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...

Hydraulic systems

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 TeflonĀ® 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

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.

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