What condition causes header tank pressure to be low in the MQ-9?

The condition that leads to low header tank pressure in the MQ-9 is when the feed solenoid fails closed. This solenoid is responsible for regulating the flow of fuel from the header tank to the engine. When it fails in the closed position, it prevents the fuel from being delivered efficiently, resulting in reduced pressure within the header tank.

Maintaining appropriate header tank pressure is crucial for the proper operation of the fuel system, as it ensures that the engines receive the necessary fuel flow for performance and reliability. Without adequate pressure, the engines may experience fuel starvation, leading to potential operational issues.

Other conditions, while they can impact fuel system performance, do not directly lead to low header tank pressure in the same manner. For instance, a return solenoid failing open would likely cause fuel to return excessively, possibly affecting the system differently rather than just limiting pressure; a fuel tank leak would lead to fuel loss in the system rather than a specific pressure issue; and low ambient temperature primarily affects fuel viscosity and flow characteristics rather than the solenoid's operational state.