In A320 Standard Operation Procedure (Parking procedure), the order of parking procedure is anti- ice off then APU bleed as required. Is there any meaning for this order? Can’t we switch the order instead? If there is any relation based on system relationship, I would like to know.
The SOP calls for
ANTI-ICE ... OFF APU BLEED pb-sw ... ON
the former covers both wing and engine anti-ice.
It is not allowed to use APU bleed for wing anti-icing, as defined in FCOM Limitations. (FCOM-LIM-APU-OPERATIONAL ENVELOPE)
As APU bleed has priority over engine bleeds the wing anti-ice valves have to be closed before APU bleed valve is opened.
For engine anti-icing the order does not really matter. Each engine uses its' own closed system for anti-icing with dedicated bleed valves.