Why doesn't the APU have a CSD?

• A. Because it runs at a constant speed.
• B. Because it uses a variable speed drive.
• C. Because CSD is not required.
• D. Because it's cooled by air.

Answer: (A)

The main idea is that a Constant Speed Drive is used when a machine’s speed varies and you need to output a steady speed for the accessory or generator. An APU is designed to run at a nearly fixed shaft speed, controlled by an internal governor and generator regulator. This keeps the electrical output frequency stable without needing a separate drive. So there’s no CSD required because the APU’s own speed control maintains the constant input speed the generator needs. The other options don’t fit because a variable speed drive would contradict the need for steady output, and cooling or the idea of “not required” without the mechanism isn’t the reason—the actual reason is the built-in constant-speed behavior.