Give Me a Brake: A look at braking systems

By Chris Grosenick

Aircraft braking systems technology has progressed from the use of gravity and simple hydraulic master cylinder applications, to extremely complex systems with lots of wiring and tubing. All aircraft use the same basic types of braking systems, with system complexity determined by aircraft size and type.

Airplanes that are King Air size and smaller typically use single or dual disk brake assemblies using several pistons. Airplanes larger than this typically use multiple disk brakes that have a full piston carrier housing, and as many as four to five rotor/stator pairs. Older large aircraft, those of WWII vintage, use expanding tube type brakes, which worked well for the slower piston powered aircraft, but didn’t have the energy absorption requirements for faster, heavier jets. Modern large aircraft braking systems, like those on the 747, have to absorb millions of foot-pounds of torque, which converts the momentum of a landing aircraft into slower rotational speeds, and of course, heat.

Brake assemblies

Brake assemblies are constructed of the same basic materials. Small disk brakes use steel rotors and aluminum pistons and housings (Figure 1). They operate like car brakes, and their construction is very simple. Some large aircraft multiple disk brake assemblies have components made from magnesium and beryllium, and those applications are primarily military. The most common components of multiple disk brakes are an aluminum piston housing, self-adjusting mechanisms, steel stator plates, carbon composite rotors, and a steel torque tube that holds everything together (Figure 2.).

Small aircraft braking systems are simple in construction and operation. Typical components include a reservoir to hold fluid, master cylinders for both rudder pedals, tubing/hoses to the brake assembly, and the brake itself. Typical installations are shown in Figure 3 on page 32. These systems are usually referred to as independent brake systems because they don’t use outside power sources.

Power brake systems

Aircraft like airliners, business jets, and military planes utilize power brake systems that use pressure from the aircraft hydraulic system. Design considerations and propulsion systems play a part in what type of brake system an aircraft uses. Small turboprop airplanes use a regular master cylinder type system because the propulsion system can be used in the reverse thrust range to provide substantial braking.

Large turboprops use reverse thrust as well, but still need a power brake system simply because of gross weight and the need for an anti-skid feature. Thrust reversers, reverse pitch, and braking systems work together to bring an aircraft to a halt, but they also provide backup systems for one another should there be a malfunction or damage to the aircraft. Some common brake system components for large aircraft include the following: power brake metering valves, slave brake metering valves, isolation valves, selector valves, anti-spin actuators, anti-skid valves, hydraulic fuses, shuttle valves, accumulators, debooster valves, swivel joints, and pressure transducers.

When the pilot/copilot steps on the top of the rudder pedals, a mechanical linkage positions a spool in the power brake metering valve (Figure 4, page 33). This valve meters pressure to the brake assemblies through a series of other valves to move pistons and compress the stator/rotor "stack" on the brake assembly. The stacked rotors are keyed to the inside of the wheel half, and the stators are keyed to the strut-mounted torque tube. The amount of required braking is selected by moving the pedals, which lets more or less pressure through the valve depending on the amount of pedal travel. This metering function determines how much pressure acts on the brake pistons and thus how much braking force is produced. (Remember that force equals pressure times area [F=PxA].) This is an important concept when anti-skid is discussed.