Glide Speed (5e Variant Rule)
From D&D Wiki
Some creatures or vehicles are powered simply by the aerodynamics of their shape and possess no ability to maintain their altitude as they move through the air. These creatures/ objects possess a glide speed. A glide speed assumes that the creature/ vehicle falls by 5 feet every turn, and must move their full glide speed movement. The creature/ pilot can choose to dive by converting horizontal movement into downward movement.
A gnomish tinkerer is testing his new glider, by jumping from a 100-foot cliff. His glider has a glide speed of 30 feet. Each turn he descends 5 feet but moves forward 30 feet. 2 minutes later he lands, 600 feet away. On his second test as he glides he spots a hippogriff swooping on him, and makes an emergency dive. He nose dives, avoiding the attack, but descending 35 feet that turn. 1 minute and 24 seconds after he starts he lands, 420 feet away.