of a snowmobile. Ships, boats, submarines, dirigibles and aeroplanes usually have a rudder for steering. On an airplane, ailerons are used to bank the airplane for directional control, sometimes assisted by the rudder. Stopping With no power applied, most vehicles come to a stop due to friction. But it is often required to stop a vehicle faster than by friction alone: so almost all vehicles are equipped with a braking system. Wheeled vehicles are typically equipped with friction brakes, which use the friction between brake pads (stators) and brake rotors to slow the vehicle. Many airplanes have high performance versions of the same system in their landing gear for use on the ground. A Boeing 757 brake, for example, has 3 stators and 4 rotors. The Space Shuttle also uses frictional brakes on its wheels. As well as frictional brakes, hybrid/electric cars, trolleybuses and electric bicycles can also use regenerative brakes to recycle some of the vehicle's potential energy. High-speed trains sometimes use frictionless Eddy-current brakes; however widespread application of the technology has been limited by overheating and interference issues. Aside from landing gear brakes, most large aircraft have other ways of decelerating. In aircraft, air brakes are aerodynamic surfaces that provide braking force by increasing the frontal cross section thus aerodynamic drag of the aircraft. These are usually implemented as flaps that oppose air flow when extended and are flush with aircraft when retracted. Reverse thrust is also used in many aeroplane engines. Propeller aircraft achieve reverse thrust by reversing the pitch of the propellers, while jet aircraft do so by redirecting their engine exhaust forwards. On aircraft carriers, arresting gears are used to stop an aircraft. Pilots may even apply full forward throttle on touchdown, in case the arresting gear does not catch and a go around is needed. Parachutes are used to slow down vehicles travelling very fast. Parachutes have been used in land, air and space vehicles such as the ThrustSSC, Eurofighter Typhoon and Apollo Command Module. Some older Soviet passenger jets had braking parachutes for emergency landings. Boats use similar devices called sea anchors to maintain stability in rough seas. To further increase the rate of deceleration or where the brakes have failed, several mechanisms can be used to stop a vehicle. Cars and rolling stock usually have hand brakes that, while designed to secure an already parked vehicle, can provide limited braking should the