What factor is primarily affected by changes in altitude during flight?

The factor that is primarily impacted by changes in altitude during flight is both the speed and fuel consumption. As an aircraft ascends, the air density decreases, leading to a reduction in aerodynamic lift and thrust produced by the engines. In general, as altitude increases, the performance of the aircraft's engines can become less efficient because they rely on denser air for combustion and generating thrust.

Moreover, as an aircraft climbs to higher altitudes, pilots often adjust their indicated airspeed (which is the speed relative to the surrounding air) to optimize fuel efficiency and performance. This adjustment is critical because it influences how the aircraft interacts with the thinner air, affecting the overall speed and fuel consumption during the flight.

While aerodynamic lift does influence an aircraft's flight performance and can be impacted by altitude changes, the more immediate and quantifiable effects during flight operations tend to revolve around speed and fuel consumption. Visibility can be affected by altitude as well, but it is not as consistent or predictable a factor compared to the more direct relationship between altitude, speed, and fuel efficiency.