If the APU is rotating above __% in flight or __% on the ground, the APU Electronic Control Unit will inhibit start

• A. 10%
• B. 11%
• C. 12.5%
• D. 15%

Answer: (C)

APU starts are blocked when the rotor is already turning, because starting against a moving turbine can cause mechanical stress or damage to the starter and geartrain. The APU Electronic Control Unit uses a fixed speed threshold to detect this condition. If the APU rotor speed exceeds 12.5% (whether the aircraft is in flight or on the ground), the ECU inhibits a new start. Below that speed, a normal start is allowed as long as other conditions are met. So 12.5% is the protective limit that prevents starting against windmilling motion, ensuring safe and reliable operation.