。
The airplane is almost certainly going to be delayed.
Literal
Airplane [possessive-の] being-late [topic-は] certainly mistake-not.
飛行機の遅れること uses の before the verb in a nominalized clause — a somewhat formal/written construction where の replaces が as the subject marker inside the clause. Modern speech would more naturally say 飛行機が遅れることは. まず間違いない ('almost certainly, no doubt') combines まず ('first of all') and 間違いない ('no mistake').