What must be off in order to perform spins?

To perform spins in a T-6 aircraft, the Power Management Unit (PMU) must be turned off. The PMU is responsible for managing engine performance and ensuring optimal operation in various flight conditions. When performing spins, the PMU would typically intervene to prevent conditions that could lead to a stall or uncontrolled spin, which means it must be disabled to allow for the full range of aircraft attitudes and behaviors.

The other systems, like the fuel, electrical, and hydraulic systems, are not specifically required to be off for spin training. In fact, those systems should remain operational for flight safety and maneuverability. If a pilot attempts a spin with the PMU engaged, the system's protective features would prevent the aircraft from spinning correctly or would recover from the spin automatically, potentially compromising the spin training experience. Therefore, disabling the PMU is essential for conducting spins safely and effectively.