Complex sentence: Tegel's permit was set to expire at the end of 2017, but if Tegel is closed before BER is opened, massive disruptions would occur, as Tegel handled over 60% of all passenger traffic in Berlin.
Split into simpler sentences: Tegel's permit was set to expire at the end of 2017 but if Tegel was closed before BER was opened, massive disruptions would occur. This disruption would be due to Tegel handling over 60% of all passenger traffic in Berlin.