Delta Air Lines plans to add 88 Boeing 717s to its fleet, picking up planes that Southwest Airlines didn’t want anymore. Delta said it will start leasing the 717s next year as long as its pilots approve a new labor contract, which isn’t certain. Also, Southwest said some of the deal’s details still need to be worked out. Delta said it will begin taking three planes a month starting in mid-2013. Delta said the deal accelerates the revamping of its fleet, which has also included the elimination of turboprops. Because the 717s are replacing other planes, Delta said its overall flying level won’t change. The airline’s shares also benefited as jet-fuel prices ­remained near a five-month low. Jet-fuel prices have fallen 40 cents per gallon since February.