Complex sentence: In September 1928, DELAG began operating the successful rigid airship LZ 127 Graf Zeppelin, which made regular, nonstop, transatlantic flights possible before airplanes had flight ranges sufficient to cross the ocean in either direction without stopping.
Split into simpler sentences: In September 1928, DELAG began operating the successful rigid airship LZ 127 "Graf Zeppelin". This craft made regular, nonstop, transatlantic flights possible before airplanes had flight ranges sufficient to cross the ocean in either direction without stopping.