I can get vexed by people spelling 'Plain Sailing'
Ahem...... "Plain Sail" means you are using what in racing is referred to as "white sails" i.e. you are sailing without a spinnaker. The order "make all plain sail" would have meant "put up every sail that can be flown on all points of sailing" i.e. exclude things that can only be flown with the wind abaft the beam, such as spinnakers. Spinnakers are fiiddly things to use but great fun and provide additional power downwind.
The expression "it was plain sailing" therefore means it was simple and straightforward.
Being "on the plane" or "planing" is something else altogether. A boat moving through the water creates a bow wave and of course a wave creates a trough. As speed increases the bow wave lengthens. When the speed is such that the trough is under the stern it takes a tremendous amount of additional power to get any addiitonal speed. If you can achieve that power the boat starts to move up the wave and is planing. It will accelerate rapidly, as anyone who has driven a planing powerboat can testify.