APU Starter cutout on the ground occurs at what percent N2?

APU Starter cutout on the ground occurs at what percent N2?

• A. 46%
• B. 50%
• C. 60%
• D. 70%

Answer: (A)

The cutout speed is the point at which the APU starter disconnects and the APU takes over as a self-sustained power source. For the CRJ-550 on the ground, the starter is removed when the core speed reaches about 46% N2. This threshold ensures the APU can light off and run reliably without relying on the starter any longer, while avoiding unnecessary starter wear or overspeed. Higher figures would keep the starter engaged longer than needed, while 46% is the designed value that safely transitions the APU to normal operation.

The cutout speed is the point at which the APU starter disconnects and the APU takes over as a self-sustained power source. For the CRJ-550 on the ground, the starter is removed when the core speed reaches about 46% N2. This threshold ensures the APU can light off and run reliably without relying on the starter any longer, while avoiding unnecessary starter wear or overspeed. Higher figures would keep the starter engaged longer than needed, while 46% is the designed value that safely transitions the APU to normal operation.