Berkshire Hathaway has taken a $2.6 billion stake in Delta Air Lines. That makes it their fourteenth largest investment as they look for places to deploy nearly $400 billion in cash. Warren Buffet is now 95 years old and has stepped down as CEO of Berkshire Hathaway but remains its chairman. Given his history, it’s hard to imagine Buffett returning to airline investing, except on the most unusually favorable of terms. Airlines are a terrible business . They’re capital-intensive, unionized and heavily regulated. Much of their product is determined or provided by the government (airports, security, air traffic control, staffing, signoff on cabin elements). There’s very little moat. And Buffett learned this lesson.…