As an airplane climbs, how does the fuel-air mixture change?

As an airplane climbs, the fuel-air mixture becomes leaner, not richer. This is because, as altitude increases, the air density decreases, leading to less oxygen being available for combustion. To maintain the appropriate fuel-air ratio for optimal engine performance, the pilot must adjust the mixture.

In a typical fixed-pitch propeller airplane equipped with a carbureted engine, the fuel-air mixture becomes leaner during ascent. Many pilots utilize this process to ensure the engine operates efficiently under changing atmospheric conditions.

The choices provided about the mixture remaining the same or fluctuating do not align with the principles of aerodynamics and engine performance. The mixture may require constant adjustment as altitude and temperature change, but it does not remain the same and would not fluctuate without intervention from the pilot.