What effect does high altitude have on the SFC of an aircraft engine?

High altitude significantly affects the specific fuel consumption (SFC) of an aircraft engine due to changes in air density. As altitude increases, the air density decreases, which has a direct impact on engine performance.

In jet engines, the amount of oxygen available for combustion diminishes with decreasing air density. This reduced availability of oxygen can lead to less efficient combustion, ultimately requiring more fuel to produce the same amount of thrust. Consequently, the SFC—defined as the amount of fuel consumed per unit of thrust produced—tends to increase at high altitudes.

Additionally, while some might think that reduced drag at higher altitudes contributes to better fuel efficiency, it is the combination of lower air density and its effect on engine power output that primarily influences the increase in SFC. The overall performance characteristics of the engine at higher altitudes, including the reduced thrust produced under these conditions, are significant factors in this relationship.

By recognizing that high altitude correlates with lower air density and that this leads to inefficient fuel usage in terms of engine output, the understanding of why SFC increases at altitude is clear.