What component maintains Np at 100+/-2% if the PMU is lost or deactivated?

The component that maintains Np at 100 +/- 2% in the event of PMU loss or deactivation is the mechanical flyweight overspeed governor. This governor is designed to function as a fail-safe mechanism, ensuring that the propeller speed remains within a safe operational range even when electronic controls, such as the PMU, are no longer active.

When the PMU is operational, it effectively manages engine performance and propeller speed by adjusting parameters based on flight conditions. However, if the PMU is lost, the mechanical flyweight overspeed governor takes over to prevent the propeller from overspeeding. It does so by automatically controlling the pitch of the propeller blades; if the propeller begins to exceed the specified RPM range, the governor detects this and alters the blade angle to reduce the engine speed back into the desired range.

The other components mentioned do not directly maintain propeller speed in the absence of PMU control. The fuel management unit and engine control unit work to govern various aspects of engine performance, while the propeller governor needs input from the PMU to operate optimally. Thus, in situations of PMU failure, it is the mechanical flyweight overspeed governor that ensures stable propeller operation.