Learn Italian: Entro il decollo, loro saranno arrivati all'aeroporto. - By the takeoff, they will have arrived at the airport.

In the Future Perfect tense, just like in the Passato Prossimo, verbs of motion like arrivare (to arrive) require essere as their helping verb. Since the subject is loro, we use the future tense of essere, which is saranno.

Notice the ending of arrivati. Why doesn't it end in -o?

Because the helping verb is essere, the past participle must agree with the subject in gender and number. Since loro refers to a plural group (masculine or mixed), arrivato changes to the plural form arrivati.

How exactly does entro work in this sentence?

When talking about time and deadlines, entro translates to "by" or "within" a specific time limit. Here, entro il decollo sets the deadline, meaning the arrival will happen no later than the takeoff.

Why is it all'aeroporto instead of just a aeroporto?

The preposition a (at/to) must combine with the definite article when referring to the airport. Since aeroporto is a masculine noun starting with a vowel, it takes the article l'. Together, a + l' form the combined word all'.