I recall UACHRIS a UA res. agent on UA board said 4pm flight means all not checked by 20 to 4 can legally have seats taken. And often do. The fact it does not LEAVE until 5 pm or 6pm is just too bad (for us!)
Thats why UA just offered all 1P and 1K members TRIPLE points til year end to apologise!
Me, I said THANKS United! I make 60,000 points extra bonus, and have had no delays. (Yet!)
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~ Glen ~