What is the range where the APU starter cut-out will occur?

• A. 46%-60%
• B. 30%-40%
• C. 12%-20%
• D. 60%-75%

Answer: (A)

When the APU starts, the starter cut-out is the point where the starter disengages after the turbine has accelerated enough to sustain operation on its own. That disengagement happens in the middle portion of the APU’s speed range—fast enough for stable combustion and ignition, but not so fast that the starter is overstressed. This mid-range window is chosen so starts are reliable across different conditions (temperature, altitude, load) and to avoid two bad outcomes: starting too early (fuel/ignition may not hold) or starting too late (starter could be damaged from over-speed). That’s why this mid-range window is the correct choice. In practice, ranges that are too low won’t guarantee a successful start, while ranges that are too high risk overstressing the starter.