I've flown UA 16 flights during the last 2 weeks and have received exit isle seats (my Choice) every time. They are considered premium seats and usually held for Premier or Alliance Gold members. I was informed many times that these seats can only be assigned on the day of the flight after the counter agent can physically see you to confirm that you are able to help in case of an emergency. You must be 15 years old, have no physical or hearing problems, understand English and be able to lift at least 50 pounds. I think all of this is a great idea but I often have my doubts about this policy when I notice an 80 year old couple sitting in these seats. I am one of those guys that likes to be at the terminal at leats 2 hours or more before flights. I think this is the secret actually. Also asking for Economy plus seating (which as a Premier, is one of your perks) as your first choice and giving the exit as an alternative will almost usually get you the exit. Not sure this helps but it always works for me. Oh yes,remember to smile at the counter. I think this help as well.
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Rolyjp