FAA Powerplant Written Practice Test

What pressure is measured by the Electronic Fuel Control Unit (EFCU) in a hydro mechanical fuel control system to determine fuel quantity?

1. Ambient air pressure

2. Compressor Discharge Pressure

3. Fuel inlet pressure

4. Exhaust pressure

The correct answer is: Compressor Discharge Pressure

In a hydro mechanical fuel control system, the Electronic Fuel Control Unit (EFCU) measures the compressor discharge pressure to determine fuel quantity. This is crucial because the fuel flow needs to be accurately metered in relation to the engine's operating conditions. The compressor discharge pressure reflects the amount of air being compressed and entered into the combustion chamber, which is directly related to the power output and efficiency of the engine. Fuel needs to be delivered in proportion to the airflow for optimal combustion; thus, the EFCU uses this pressure measurement to adjust fuel flow accordingly. The accurate correlation between compressor discharge pressure and the demand for fuel ensures that the engine performs efficiently and within its designed parameters, leading to effective powerplant operation.