Complex sentence: The Douglas DC-6B aircraft remained in use with SABENA in the mid- 1960s although they were no longer used on the airline's main routes; the Boeing 707s and Caravelles became the mainstay types during this decade.
Split into simpler sentences: The Douglas DC-6B remained in service with SABENA in the mid- 1960s although they were no longer used on the airline's main routes. The Boeing 707s and Caravelles became the mainstay types during this decade.