As title what does it mean and when should it be used.? I have googled it but still confused. I can land perfectly ok using ILS, I struggle a bit on visual. 

LOC means localiser (which is the lateral guidance of an ILS approach). Do you mean the LOC button? It‘s used to capture the localiser, but not yet the glideslope (GS).
Use cases:
1) You‘re cleared for the LOC only, and are supposed to stay off the GS until a certain distance / waypoint. In this situation you can only use the LOC button.

2) LOC only approach (no GS available). If you use the APPR button, the A320 might fly an RNAV overlay instead of using the raw data localiser. In this case hitting LOC enables you to actually use the localiser.

3) There is a GS, but it‘s faulty / under maintanance. In this case using the GS could be potentially dangerous and therefore you can not hit the APPR button, but use the LOC button instead.

4) Some (like myself) like to first capture the LOC only and then hit APPR. It feels more natural to me and gives more control over the aircraft in certain situations.

5) When you‘re too high and need to intercepting the glideslop from above, this can lead to dangerous descents and might be hard to control if you use APPR mode (with GS armed). Instead it‘s recommended to use LOC only, descent with VS and wait until the actual GS indicator on the PFD says you‘re now at/below the GS, and only then hit the APPR button. 

That said, if you only fly offline you‘ll practically never have to use LOC, since all its use cases are induced by ATC or real life maintainance stuff.

You'd use LOC if you've been instructed to join the localizer but not cleared for the ILS approach (probably the most common case these days) or when flying a localizer back course or localizer-only approach (though these are increasingly rare given the prevalence of RNP approaches).

Edit: What @Fiorentoni said!

Edited February 27, 20242 yr by martinboehme

On 2/27/2024 at 11:37 AM, Fiorentoni said:

LOC means localiser (which is the lateral guidance of an ILS approach). Do you mean the LOC button? It‘s used to capture the localiser, but not yet the glideslope (GS).
Use cases:
1) You‘re cleared for the LOC only, and are supposed to stay off the GS until a certain distance / waypoint. In this situation you can only use the LOC button.

2) LOC only approach (no GS available). If you use the APPR button, the A320 might fly an RNAV overlay instead of using the raw data localiser. In this case hitting LOC enables you to actually use the localiser.

3) There is a GS, but it‘s faulty / under maintanance. In this case using the GS could be potentially dangerous and therefore you can not hit the APPR button, but use the LOC button instead.

4) Some (like myself) like to first capture the LOC only and then hit APPR. It feels more natural to me and gives more control over the aircraft in certain situations.

5) When you‘re too high and need to intercepting the glideslop from above, this can lead to dangerous descents and might be hard to control if you use APPR mode (with GS armed). Instead it‘s recommended to use LOC only, descent with VS and wait until the actual GS indicator on the PFD says you‘re now at/below the GS, and only then hit the APPR button. 

That said, if you only fly offline you‘ll practically never have to use LOC, since all its use cases are induced by ATC or real life maintainance stuff.

Thank you so much for that infomation.

On 2/27/2024 at 11:38 AM, martinboehme said:

You'd use LOC if you've been instructed to join the localizer but not cleared for the ILS approach (probably the most common case these days) or when flying a localizer back course or localizer-only approach (though these are increasingly rare given the prevalence of RNP approaches).

Edit: What @Fiorentoni said!

Thank you, much clearer now much appreciation for your response.

LOC is half an ILS 😄  .... the "lefty/righty bit".   GS in the "uppy/downy bit".      LOC + GS = ILS.

There are though some approaches that are LOC only, where the ground equipment includes a localizer but not a glideslope.

Also, you don't have to arm LOC for an ILS.  Just press APPR.