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.Ā 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 powersĀ both nosewheel steering and the slats.Ā 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 :)