Answer: airtravel

AnswerCrossword Clue
AIRTRAVELDelta's business
AIRTRAVELFlights
AIRTRAVEL20th century phenomenon.
AIRTRAVELDelta's concern with a N.Y.C. subway tangle (3,6)
AIRTRAVELSouthwest's business
AIRTRAVELIt may involve connections
AIRTRAVELBusiness for Delta or Southwest