FAA Powerplant Written Practice Test

What is a primary concern if the engine pressure ratio becomes excessively low?

1. Fuel efficiency improves

2. Engine thrust decreases

3. Engine maintenance costs rise

4. Fuel consumption decreases

The correct answer is: Engine thrust decreases

When the engine pressure ratio becomes excessively low, the primary concern is a decrease in engine thrust. The engine pressure ratio is a measure of the engine's efficiency and performance, defined as the ratio of the pressure of the exhaust gas leaving the engine to the pressure of the air entering the engine. A low pressure ratio indicates that the engine is not effectively compressing the air entering it, which directly impacts the engine's ability to produce thrust. As the pressure ratio declines, there is less energy available to convert into thrust, leading to reduced overall performance. This drop in thrust can affect aircraft operations, particularly during critical phases such as takeoff and climb, where maximum thrust is essential. Therefore, maintaining an appropriate engine pressure ratio is crucial for ensuring optimal engine performance and safety. The other options do not accurately represent the primary concern associated with a low engine pressure ratio. Fuel efficiency may not improve as thrust decreases; rather, it might worsen due to inefficient engine operation. Similarly, rising engine maintenance costs and decreasing fuel consumption are not immediate concerns related to a low pressure ratio, as these factors are influenced by other operational parameters and engine conditions.