Under what condition can aircraft begin their descent when within a TAA?

Aircraft can begin their descent when they are cleared for the approach. This means that the pilot has received explicit authorization from air traffic control to proceed with the approach procedure, which encompasses the descent to the runway or a predetermined altitude.

During the approach phase, especially within a Terminal Arrival Area (TAA), pilots must adhere to specific altitudes and procedures designed to ensure safe and efficient traffic flow. The clearance for the approach signifies that the pilot should now follow the published procedures, including descending to the appropriate altitudes as laid out in the approach charts.

While reaching a specified altitude is a key part of executing the approach, it is the clearance for the approach that officially authorizes the descent and regulates the aircraft's altitude throughout the maneuver. Similarly, the conditions of visual flight do play a role in approach operations, but the initiation of descent is fundamentally linked to the approval from air traffic control. Therefore, the correct action is to wait for that explicit clearance before descending, ensuring compliance with regulations and safety protocols.