Answer: The flights' passengers.
Explanation: Since the word already ends in 's' you just add the apostrophe after the 's'.
Example: The kids don't have work. But the kids' parents do.
Since kids is plural and it already has an 's' the apostrophe follows the 's'. But if the word 'kids' isn't plural it would be like this:
The kid doesn't have work. But the kid's parents do.