I agree with R99, but in addition, Tina was supposed to leave as well. In fact, in her interview in PA, she states "that she was taking information for the hijacker from the pilot, and she told the other stewardesses to go ahead and she would be with them in a second. They went forward to the cockpit." When Cooper found out that the aft air stairs would need to be lowered after takeoff, he had Tina stay.
The timing is not explicit, but I believe that after Flo and Alice go forward, Flo remembers that she left her pocketbook in the aft section of the 727 and returns to get it. She asks Cooper if she can retrieve her pocketbook, and Cooper says something like, "Go ahead, I don't bite".