A steep, controlled dive, especially by an aircraft with the engine off.
verb
To perform a volplane.
1916, Wilfred Percy ("Billie") Nevill, letter, in 1991, Ruth Elwin Harris (editor), Billie: The Nevill Letters, 1914-1916, Julia Macrae, page 143,
[…] when well over the bosche lines & he volplaned home, just skimmed over our & the Bosche trenches, they all fired at him like sin, of course, and landed about 1000 yards back on a hill.