expression, godan verb (-ru)
board an airplane; get on an airplane
A formal or literary set phrase meaning to board an aircraft. Often used in announcements or written descriptions.
まもなく機上の人となります。
We will soon be boarding the aircraft.
彼は初めて機上の人となった。
He boarded an airplane for the first time.
搭乗する is the standard neutral verb for boarding a plane or ship. 機上の人となる is a more formal, literary expression specifically for airplanes.
A set phrase combining 機上 (on board an aircraft) and 人となる (to become a person). The exact origin is uncertain, but it is used in formal contexts.