Airline Transport Pilot Test 2025 – 400 Free Practice Questions to Pass the Exam

At higher altitudes, the density of air decreases due to lower atmospheric pressure. In a reciprocating engine, the fuel-to-air mixture is critical for optimal operation. As air density decreases, the air molecules become less prevalent in a given volume, which directly impacts the amount of air available for combustion.

In this context, when stating that the amount of fuel remains constant as air density decreases, it's crucial to recognize that the engine’s fuel metering system is designed to deliver a specific amount of fuel regardless of air density. Therefore, as altitude increases and air density decreases, the volume of air entering the engine decreases while the fuel flow remains unchanged. This results in a richer fuel-air mixture because there is less air available to mix with the constant amount of fuel.

Understanding this dynamic is vital for pilots, as maintaining the correct fuel-to-air ratio is essential for engine performance, especially at high elevations where altered atmospheric conditions can affect engine efficiency and power output.