What happens when the APU switch is moved to OFF?

What happens when the APU switch is moved to OFF?

• A. The APU generator trips, the APU bleed air valve closes, the APU GEN OFF BUS light extinguishes, and shutdown occurs automatically after a 60 second delay
• B. The APU generator remains engaged; no change
• C. The APU bleed air valve opens and system remains
• D. The APU GEN OFF BUS light illuminates; shutdown does not occur

Answer: (D)

When you move the APU switch to OFF, the APU is taken off the electrical bus. The GEN OFF BUS light comes on to show that the generator is no longer supplying electrical power to the bus. In this position, the indication reflects isolation from the electrical system, and the APU begins its shutdown sequence rather than an immediate, instantaneous cessation. This is why the light illumination is the visible cue, while the actual shutdown may occur after a brief, designed delay to ensure a safe cooldown and sequence.

When you move the APU switch to OFF, the APU is taken off the electrical bus. The GEN OFF BUS light comes on to show that the generator is no longer supplying electrical power to the bus. In this position, the indication reflects isolation from the electrical system, and the APU begins its shutdown sequence rather than an immediate, instantaneous cessation. This is why the light illumination is the visible cue, while the actual shutdown may occur after a brief, designed delay to ensure a safe cooldown and sequence.