What does a flashing "VV-400" symbol in the pitch or thrust FMA indicate?

The flashing "VV-400" symbol in the pitch or thrust Flight Mode Annunciator (FMA) indicates that the vertical speed mode has been engaged without an altitude being armed. This situation is critical for pilots to recognize because it signifies that the aircraft is set to descend at a specified rate but without a reference altitude to level off at. Therefore, the aircraft could continue to descend indefinitely, which could lead to safety issues if not managed properly.

Understanding this alert is essential for ensuring the aircraft trajectory remains controlled and that altitude changes are intentional and safe. It prompts the pilot to intervene if needed, either by selecting an altitude to level off at or by adjusting the vertical speed to prevent unintentional descents. Being aware of these notifications helps maintain situational awareness and enhances flight safety.