Learn Turkish: Herkes havacılık uzmanlarının o sistemi kullandığını biliyor. - Everyone knows that the aviation experts use that system.

This is the core of how embedded clauses work in Turkish. The phrase expressing what is known is turned into a large noun phrase. In these nominalized clauses using the -dIk suffix, the logical subject doing the action must take the genitive case. So, the experts (uzmanları) become the grammatical owners of the action (uzmanlarının).

Let's look at kullandığını. Why does it end with the accusative suffix -ı?

The entire embedded clause acts as the direct object of the main verb biliyor (knows). Because everyone knows a specific, defined fact, the accusative -ı is attached to the very end of the nominalized verb, right after the buffer letter -n-.

Can you break down all the suffixes on kullandığını?

It breaks down as kullan (use) + -dığ (the -dIk participle, with k mutating to ğ) + -ı (3rd person possessive, linking back to the experts) + -n- (buffer letter) + -ı (accusative case, showing it is the object of the main verb).

In havacılık uzmanlarının, why are there two suffixes after uzmanlar?