Minecraft wiki entry for Elytra, associated text: Directly hitting any surface while gliding too fast causes damage to the player and their armor proportional to flight speed (although this may be calculated from another factor). The player does not take fall damage from colliding at a shallow angle or a low enough speed. The critical, damaging angle is about 50° with the collision surface, and the calculation for damage seems to be as if the player had fallen from their maximum potential glide height to the height of the struck surface.