When does the AOA indexer illuminate on the T-6A?

The Angle of Attack (AOA) indexer on the T-6A is designed to provide critical visual feedback to the pilot about the aircraft's angle of attack relative to its critical values, which aids in maintaining safe flight operations, especially during approach and landing phases. The indicator illuminates specifically when the landing gear is down and locked to signal that the aircraft is in a configuration suitable for landing.

This is important as the landing gear position correlates with the aircraft's configuration and flight characteristics. When the landing gear is extended, it alters the aircraft's aerodynamic performance, increasing drag and affecting the ideal angle of attack for safe landing. Thus, the illumination of the AOA indexer when the gear is down and locked serves as a reminder to the pilot that careful management of angle of attack is vital to avoid stall conditions during this critical phase of flight.

The other options relate to conditions that do not specifically require the pilot to monitor the angle of attack in a meaningful way. For instance, the engagement of speed brakes, autopilot, or full extension of flaps may involve operational changes, but they do not provide the same critical safety reminder regarding angle of attack as the landing gear position does.