The PDI Gauge is used closely with the C-1 Autopilot. PDI stands for Pilot Directional Indicator. The gauge is used to show the pilot when the aircraft is properly trimmed. When the aircraft is trimmed and the PDI is centered, then the autopilot may be engaged. The autopilot will then make corrections automatically.
The PDI gauge is rated at 26 volts, however it only recieves 24. It is also painted with UV and glow-in-the-dark paint, as are the other instruments.