Starting a descent on an Airbus A320 involves careful planning and execution. The procedure is based on airline Standard Operating Procedures (SOPs), ATC clearances, and operational conditions. Here's a general guide:

1. Plan the Descent

• Descent Point: Use the Flight Management System (FMS) to calculate the Top of Descent (TOD). This is typically displayed on the Navigation Display (ND) as a green circle.

• Descent Profile: Review the descent profile in the FMS, which includes altitude constraints, speeds, and waypoints.

• Clearance: Obtain ATC clearance for the descent. Note any altitude or speed restrictions.

2. Set the Descent Parameters

• Altitude: Dial the cleared descent altitude on the FCU (Flight Control Unit).

• Speed: Verify or adjust managed/selected speeds based on SOPs or ATC instructions.

• Descent Mode: Decide on a descent mode:

  • Managed Descent (Default): Push the altitude knob. The aircraft follows the vertical profile in the FMS.

  • Open Descent: Pull the altitude knob. The aircraft descends at idle thrust to the selected altitude, prioritizing speed control.

3. Initiate the Descent

• Monitor the aircraft as it transitions into the descent phase.

• If in managed descent:

  • Verify the "DES" mode is active on the FMA (Flight Mode Annunciator).

• If in open descent:

  • Confirm "OP DES" is displayed on the FMA.

4. Monitor the Descent

• Autothrust: Ensure autothrust is managing power, typically at idle during descent.

• Speed and Altitude Constraints: Monitor adherence to FMS constraints or ATC-imposed restrictions.

• Vertical Speed: Check the vertical speed indicator to ensure the rate of descent is appropriate.

• Energy Management: Use speed brakes if needed to increase the descent rate or reduce speed.

5. Communicate with ATC

• Report leaving current altitude and passing significant levels.

• Acknowledge further clearances or changes to the descent profile.

6. Approach Preparation

• Review and brief the approach procedure.

• Program the FMS for the expected approach and landing configuration.

• Transition to approach phase as per SOPs.

Key Points to Remember:

• Always verify the active modes on the FMA.

• Avoid excessive use of speed brakes unless necessary.

• Maintain situational awareness, especially in non-managed descent scenarios.

Let me know if you want more detailed guidance or specifics about a particular aspect of descent!