Fall of Saigon Metal Staircase

One of the staircases used to rescue Vietnamese people into American helicopters during the Fall of Saigon. When someone reaches the top of the staircase, they're transported to the city during the Vietnam War. Anyone there can not interact with those in the past, only view. They have to get on one of the leaving American choppers if they are to return back to their time and home. If they miss the last helicopter, they become real and stranded in the past.