- The FDAI (Flight Director/Attitude Indicator) was a critical Apollo instrument that displayed spacecraft orientation via a rotating black-and-white ball called the '8-ball'.
- The ball rotates in three axes (roll, pitch, yaw) using a clever mechanism: two hollow hemispherical shells provide yaw rotation, while the inner mechanism handles pitch and roll via motors and gimbals.
- Slip rings and two sets of electrical contacts prevent wiring tangles as the ball rotates in multiple axes.
- The FDAI uses synchros and servo loops—with control transformers, amplifiers, and motor/tachometers—to precisely position the ball and needles based on input signals.
- The instrument evolved from Bill Lear's designs for the X-15 and F-4 fighter, through Gemini and Apollo, and later influenced the Space Shuttle ADI.
- The specific unit examined was built for Apollo (dated 1968) but modified for a Shuttle simulator, including replacement of resolvers with synchros, incandescent lighting, and repainted dial.
- Key differences from the Apollo design include modified electronics, removal of pitch trim (irrelevant for spacecraft), and addition of connectors for easier simulator reconfiguration.
- The Shuttle ADI had more advanced features like error detection, LVDT feedback for needles, and integrated circuits, but shared the same basic ball-rotation mechanism.