Learn Italian: Al nostro arrivo, la ragazza sarà già andata alla dogana. - Upon our arrival, the girl will have already gone to customs.

Because the main verb is andare (to go), which is a verb of motion. Just like in the passato prossimo (present perfect), verbs that indicate movement from one place to another take essere as their auxiliary (helping) verb in all compound tenses, including the futuro anteriore.

Why does the past participle end in "-a" (andata)?

Whenever a compound tense uses essere as the helping verb, the past participle must agree in gender and number with the subject. Since the subject here is la ragazza (feminine singular), the default participle andato changes to andata.

Why is già placed right in the middle of sarà and andata?

In Italian compound tenses, short adverbs of time like già (already), ancora (yet/still), and mai (ever/never) are typically placed directly between the auxiliary verb (sarà) and the past participle (andata).

Why do we use al in Al nostro arrivo to mean "Upon our arrival"?