Describe the role of the fuel control unit in meeting engine speed commands.

• A. It meters fuel flow to the combustor to maintain the commanded speed and stability across operating conditions.
• B. It meters fuel flow to the combustor to adjust exhaust emissions.
• C. It meters air flow to regulate compressor speed.
• D. It meters fuel flow to the combustor to maintain the commanded speed.

Answer: (D)

The fuel control unit’s job is to convert the engine’s speed command into a fuel metering schedule that the combustor can follow. By adjusting the fuel flow with a metering valve or servo, it changes the amount of fuel entering the combustor so the turbine work and shaft speed rise or fall toward the commanded value. It uses feedback from engine speed (and sometimes other sensors) to trim fuel: if speed tends to drop, fuel flow increases; if speed tends to rise, fuel flow decreases. This keeps the engine at the desired speed and helps maintain stable operation across different inlet conditions, loads, and temperatures. It does not directly regulate air flow or emissions; those are managed by other subsystems, while the FCU focuses on supplying the right fuel amount to meet the speed command.