Contrary to some descriptions, Schonefeld airport, to the south of the city, was not in East Berlin, but just outside it in the mainstream GDR, and thus the whole additional panoply of restrictions for the country were applied to those using it. At least the runways were parallel to Tempelhof/Tegel. There was a special regular GDR bus which left from by the Zoo station in West Berlin, direct to the airport, on which documents and tickets were substantially checked at boarding, then again at the southern zone boundary, then again on alighting at the airport. It appeared to operate mostly empty.
I flew out of Schönefeld a few times in the 70s. It sat in the state of Brandenburg. Possibly the most miserable and underused airport in Eastern Europe in those days. Since I was already in East Berlin, I would take the train to the airport, but it seemed to take forever to get there. Then through the dimly-lit terminal to an Interflug IL-18 to Sofia or Bucharest. Oh, the joys of travelling in the GDR!