No rule for this one
No ending or word group predicts the article of Flug. Learn it as a pair and say it out loud: der Flug.
der Flug
It's der Flug. Flug is masculine, plural die Flüge.
Der Flug nach Frankfurt dauert sieben Stunden.
The flight to Frankfurt takes seven hours.
No ending or word group predicts the article of Flug. Learn it as a pair and say it out loud: der Flug.