Airline Transport Pilot Written Knowledge Practice Test

When an aircraft climbs to higher altitudes, the atmospheric pressure decreases, leading to a reduction in air density. If the fuel/air mixture is not adjusted during this ascent, the proportion of fuel to the reduced amount of air becomes higher. This means that there is relatively more fuel for each unit of air entering the engine, which results in a richer fuel/air mixture.

This is significant for engine performance, as a rich mixture can cause issues such as increased fuel consumption and potential fouling of spark plugs. Pilots must be aware of this effect and adjust the mixture accordingly, especially when flying at altitude, to ensure optimal engine performance and efficiency.