The most important system used to stop the aircraft is the wheel brakes. Modern airplanes use disc brakes like cars; but instead of having only one rotor disk, they usually have 4 or more rotor disks. These discs are inside the wheels. The reason big jets have so many wheels is because they have so many brakes. The landing speed of the planes is about 270 km per hour. When the plane lands on the ground, deceleration panels called spoilers are opened on the wings to prevent it from flying again. These panels change the direction of the air flow and prevent the aircraft from taking off again when it lands. Therefore, when the plane lands, the wheel brakes can do their job. The panels on the wings are called "decelerator".