"Destroy the planes" is a mission objective in Assassin's Creed Unity: Server Bridge Paris 1944
There are a few planes that you need to deal with. Thankfully it is just one at a time. As much as possible, lead your shots. This will improve the time you need to take down any given plane. There are 2 planes to deal with here. Each have their own health meter. Remember, whenever Arno is under attack, to take cover behind the machine gun. You need to try to get them while they are lining up to come at Arno. You only get a short while to shoot at them before the bullets start hitting you.