Key Takeaways Delta Air Lines (DAL) bills itself as the "premium" U.S. airline. Passengers appear to be buying in. With customers increasingly paying for seats at the front of the plane, leaving upgrades in short supply, Delta reported record fourth-quarter revenue and operating profit on Friday. Do you have a news tip for Investopedia reporters? Please email us at tips@investopedia.com Delta hammered home the "premium" messaging in its report, using the word 11 times. But there is substance behind it: Whil ...