Which of the following pairs best describes the two sections of the fuel control unit?

• A. Manual control and hydro-mechanical control
• B. Automatic control and hydraulic control
• C. Manual control and hydraulic control
• D. Automatic control and hydro-mechanical control

Answer: (D)

The fuel control unit is built around two parts: an automatic control section and a hydro-mechanical control section. The automatic section handles the programmed fuel schedule based on throttle input and engine conditions, deciding how much fuel should be allowed to meet the desired thrust and keep speeds within safe limits. The hydro-mechanical section then uses hydraulic pressure and mechanical linkages to move the actual metering valve, converting that automatic command into precise fuel flow and maintaining proper fuel pressure and response during changes in demand. This combination lets the system automatically schedule fuel while the hydro-mechanical part provides the physical control and safeguards needed for real-time operation. Other pairings don’t fit how these units are typically organized, since manual control isn’t one of the two major sections, and hydraulic or manual-only descriptions don’t capture the automatic scheduling paired with precise metering.