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.
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