The APU starter cut-out range is used to define what RPM range?

• A. 30%-40%
• B. 46%-60%
• C. 12%-20%
• D. 60%-70%

Answer: (B)

The concept here is when the APU’s starter disengages during a start, which is defined by a specific RPM window. During an APU start, the starter spins the turbine up to a point where the APU can sustain operation on its own. That point is a percentage range of the nominal RPM, and it’s specified as about 46% to 60%. This window gives enough speed for reliable ignition and stable operation, while avoiding overspeed or prolonged starter wear.

Why this range fits best: 46% to 60% represents the mid-to-upper portion of the start spectrum where the turbine is spinning fast enough to light off and run independently, but not so close to the upper limit that the system risks overspeed or unnecessary starter stress. The other ranges are either too low for reliable ignition or too high, making them unsuitable as the cut-out point.