Relevant extracts from FCOM
The FCU APPR pushbutton arms or engages LOC and G/S modes, if:
‐ An ILS approach is entered in the flight plan, or
‐ No approach, or only a runway, is entered in the flight plan, and an ILS is manually-tuned on the RAD NAV page, or
‐ Both RMPs are set to NAV, and an ILS is selected.
PRO- NOR - SRP - 01- 70 P5/12
CAUTION
If the pilot decides to fly the ILS approach without revising the arrival of the primary flight plan (a non ILS approach is in the F-PLN), LOC and G/S modes will not be available when he presses the APPR pushbutton.
Consequently, he should:
• Manually TUNE in the ILS on the RAD NAV page: CHECK that the "CHECK APPR SELECTION" message comes up.
• Press the ILS pushbutton and select ROSE ILS on the EIS CONTROL panel.
• Use HDG, V/S or TRK, FPA modes to fly the ILS
P 10/12 same section
So, when only runway and no approach is selected. Then manually tune ils, course, arm approach and go.
But if another approach is already in, then will have to fly hdg, vs/trk fpa.
This explains.