American Airlines, Philadelphia's largest air carrier after merging with US Airways, will join the Standard & Poor's 500 stock index after the stock market closes on Friday. American will replace Allergan Inc., which is being acquired by Actavis P.L.C. in a $66 billion deal.

Shares of American opened Tuesday at $52.64, up 5 percent from the closing price Monday. The stock rose 5.9 percent to $53.19 in mid-morning trading.

American joins other U.S. airlines, Delta and Southwest, on the S&P 500 which is seen by some money managers and pension plan administrators as a reflection of the overall performance of the stock market.

Speaking on CNBC Tuesday morning, American Chief executive officer Doug Parker said as "the largest airline in the world, we certainly knew we would be in the S&P 500 at some point in time, but this is earlier, I think, than most people expected. It's really good news. It is good news for our shareholders. It's good news for American."

American has a market cap of $37 billion and merged with US Airways in December 2013 to create the world's largest airline, as measured by passenger traffic.

"The addition comes at a good time," analyst Hunter Keay of Wolfe Research L.L.C. said in a client note. Airline stocks "overall did incredibly well in the six months after Delta's addition to the S&P 500 in September 2013," Keay wrote. Stocks soared 78 percent "but industry fundamentals were also doing well - causality can be a tricky thing to assess."