From the American Airlines non-rev travel site:
Non-rev travel: Heartburn routes
No matter your destination, spring and summer travel may become a little more difficult for you and your guest travelers. Many of our domestic and international routes are likely to see more travelers so it’s important to be mindful of loads as you plan trips. Although all routes could experience high load factors, to help you and your guests with non-rev planning, we’ve put together a list of international routes that historically have extremely high load factors during the summer season and may cause you and your non-rev guest travelers some heartburn.
Cities are listed in order of most heartburn to least heartburn.
International Routes
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Europe: CDG ATH FCO VCE BCN BUD FRA ZRH MAD AMS DUB PRG MUC EDI LHR
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