Das Flugzeug, das gestern gelandet ist, war international unterwegs.

The first das is the definite article for the neuter noun Flugzeug (i.e. “the plane”).
The second das is a relative pronoun that refers back to Flugzeug and introduces the relative clause das gestern gelandet ist (“that landed yesterday”).

Relative clauses in German are subordinate clauses, and subordinate clauses always put the finite verb (here ist) at the end, after any participles or objects. Since this clause is in the perfect tense, you have the past participle gelandet + auxiliary ist at the very end.

In spoken and many written contexts, German uses the perfect tense (Perfekt) to describe past events. Gelandet ist tells us that the landing is a completed action. The simple past (landete) is also correct but is more common in formal or literary styles.

Verbs of movement or change of state (like landen, gehen, fahren) form the Perfekt with sein. So you say ist gelandet, not hat gelandet.

In German, all relative clauses are set off by commas. You place a comma before the relative pronoun (here das) and another comma after the end of the clause (just before war).

unterwegs literally means “on the way” or “en route.” Pairing it with war (simple past of sein) gives war unterwegs, meaning “was traveling” or “was in transit.”

Here international is an adverb modifying unterwegs, so it directly describes how the plane was traveling (“internationally”). Adverbs in German often appear without a preposition when modifying verbs or other adverbs.

German frequently uses Präteritum (simple past) for auxiliary verbs like sein, especially in written or narrative contexts. That’s why we see war unterwegs rather than ist unterwegs gewesen.

In a subordinate clause, the finite verb must stay last. A natural order is: relative pronoun (das) – time adverb (gestern) – participle/auxiliary (gelandet ist). You could also say das gelandet ist gestern, but that sounds awkward. Time expressions typically come directly after the subject or relative pronoun.