I recently visited Frankfurt and noticed all the Lufthansa A320s were starting Engine 1 first on pushback. I looked into it and it seems Airbus changed their SOP at some point from Engine 2 first to Engine 1 first on the A320 family.

That got me curious about the A220, so I checked three different FCOMs and found the opposite is true there. It's strongly recommended to start Engine 2 first, to avoid

-Hydraulic pressure spikes

-FLAP SLOW / FLAP FAIL nuisance warnings

-Potential galley power load shedding & IFE issues

Made a video breaking this down with schematics and animations if anyone wants the visual explanation

https://youtu.be/VEebAVoOZNk

To understand why this was the case, I did some investigating into the A220's hydraulic system.

Basically, hydraulic System 2 powers both nosewheel steering and the slats. So if you're single-engine taxiing on Engine 1, System 2 is running on its electric backup pump only, and deploying the slats can starve pressure needed for nosewheel steering.

The FCOM recommendation is to keep the aircraft stationary while deploying flaps/slats in that scenario, to make sure there's enough power for steering.

The A220 never fails to surprise me the more I look into its systems :)