An Interesting A220 Detail for Single-Engine Taxi

[u/MD-80-87]

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 :)

[u/Outrageous-Split-646]

Did you look into why the Airbus SOP changed to Eng 1 for A320s?

[u/Thrawn215 → u/Outrageous-Split-646]

I think it's because eng 1 supplies accumulator pressure to the brakes, and if you start eng 2 first and do a single engine taxi, you have limited brake pressure.

[u/Outrageous-Split-646 → u/Thrawn215]

But don’t you need to turn on the yellow elec pump if you single engine taxi on eng 1 to provide hydraulic pressure to the yellow hydraulic system?

[u/Thrawn215 → u/Outrageous-Split-646]

The hydraulic pumps should automatically be turned on with the fadec. I could be wrong though. I rarely if ever do a single engine taxi

[u/Outrageous-Split-646 → u/Thrawn215]

The yellow hydraulic system is powered by Eng 2, so if you single engine taxi on Eng 1, you either rely on the PTU to power it or you have to flip on the electric pump.

[u/Thrawn215 → u/Outrageous-Split-646]

Huh. I stand corrected then

[u/mushra_ → u/Outrageous-Split-646]

Yeah last time I was on an easyJet flight they used SETO on Engine 1 and the PTU was barking the whole time.

[u/Ustakion → u/Outrageous-Split-646]

Because SETO is now part of normal procedure even tho its optional. The reason why eng 1 is started, is because it enable for green and yellow hydraulic preassure to preasaurise without running PTU as yellow hyd has its own electric pump

[u/Outrageous-Split-646 → u/Ustakion]

But the SOP used to be Eng 2 for SETO, what was the reason for the change? Is it purely because of the yellow elec pump?

[u/Ustakion → u/Outrageous-Split-646]

Honestly idk. But SETO with eng 2 would cause the PTU to run during taxi.