Which valve fails in the closed position?

In aircraft systems, particularly those involving engines and auxiliary power units, certain valves are designed to fail in a closed position for safety and operational integrity. The feature of failing closed helps prevent unintentional flow or operation when there is a malfunction.

The Engine PRSOV (Pressure Regulating Shutoff Valve) is designed to close to stop fuel flow to the engine if there is an issue in the system, providing a safeguard against potential hazards such as fuel leaks. The APU LCV (Auxiliary Power Unit Load Control Valve) also works on similar principles, closing to prevent unneeded fuel flow to the APU during a failure scenario. Additionally, Engine HP (High Pressure) valves are critical components that help regulate fuel and airflow to the engines; if a failure occurs, these valves closing limits potential damage and maintains engine safety.

Thus, the characteristic of failing closed is common across these components, ensuring that under fault conditions, systems revert to a safe state rather than allowing unregulated flows that could compromise the aircraft's safety. Knowing that all these valves are designed with this fail-safe mechanism reaffirms the selected answer.