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 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.
Depending on the aircraft, multiple hydraulic systems provide reliability and redundancy, with some systems using as many as three power sources for the brakes. Isolation valves, selector valves, and shuttle valves are used to route and control these other pressure sources. The next major metering device in the system is the anti-skid valve, which meters pressure depending on anti-skid system inputs and conditions. Downstream of the anti-skid valves, pressure moves through fuses, debooster valves, and shuttle valves to the brakes. When rubber hoses were used more in brake systems, fuses and shuttle valves were required downstream of the anti-skid valves because rubber hoses failed regularly. The widespread use of Teflon® hoses has allowed systems design to relocate fuses, because they are much more reliable and burst resistant. Fuses are flow sensitive devices that allow a certain volume of flow through before shutting off flow and pressure downstream, and shuttle valves are used to separate normal and alternate/emergency sources of pressure, with some installed at the brake assembly instead of farther upstream.
Anti-skid systems are an integral part of large aircraft brake systems, and are used during taxi, take-off, and landing. When an aircraft enters a skid, directional control is compromised and these systems work on the principle of metered pressure dump, which allows the pilot to apply maximum braking force. Components of an anti-skid system include the wheel speed detectors, computer/processor, fault indication, and the anti-skid valve (Figure 5, page 34.). These systems have several modes as well, and they include touchdown protection, locked wheel protection, and differential pressure dump. During landing, touchdown protection prohibits the brakes from being applied before there is weight on the wheels. Locked wheel protection is a function of the regular metering process of the system, and differential pressure dump uses aircraft speed vs. individual wheel speed to keep the brakes from pulling to one side during maximum braking.
On take-off or landing, wheel spin-up drives the wheel speed detectors which are nothing more than DC generators. These little generators send a signal (voltage) to the anti-skid computer/processor, and the computer processes and compares the signals coming in from the separate detectors. When a rapid drop in voltage is detected, the computer interprets this as an imminent skid condition and sends a command to the electro-hydraulic servovalve torque motor in the anti-skid valve. Internally, the servovalve ports pressure to a spool, which shifts and vents some or all of the applied pressure from the brake metering valve to return. The braking action is less due to decreased pressure and the wheel speeds up. The wheel speed detector senses increased rotation and passes more voltage to the computer, which tells the anti-skid valve to meter the spool the other way and apply more pressure to the brake assembly. This closed loop system meters pressure down to the 20- to 40-knot groundspeed range depending on the aircraft, and cycles many times a second to provide smooth deceleration. On airliners with multiple brakes, two to four per truck in most cases, the anti-skid system senses the wheel speeds on both gears, and if one gear set is slowing down more than the other, the system will dump pressure on the opposite side to keep the aircraft from pulling off to one side of the runway. Older aircraft use a more primitive system that uses three-way solenoid valves instead of an electro-hydraulic servovalve, and an analog control with the same wheel speed detectors in use on newer aircraft.
Most current airliners have an autobrake system that works in conjunction with the anti-skid system. Inputs to the processor come from the wheel speed detectors and the inertial reference system, and stops metering pressure below about 10 knots. The pilot still has to maintain directional control with the tiller or the rudder pedals, and the level of braking is selectable to provide a range of braking options. Large aircraft are also equipped with an anti-spin system. This system is usually independent of the anti-skid, and it is used to stop wheel rotation prior to gear retraction. Large spinning wheels develop a gyroscopic moment that can hinder or stall gear retraction, especially when there are two to four spinning tires per gear strut. These systems use gear-up hydraulic pressure ported through shuttle valves to the brakes, or actuators that position the power brake control valves to apply the brakes normally.
Emergency braking systems
Emergency braking systems are employed differently on various aircraft, but they all have a few things in common. On GA aircraft, there is not much need for a backup hydraulic brake system due to low landing speeds, but as the weight of the aircraft increases, even with low landing speeds, some sort of backup braking system is needed. Turboprops have the reverse pitch feature, and larger planes use multiple pressure sources to provide braking redundancy. Multiple systems usually consist of an accumulator and one or two alternate sources from other onboard hydraulic power systems (Figure 6). Alternate sources tie into the system upstream of the anti-skid valves and do not operate any differently than the normal system. Emergency systems usually bypass the anti-skid system, and depending on accumulator capacity, have from five to 20 applications before braking is lost. Naturally, as the size and complexity of the aircraft increases, so do the number of alternate/emergency braking options and features.
Other safety notes
Some safety notes on hot brakes and brake system air bleeding are in order here. Hot brakes are not much of a problem on light aircraft, but on heavier planes they can cause aircraft damage and personal injuries. Never approach suspected or actual hot brakes facing the wheel half or hub, always approach facing the tread edge-on, and don’t spray hot brakes with water or extinguishing agent unless fire or imminent personal danger are present. One indication of hot brakes on large/high performance aircraft is a flat tire. These tires have melt plugs that release pressure from an overheated tire before pressure builds enough to cause rim failure and explosion. Brake bleeding is an important maintenance function that sometimes gets neglected. Air in the brake system causes malfunctions in the anti-skid system that could set fuses and degrade braking efficiency. Always follow the maintenance manual and use the right equipment to bleed brakes, shortcuts here can have disastrous results.
With so many different aircraft, it’s difficult to adequately describe all braking systems here. Some are more complex than others, and this affects reliability more than it does redundancy. Always study the specific aircraft maintenance manual, and use experience and general system knowledge to understand and service braking systems.
About the author
Chris Grosenick is a Quality Assurance Specialist at NASA Langley Research Center, Hampton, VA. He holds an A&P Certificate and is also a private pilot.
Parker Aerospace, Cleveland Wheels & Brakes: www.parker.com/cleveland