After hitting the APU start button, when does the starter cut out?

After hitting the APU start button, when does the starter cut out?

• A. 46% on the ground; 46 - 60% in the air
• B. 50% on the ground; 60 - 70% in the air
• C. 40% on the ground; 40 - 50% in the air
• D. 60% on the ground; 70 - 80% in the air

Answer: (A)

The key idea is that the APU starter is cut out when the APU compressor reaches a specific speed, to stop the starter from overworking and to let the APU take over smoothly. For the CRJ-550, the automatic starter cutout is at about 46% N2 when the aircraft is on the ground. In flight, the system allows a higher cutoff, in the range from 46% to 60% N2, to accommodate the different operating conditions you meet in the air (airflow, engine bleed demands, electrical load, and startup dynamics). Once the N2 reaches the cutoff threshold, the starter disengages and the APU continues running as a self-sustaining power source. This explains why the ground value is a fixed 46% while in the air it spans 46–60%—to ensure a reliable start across environments while protecting the starter.

The key idea is that the APU starter is cut out when the APU compressor reaches a specific speed, to stop the starter from overworking and to let the APU take over smoothly. For the CRJ-550, the automatic starter cutout is at about 46% N2 when the aircraft is on the ground. In flight, the system allows a higher cutoff, in the range from 46% to 60% N2, to accommodate the different operating conditions you meet in the air (airflow, engine bleed demands, electrical load, and startup dynamics). Once the N2 reaches the cutoff threshold, the starter disengages and the APU continues running as a self-sustaining power source. This explains why the ground value is a fixed 46% while in the air it spans 46–60%—to ensure a reliable start across environments while protecting the starter.