PARIS — The new head of Airbus, the troubled European plane maker, pledged Tuesday to go ahead with plans for a new mid-sized jet and with cost savings identified by former the former chief executive, Christian Streiff, who quit amid a leadership struggle after little more than three months in the job. Louis Gallois, appointed to succeed Streiff at Airbus while also staying on as co-chief executive of European Aeronautic Defense & Space, the parent of Airbus, said he was in favor of pressing ahead with the planned A350 jet to rival Boeing's 787. "I believe that Airbus has to be present across the whole market, and the A350 is the middle of the market," Gallois said in an interview broadcast by Europe-1 radio Tuesday after his appointment was announced. The A350's mid-size category accounts for "40 percent of the market" by value, he added.