Passenger boarding stairs, sometimes referred to as 'air-stairs', 'boarding ramps', 'stair car' or 'aircraft steps', provide a mobile means to traverse between aircraft doors and the ground. Because larger aircraft have door sills 5 to 20 feet high, stairs facilitate safe boarding and deplaning. While smaller units are generally moved by being towed or pushed, larger units are self-powered. Most models have adjustable height to accommodate various aircraft. Optional features may include canopy, heat, supplementary lighting and red carpet.