Figure 13-78.  A single disc brake is a floating-disc, fixed caliper  brake. Types and Construction of Aircraft Brakes  Single Disc Brakes Floating Disc Brakes 13-43 during engine run-up and, in many cases, steer the aircraft  during taxi. On most aircraft, each of the main wheels is  equipped with a brake unit. The nose wheel or tail wheel  does not have a brake.  In the typical brake system, mechanical and/or hydraulic  linkages to the rudder pedals allow the pilot to control the  brakes. Pushing on the top of the right rudder pedal activates  the brake on the right main wheel(s) and pushing on the top  of the left rudder pedal operates the brake on the left main  wheel(s). The basic operation of brakes involves converting  the kinetic energy of motion into heat energy through the  creation of friction. A great amount of heat is developed  and forces on the brake system components are demanding.  Proper adjustment, inspection, and maintenance of the brakes  is essential for effective operation. Modern aircraft typically use disc brakes. The disc rotates  with the turning wheel assembly while a stationary caliper  resists the rotation by causing friction against the disc when  the brakes are applied. The size, weight, and landing speed  of the aircraft influence the design and complexity of the  disc brake system. Single, dual, and multiple disc brakes are  common types of brakes. Segmented rotor brakes are used  on large aircraft. Expander tube brakes are found on older  large aircraft. The use of carbon discs is increasing in the  modern aviation fleet.  are located opposite of these pucks on the inboard side of  the caliper. These linings are stationary. The brake disc is keyed to the wheel. It is free to move  laterally in the key slots. This is known as a floating disk.  When the brakes are applied, the pistons move out from the  outboard cylinders and their pucks contact the disc. The disc  slides slightly in the key slots until the inboard stationary  pucks also contact the disc. The result is a fairly even amount  of friction applied to each side of the disc and thus, the  rotating motion is slowed. When brake pressure is released, the return spring in each  piston assembly forces the piston back away from the disc.  The spring provides a preset clearance between each puck  and the disc. The self-adjusting feature of the brake maintains  the same clearance, regardless of the amount of wear on the  brake pucks. The adjusting pin on the back of each piston moves with the piston through a frictional pin grip. When  brake pressure is relieved, the force of the return spring is  sufficient to move the piston back away from the brake disc,  but not enough to move the adjusting pin held by the friction  of the pin grip. The piston stops when it contacts the head of  the adjusting pin. Thus, regardless of the amount of wear,  the same travel of the piston is required to apply the brake.  The stem of the pin protruding through the cylinder head  serves as a wear indicator. The manufacturer’s maintenance  information states the minimum length of the pin that needs  to be protruding for the brakes to be considered airworthy.  [Figure 13-80] Small, light aircraft typically achieve effective braking using a  single disc keyed or bolted to each wheel. As the wheel turns,  so does the disc. Braking is accomplished by applying friction  to both sides of the disc from a non-rotating caliper bolted to  the landing gear axle flange. Pistons in the caliper housing  under hydraulic pressure force wearable brake pads or linings  against the disc when the brakes are applied. Hydraulic master  cylinders connected to the rudder pedals supply the pressure  when the upper halves of the rudder pedals are pressed. A floating disk brake is illustrated in  Figure 13-78 . A more  detailed, exploded view of this type of brake is shown in  Figure 13-79 . The caliper straddles the disc. It has three  cylinders bored through the housing, but on other brakes this  number may vary. Each cylinder accepts an actuating piston  assembly comprised mainly of a piston, a return spring, and  an automatic adjusting pin. Each brake assembly has six  brake linings or pucks. Three are located on the ends of the  pistons, which are in the outboard side of the caliper. They  are designed to move in and out with the pistons and apply  pressure to the outboard side of the disc. Three more linings  The brake caliper has the necessary passages machined into  it to facilitate hydraulic fluid movement and the application  of pressure when the brakes are utilized. The caliper housing  also contains a bleed port used by the technician to remove 