FAA Powerplant Written Practice Test

Which statement relating to fuel/air ratios is true?

1. The best power mixture is leaner than the best economy mixture

2. The mixture for best power is richer than that for best economy

3. Both mixtures are identical

4. The best economy mixture is always richer than the best power mixture

The correct answer is: The mixture for best power is richer than that for best economy

The statement that the mixture for best power is richer than that for best economy is correct because of the differing demands of the engine under these two operating conditions. In aviation engines, the best power mixture is adjusted to achieve maximum performance, which typically involves a richer fuel/air ratio. This richer mixture provides more fuel relative to air, allowing for a more powerful combustion reaction. When operating in a rich mixture, there is enough fuel present to ensure that the engine produces maximum thrust and power, especially during critical phases of flight such as takeoff or climb. On the other hand, the best economy mixture is leaner, meaning there is less fuel relative to air. This leaner setting is intended to optimize fuel consumption rather than maximize power. It allows the engine to operate more efficiently for longer durations, such as during cruise flight, where the priority shifts from power output to fuel efficiency to extend range. This fundamental difference between the objectives of power and economy explains why the mixture for best power is richer than that for best economy.