At what Np does the PMU switch to the battery bus?

The correct choice indicates that the Power Management Unit (PMU) switches to the battery bus at an engine speed (Np) between 40-50%. This is significant because the PMU is responsible for managing electrical systems and ensuring that the engine control and avionics functions remain powered during various phases of flight, particularly during low power conditions or engine failure scenarios.

At lower engine speeds, such as those within the 40-50% Np range, the electrical system can experience variations in output and efficiency. As such, switching to the battery bus at this point helps to maintain system stability and ensures that critical components receive reliable power despite potential fluctuations in the generator's performance. This transition is a protective measure to guard against the risk of losing power to essential systems when the engine is not producing enough output.

Understanding this transition point is crucial for T-6A pilots and helps them manage their aircraft's systems effectively, especially in critical phases of flight or emergencies.