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.With 117 seats, the 717 fills a hard-to-fit niche in Delta’s fleet. Delta will use the 717s to replace 50-seat regional jets, which are increasingly unprofitable at higher fuel prices. They’ll also replace the DC-9s that Delta got when it bought Northwest. Those planes were similar in size to the Boeing 717, but they are getting close to retirement age. Planes in that size are often used for domestic flying on routes that aren’t busy enough to fill a larger Boeing 737. Some of Southwest’s 737s seat as many as 143 people.