Originally Posted by
DanTravels
NWA.com says (red text mine):
Domestic Flights: Passengers must be checked in at least 30 minutes and
on-board at least 15 minutes before scheduled departure time for flights within the US. Exception: Check-in 45 minutes before departure in Atlanta, Denver, Los Angeles, Newark, Tampa and Washington D.C-Dulles and 60 minutes before departure in Las Vegas.
International Flights: Passengers must be checked in at least 60 minutes and on-board at least 30 minutes before scheduled departure time for flights to/from Canada, Mexico, the Caribbean, and all other International flights.
They want you on-board 15 minutes before departure so they have the last 10 minutes to sort out standbys, non-revs, figure out if they have to hold the flight, etc. Typically, if you have a late connection and you run to your gate and it's 6 minutes or more before departure, you can still get on, although there's no guarantee since you technically lost the guarantee to your seat when you weren't there 15 minutes prior.