How does fuel consumption change at high altitudes?

At high altitudes, fuel consumption typically decreases due to a combination of several factors related to the performance of the aircraft and the properties of the atmosphere. As altitude increases, the air density decreases, which can lead to improvements in the efficiency of the engine.

In a piston engine, for example, the reduced air density can result in less drag on the airframe and more efficient fuel burning, as the engine can operate at a more optimal mixture with cooler temperatures. In jet engines, the thinner air allows them to operate more efficiently, often resulting in lower specific fuel consumption at cruise altitudes.

Additionally, the higher altitudes allow aircraft to take advantage of the jet stream for more favorable wind conditions, which can further reduce the overall fuel consumption compared to lower altitudes where atmospheric resistance and turbulence may be higher. Thus, flying at altitude makes better use of the aerodynamic characteristics of the aircraft, contributing to decreased fuel consumption during cruise phases.