。
I watched five airplanes fly away just like birds.
Literal
Airplane [subject-が] 5-aircraft just-like birds [like-のように] fly-away [nominalizer-の] [object-を] saw.
さながら is a literary adverb meaning 'just like, exactly as if' — more elevated than まるで. 飛び去る is a compound verb: 飛ぶ ('fly') + 去る ('depart'). ~のを見た uses the perception construction: seeing (見た) the event (飛び去るの) happen. 機 (き) is the counter for aircraft.