You have to not kill the guy at the kareoke bar, and do a fetch quest for a first aid kit and then return with his ex-wife. After that you bring The girl to the airplane and it should start overtime after a couple of cutsceens.
Huh, I was wondering how to get the other endings. It never occurred to me to kill that one guy (Pete was it?). I know failing the time limit is ending F; figured failing that one part in overtime is another bad end (letting the bar fill up). So that's three endings there (if there's one for each letter grade that means there should be 3 more).
Xbox GT - fallouthirteen 3DS FC - 5112 - 4839 - 8419