What mode does an engaged autothrust system revert to when the flight directors and autopilots are switched off?

When the autothrust system is engaged and both the flight directors and autopilots are turned off, it reverts to "Speed" mode. This transition occurs because the autothrust system is designed to maintain the aircraft's current speed as a means of providing thrust management without the additional guidance and control offered by the flight directors and autopilots.

In this scenario, the system continues to operate by making adjustments to the thrust in response to changes in speed, ensuring that the aircraft remains as stable as possible during flight. This is particularly useful in situations where manual control is preferred or required, allowing the pilot to focus on other aspects of flying while relying on the autothrust to manage speed effectively.

Other modes, like "Thrust," "Armed," or "Fixed," do not apply in this situation. "Thrust" mode would indicate a specific thrust setting rather than speed management, while "Armed" typically refers to a readiness state, and "Fixed" would imply a static thrust setting that doesn't adjust based on aircraft speed. Thus, "Speed" mode is the correct answer as it reflects the operational logic of the autothrust system when the autopilot and flight director systems are deactivated.