FAA Powerplant Written Practice Test

In a hydro mechanical fuel control system, what pressure does the EFCU measure to determine fuel quantity?

1. A Fuel tank pressure

2. B Compressor discharge pressure

3. C Inlet manifold pressure

4. D Ambient pressure

The correct answer is: B Compressor discharge pressure

In a hydro mechanical fuel control system, the Electronic Fuel Control Unit (EFCU) utilizes compressor discharge pressure to determine fuel quantity. This pressure is critical because it directly correlates with the aircraft engine's performance and the amount of fuel required for optimal operation. As the compressor discharge pressure changes, it provides important feedback regarding the engine's operational state. In responding to varying power demands and environmental conditions, the EFCU adjusts the fuel flow accordingly, ensuring the engine operates efficiently and safely. Understanding that fuel management relies on the dynamic relationship between airflow (indicated by compressor discharge pressure) and fuel flow makes this choice appropriate. Other pressures, such as fuel tank pressure, inlet manifold pressure, or ambient pressure, do not offer the same level of correlation with the fuel control requirements necessary for the engine’s operational efficiency.