Definition of plane sailing:
(n) :
(nautical) A technique for navigation using the assumption that the journey occurs over a plane or flat surface rather than the actual curved surface of the Earth, which is sufficiently accurate over short distances.
(n) :
(figurative, archaic) Alternative spelling of plain sailing (“navigation of waters free from hazards or unfavourable winds; (figurative) something that is easy, simple, or straightforward”). [(nautical) The navigation of waters free from hazards or unfavourable winds.]