What does the display of flags such as NAV, HDG, or GS on an HSI indicate?

When flags such as NAV, HDG, or GS appear on a Horizontal Situation Indicator (HSI), they serve as alerts that indicate that a specific function is inoperative. These flags are critical for pilots, as they signal a lack of reliability or data from the navigation systems, such as the navigation (NAV) system, heading (HDG) sensor, or ground speed (GS) information.

Each type of flag corresponds to a specific function of the HSI, and their display means the pilot should not rely on that particular information for navigation or flight control. Recognizing these indicators allows for the assessment of the aircraft's navigational status and helps pilots take necessary precautions or procedures, such as cross-checking with other instruments or reverting to manual navigation methods.

Other choices do not accurately represent the implications of these flags: they do not indicate manual mode, signal loss across the board, or standard operating conditions. Understanding this functionality is essential for maintaining safe flight operations.