WASHINGTON - Southwest Airlines said it's conducting an internal investigation after one of its pilots used a phrase that's become a stand-in for insulting President Joe Biden during the pilot's greeting to passengers over the plane's public address system.The airline announced its investigation Sunday after The Associated Press reported the incident in a story about the growing use of the phrase "Let's go, Brandon," a euphemism in conservative circles for a vulgarity targeting Biden.RELATED: 'Let's Go Brandon:' Find out what it means, and how it got onto 2 Virginia electronic road signsThe pilot's use of the phrase drew audible gasps from some passengers on the flight from Houston to Albuquerque, New Mexico, on Friday.