Both are correct, but they don't mean quite the same thing.
"Is arriving" (the traditional "present progressive") need not only reference the present time. It can also reference an action arranged for the future.
"Will arrive" (the traditional "future tense") references not just an action in the future but also an assumption with regard to the future.
So the difference is between whether you are thinking about the arrival as an "arrangement" or an "assumption". The conditionality of the "unless" clause does not affect this.
You could also use "should arrive" (emphasising the obligation of the airline), or plain "arrives" (emphasising action set by a timetable or schedule).
This is a good list of uses for the English tenses which helped me: