FRANKFURT, June 13 (Xinhua) -- A massive NATO exercise that kicked off on Monday is set to cause major disruptions to passenger flights in Germany, raising concerns from air travellers and also protesters. Jens Bischof, chief of Lufthansa Group's low-cost airline Eurowings, said he was concerned about the fallout of the exercise, which had led to the closure and heavy restriction of a large swath of German airspace. The military drill, known as "Air