Delta Air Lines has earned a stellar reputation for on-time performance, even promoting its excellence in its marketing campaigns. That's what makes their second place performance in September so noteworthy. During the month, Delta was second to Southwest with 87.2% of flights arriving on-time versus 87.5% by Southwest. Hence, this is really the story of a great month by Southwest more than Delta slipping. For corporate customers on contract, Delta has promised refunds if their on-time performance falls behind rivals American and United. Since Southwest is not benchmarked for that offer, Delta won't have to pay out any rebates.