What is the purpose of the APU fuel system and how does it prevent fuel leaks or uncommanded starts?

• A. It stores fuel and uses no regulation.
• B. It meters and delivers fuel to the combustor via a controlled fuel pump, filters, and shutoff valves with safeguards to prevent flow unless commanded.
• C. It only filters fuel; no flow control.
• D. It relies on gravity to deliver fuel without valves.

Answer: (B)

The key idea is that the APU fuel system is responsible for delivering the exact amount of fuel the combustor needs, only when the APU is commanded to start or run, and it does this with regulators, sensors, and isolation devices to prevent leaks and unintended starts. The fuel is metered and delivered through a controlled fuel pump, with filters to remove contaminants and shutoff valves that isolate the fuel path when the APU is not demanding fuel. Safeguards—such as interlocks that require a start command, proper ignition, and positive isolation when the APU isn’t running—prevent fuel from flowing unless commanded. This combination ensures stable startup and operation, while keeping leaks or uncommanded starts from occurring.

Saying it simply, you need controlled flow and isolation for safe, reliable starts and operation. The other options skip essential parts: just filtering doesn’t manage flow or prevent leaks; gravity delivery isn’t reliable or controllable in an aircraft environment; and storing fuel without any regulation would allow uncontrolled flow and potential leaks.