How is DC power maintained during start if AC power is not yet available?

How is DC power maintained during start if AC power is not yet available?

• A. The standby power system powers the essential DC bus.
• B. The APU provides DC power directly to all buses.
• C. There is no DC power during start.
• D. The battery powers the essential DC bus until the generators/TRUs come online.

Answer: (D)

During start, when no AC power is available yet, the essential DC bus must stay energized to keep the important systems alive. The aircraft battery provides that DC power directly to the essential DC bus, ensuring flight controls, instruments, and other critical equipment have power while the engines or APU are starting. As the APU or engines spool up, their generators begin producing AC power. Transformer-rectifier units then convert that AC into DC power to feed the DC buses, including the essential DC bus, allowing the electrical system to transition from battery-only to generator-supplied operation. This is why the battery powers the essential DC bus until the generators and TRUs come online. The other options aren’t correct because the APU doesn’t normally power all DC buses directly without conversion, there isn’t a separate standby system powering the essential DC bus in this scenario, and there is DC power during start to support those critical systems.

During start, when no AC power is available yet, the essential DC bus must stay energized to keep the important systems alive. The aircraft battery provides that DC power directly to the essential DC bus, ensuring flight controls, instruments, and other critical equipment have power while the engines or APU are starting.

As the APU or engines spool up, their generators begin producing AC power. Transformer-rectifier units then convert that AC into DC power to feed the DC buses, including the essential DC bus, allowing the electrical system to transition from battery-only to generator-supplied operation. This is why the battery powers the essential DC bus until the generators and TRUs come online.

The other options aren’t correct because the APU doesn’t normally power all DC buses directly without conversion, there isn’t a separate standby system powering the essential DC bus in this scenario, and there is DC power during start to support those critical systems.