What is the meaning of "out of ~"?
"Out of" means that something occurs among a range of something or "out of" a total of something. So in this case, "11 hours out of a 16-hour flight" means that among a total of 16 hours, she is on her feet for about 11 hours. This phrase is often used when something is picked or selected from a group of many. Ex: Out of all the animals in the world, lions are my favorite. Ex: I would choose to hangout with you out of everyone. Ex: She slept for four hours out of the total five-hour drive.