прилетели - past tense prefect aspect.
We have arrived, we had arrived, we arrived etc.
прилетаем,- present, imperfective aspect, but it's used to mean 'future' here. Well... It's possible to say 'we're arriving' in English.
Most probably this means - we haven't arrived yet, but we are sure now, that it'll happen точно по расписанию. "We're going to..."
It may mean just present (we're arriving right now, or we arrive usually), but this 'present as future' meaning is the most probable in the context.
Cf завтра мы идем в кино!
(we're going to...)