What does a reciprocating engine automatic mixture control respond to?

A reciprocating engine's automatic mixture control is primarily designed to respond to changes in air density. As altitude increases, the density of the air decreases, which affects the amount of oxygen available for combustion. The automatic mixture control adjusts the fuel-air mixture accordingly to ensure optimal performance and to maintain efficient combustion across varying environmental conditions.

In contrast, other factors mentioned, such as fuel viscosity, fuel temperature, and oil pressure, do not directly influence the operational adjustments of the automatic mixture control. While these factors can affect engine performance in other ways, they are not the primary variables that the automatic mixture control is designed to monitor and adjust in response to changes in atmospheric conditions.