DFW Instructors 300/350 Practice Test

When the NAV mode is used for navigation and the flight director detects a loss of the navigation source, it automatically defaults to BASIC ROLL mode. This mode is essential for maintaining some level of control and stability during flight when the aircraft's intended navigation reference becomes unavailable. BASIC ROLL mode provides the pilot with a means to fly and maintain level flight or perform turns based solely on the aircraft's current heading and roll inputs, rather than relying on external navigation inputs that might no longer be valid.

In contrast, the other modes listed are designed for specific functionalities. HEADING mode allows the aircraft to maintain a specific heading, but it does not automatically engage when the navigation source is lost. ALTITUDE HOLD mode is focused on maintaining a specified altitude rather than heading or navigation. GLIDE SLOPE mode is specific to instrument landing systems and is not applicable when operating under lost navigation conditions. Therefore, BASIC ROLL mode remains the most suitable and logical default when navigating without a valid navigation source.