Which of the following indicates a richer fuel/air mixture at higher altitudes?

To understand why maintaining a constant amount of fuel indicates a richer fuel/air mixture at higher altitudes, it’s essential to consider how altitude affects air density and how that impacts engine performance and fuel mixtures.

As altitude increases, air density decreases, meaning there are fewer air molecules in a given volume of air. For optimal combustion within an aircraft engine, a specific ratio of fuel to air is necessary. If the amount of air decreases due to lower density, keeping the amount of fuel constant results in a higher fuel/air ratio. This scenario leads to a "richer" mixture, which is defined as having a higher concentration of fuel relative to air.

In contrast, if the fuel amount decreased or air amount increased, it would lead to a leaner fuel/air mixture, which could potentially affect engine performance negatively. Therefore, maintaining a constant amount of fuel in the context of decreasing air density effectively points to a richer mixture, which can improve engine performance in certain situations, especially during ascent to high altitudes where engine efficiency becomes critical.