This question stems from both the DMG (Tactical Aerial Movement) and the Monster Manual (Wyvern). The wyvern has poor maneuverability and must maintain minimum forward speed of 30 ft to stay aloft. It has to be flying in order to use its claw attacks, as far as I can tell. How is it ever possible for a wyvern to get both claw attacks (requires a full-attack action, hence, no movement over 5') in order to use its improved grab?