When does header tank overpressure occur in the MQ-9?

Header tank overpressure in the MQ-9 occurs when the return solenoid fails closed. This component is essential in regulating the fuel that returns from the engine to the header tank. If the solenoid does not operate correctly and remains closed, it prevents the proper return of fuel, causing pressure to build up in the header tank. This overpressure situation can lead to potential damage or malfunction of the fuel system, as excess pressure may compromise seals and other fuel system components.

In contrast, the other options do not directly lead to header tank overpressure. Fuel pump failure would typically result in insufficient fuel delivery rather than an increase in pressure. Excessive fuel flow could refer to too much fuel entering the system, but it wouldn't necessarily cause a closed-system pressure build-up without the return path being obstructed. Fuel contamination could affect engine performance and fuel quality but does not relate specifically to the mechanical operation that would cause the header tank to overpressurize. Understanding how these components work together is critical for maintaining the MQ-9's fuel system integrity.