Learn Portuguese (Portugal): Os aviões partem hoje.

The singular word for airplane is avião. In Portuguese, the most common way to make words ending in -ão plural is by changing the ending to -ões. So, avião becomes aviões.

Why did the tilde (~) move from the a to the o in aviões?

In the singular word avião, the tilde is over the a to show a nasal sound. When the ending changes to the plural -ões, the nasal sound shifts to the o, so the tilde moves there too!

What is the dictionary form of the verb partem?

The verb is partir (to leave or to depart). Because we are talking about multiple airplanes (a 'they' subject), we drop the -ir and add the third-person plural ending -em, giving us partem.

Doesn't partir also mean 'to break'?

Yes, it does! Partir is a very versatile verb that can mean either 'to leave' or 'to break'. You have to rely on context to know which one is being used. Here, airplanes scheduled for today clearly mean 'to leave' rather than 'to break'.