude and higher, it becomes increasingly important to lean the mixture to maintain the appropriate fuel-to-air ratio for efficient engine operation. By leaning the mixture, pilots can prevent the engine from running too rich, which can lead to rough running, spark plug fouling, and increased fuel consumption.

This practice is especially critical in ensuring that the engine operates efficiently and effectively in less dense air conditions. Thus, density altitudes starting from around 5,000 feet serve as a general guideline for when pilots should begin to consider adjusting the mixture.