this expression derives from plane sailing, a simple former method of navigation based on the working hypothesis that the earth's surface is a plane rather than a sphere. Although it is first recorded in English (in the late 17th cent.) in the form plain sailing, this stems from the lack of clear distinction between the spellings plane and plain at the time. The spelling plane sailing became usual in the 19th cent., but has now been replaced by plain sailing, which arose by association with plain meaning ‘simple, straightforward’