The RPM of a gas turbine APU in a large jet transport aircraft is controlled by which device?

• A. Fuel (Governor) FCU
• B. Electronic Control Unit
• C. Throttle Lever
• D. Mechanical Governor

Answer: (A)

Fuel metering to the combustor governs the APU’s speed. The fuel control unit (FCU) includes a governor that continuously compares the actual RPM with the commanded value and adjusts fuel flow to keep the APU running at the desired speed. When the speed tends to rise, the governor cuts fuel flow; when it tends to fall, it increases fuel flow. The throttle lever simply sets the requested speed, but the actual regulation is performed by the FCU. While some systems may incorporate electronic controls, the device that directly controls RPM in a large jet APU is the Fuel Control Unit with its governor.