A lighter-than-air aircraft (1) having a gas-filled hull (2) and a pair of spaced apart landing gear units (11, 12) on the underside of the hull arranged on opposite sides of a longitudinally extending central vertical plane of the hull. Each landing gear unit (11, 12) comprises bag skirt means (5-7, 5-7), means for supplying air to and removing air from the bag skirt means and actuating means operable to move the bag skirt means between an operative configuration for containing one or more air cushions and an inoperative configuration.