Starting engine 2 first on an Airbus A320 is a standard practice in many operational scenarios, and it is primarily due to the following reasons:

1. Hydraulic System Consideration

• Engine 2 drives the Yellow Hydraulic System, which powers critical systems such as nosewheel steering and the parking brake. Starting engine 2 first ensures that these systems are operational if the aircraft needs to taxi or if there's a need to release the parking brake during ground operations.

2. Operational Efficiency

• On some airport layouts, the pushback operation or parking positions may require nosewheel steering functionality earlier. Starting engine 2 first allows crews to establish hydraulic power for these operations while still on the ground.

3. Standardization

• Airbus specifies this sequence as part of its standard operating procedures (SOPs) to streamline pilot workflow, reduce error, and provide consistency across fleets and airlines.

4. APU Location

• The APU bleed air system used for engine start is located closer to engine 2. Starting the closer engine first can slightly reduce pneumatic load on the APU during the start sequence.

5. Pushback Procedures

• In certain operational scenarios, only one engine may be started during pushback to minimize noise and fuel consumption. Starting engine 2 first allows the aircraft to taxi immediately after pushback is completed (if necessary), given the Yellow Hydraulic System's role.

Exceptions

In some cases, such as operational constraints or maintenance requirements, engine 1 may be started first. Always refer to the airline’s SOPs or the Airbus FCOM for specific guidance.