What occurs to the engine if fuel is not leaned at high altitudes?

If fuel is not leaned at high altitudes, the mixture can become too rich, which leads to a drop in RPM and fouled spark plugs. At higher altitudes, the air density decreases, resulting in less oxygen available for combustion. If the fuel mixture is not adjusted (leaned), the engine continues to receive the same amount of fuel as it would at lower altitudes, creating an overly rich mixture.

An overly rich mixture can lead to incomplete combustion, which not only causes a drop in engine RPM due to loss of efficiency but also results in unburned fuel accumulating on the spark plugs. This can lead to fouling, which can impede the spark plugs' ability to ignite the fuel-air mixture effectively, further exacerbating performance issues.

By leaning the mixture, the pilot can achieve a more optimal fuel-to-air ratio, ensuring better combustion and maintaining engine performance, especially critical when operating at high altitudes where the engine is already facing challenges due to thinner air.