What component regulates the prop RPM in the event of a PMU or PIU failure?

The component that regulates the propeller RPM in the event of a PMU (Propulsion Management Unit) or PIU (Propeller Interface Unit) failure is the mechanical flyweight. The mechanical flyweight operates as a backup system that ensures the propeller maintains a specific RPM range. It uses centrifugal force to adjust the blade pitch, thereby managing the engine's power output and propeller speed.

When the primary electronic control systems like the PMU and PIU are compromised, the mechanical flyweight takes over the control of the propeller pitch. This allows the propeller to function safely and efficiently by providing a more passive and direct method of controlling the RPM, which is particularly essential for maintaining performance and safety in flight.

The other options, while related to engine and propeller performance, do not fulfill the same role in the event of the PMU or PIU failing.