Plane-sailing is a nautical term that describes the idea of navigating using a plane or level surface, without taking into account the Earth's curvature. However, there are several synonyms that can be used instead of this term, including smooth sailing, plain sailing, clear sailing, and easy sailing. All these terms are used to express the idea of a journey or task that is accomplished with ease and without complications. While they may not be specific to nautical navigation, they can be used in a variety of contexts to convey a sense of smooth progress or trouble-free progress. These synonyms can be used in various situations, from describing a straightforward work project to a simple and pleasant boat ride.