As commercial aircraft have gotten larger and heavier, the need for stronger aircraftbraking system has only grown greater. In moments of mechanical failure or malfunction, the emergency braking systems are critical in bringing the aircraft to a safe and controlled stop.
On a typical commercial airliner, each main wheel assembly has two wheels, each of which has its own brake. The inboard wheel brake and the outboard wheel brake, located in their respective wheel rims, are independent of each other. This means that if the hydraulic system or brake fails on one side, the other is independently supplied to adequately slow and stop the aircraft without the other. More complicated aircraft may have another hydraulic system for back-up or use a similar alternation of sources and brake assemblies to maintain braking.
In addition to supply system redundancy, the brake accumulator is an emergency source of power for the brakes in many power brake systems. The accumulator is pre-charged with air or nitrogen on one side of an internal diaphragm, with enough hydraulic fluid on the other side of the diaphragm to operate the brakes in an emergency. When the need arises, this fluid is forced out of the accumulator into the brakes through system lines under enough pressure to slow the aircraft.
Some simpler power brake systems use an emergency source of power that is delivered directly to the brake assemblies and bypasses the remainder of the brake system. A shuttle valve immediately upstream of the brake unit shifts to accept this source when pressure is lost from the primary supply sources.
The parking brake system is a combined operation. The brakes are applied with the rudder pedals, and a ratcheting system holds them in place while the parking brake lever on the flight deck is pulled. At the same time, a shut-off valve is closed in the common return line from the brake to the hydraulic system, trapping the fluid holding the rotors stationary. Depressing the pedals further releases the pedal ratchet and opens the return line valve, letting the brakes return to normal.
Some brake assemblies that operate on aircraft hydraulic systems are not designed for such high pressure, however. They provide effective braking through a power brake system but require less than maximum hydraulic system pressure. To supply the lower pressure, a brake debooster cylinder is installed downstream of the control valve and anti-skid valve. The debooster reduces all pressure from the control valve to within the working range of the brake assembly.
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