FAA Powerplant Written Practice Test

As aircraft speed increases from static, what happens to the thrust output temporarily?

1. It increases steadily with speed

2. It temporarily decreases

3. It remains constant at all speeds

4. It becomes erratic and unpredictable

The correct answer is: It temporarily decreases

As aircraft speed increases from a static position, the thrust output temporarily decreases due to the effects of airflow and engine performance characteristics. At lower speeds, an aircraft's engines are designed to produce maximum thrust at static conditions. However, as the aircraft accelerates, the increased airflow over the engine's components can lead to a condition known as "compressor stall" in high-bypass turbofan engines or increased drag effects that may temporarily reduce the effective thrust output. This phenomenon can be attributed to several factors, including changes in the engine's operating point, airflow disruptions through the inlet, and the influence of drag on the aircraft's overall performance. The net result is that there can be a momentary dip in thrust output as speed increases, until the engine reaches a more favorable operational condition and begins to produce thrust more efficiently at higher speeds.