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According to the answer in this post Taxiing with one engine: Is engine #1 always used or do they switch? Engine #1 is used because it pressurizes the green HYD system which powers the Nose Wheel Steering. I found that, in latest model of A320 and A321, NWS is driven by yellow HYD system that is pressurised by engine #2. However, pilots still use only engine #1 if they want to taxi with one engine running. Is there other reason for using engine #1?

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Source: Airbus AMM

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According to the "Supplementary Procedures / Green Operating Procedures - One Engine Taxi" in section PRO-SUP-93-20 of the A320 FCOM:

Use Engine 1 for taxiing because it pressurizes the green hydraulic system (normal braking).

and

Y ELEC PUMP.............................ON
This pressurizes the yellow hydraulic system (nosewheel steering) without using the PTU.

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    $\begingroup$ Also note that Green does not have ELEC PUMP, so the reverse configuration is not possible. $\endgroup$
    – Jan Hudec
    Mar 17 '20 at 19:15
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    $\begingroup$ The key thing is that in this configuration, a single point of failure still leaves all hydraulic systems pressurized. If the left engine or its pump fails, green is pressurized via the PTU. If the ELEC PUMP fails, yellow is pressurized via the PTU. If only the PTU fails, you don't need it. $\endgroup$
    – Bianfable
    Mar 18 '20 at 7:39

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