What happens when you lean the mixture at cruising altitude?

Leaning the mixture at cruising altitude is a necessary action to adapt to the changes in air density. As altitude increases, the air becomes less dense, which means that there are fewer air molecules in a given volume of air. This lower density affects the fuel-to-air ratio that is optimal for combustion in the engine. By leaning the mixture, the pilot decreases the amount of fuel introduced into the combustion chamber relative to the amount of air, thereby compensating for the less dense air. This adjustment ensures that the engine continues to run efficiently, maintaining optimal performance and reducing the risk of spark plug fouling or engine roughness due to an overly rich mixture.

In contrast, increasing fuel consumption would be a result of a richer mixture, which is not the goal when adjusting for altitude. Maintaining the same fuel-to-air ratio would not acknowledge the change in air density and could lead to inefficient engine operation. Completely cutting off the fuel supply would lead to engine failure, making it an inappropriate action during flight. Thus, the adjustment allows for effective performance by reflecting the changes in the atmosphere as altitude increases.