Hello Support Team,
I happened to notice that I can attach weapon mods during a flight.
Since the item page is locked, I also thought that you couldn't use mods that you didn't attach before starting. :)
Kind Regards
also found this thread but maybe something changed in the meantime :)
https://www.torn.com/forums.php#/p=threads&f=19&t=16193053&b=0&a=0&start=0&to=20899737
Edit: The weapon mod page was still accessible after landing, and I was able to apply the mods.
Mentions: Able to Equip Weapon Mods While Flying