Final answer:
A green "F/D" (Flight Director) annunciator is commonly used to indicate when the flight director mode is active and providing guidance to the pilots, as determined by the FCC.
Step-by-step explanation:
When the Federal Communications Commission (FCC) has determined that conditions are correct for a pilot-selected flight director mode, the indication provided is usually through the aircraft's flight director system on the cockpit displays. The specific indication for engagement of a pilot-selected flight director mode can vary by aircraft type and avionics system, but commonly, an illuminated green "F/D" (Flight Director) annunciator is used to show that the flight director mode is active and providing guidance to the pilots.