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Power braking

Manufacturers are looking at new technology to improve the efficiency of aircraft brakes, but this may have an effect on their maintenance. George Marsh reports

Just when the MRO community had come to terms with carbon brakes, another revolution in aircraft braking is now taking place. Electric braking, already equipping the Boeing 787 Dreamliner and entering the single-aisle field with the Bombardier CSeries, is likely to see future wheel and brake shops become just as ‘electric’ as the aircraft they are servicing. Electrical technicians, with their specialist multimeters, oscilloscopes, digital testers, will become part of the scenery, as familiar as hydraulic rigs and fluid are now. Digital controls will introduce elements that are more in keeping with an avionics environment, rather than the conventional wheels and brakes shop.


Electric braking, just like the carbon brake revolution that preceded it, is a technology advance that will have a significant effect on MRO practice. For a start, it promises to make future brakes easier to maintain than present hydraulically actuated types. Electric brakes are lighter and therefore more manageable in the shops, meaning they should be more reliable, and require less frequent attention. The fluids, pressures and leakage possibilities that exist with hydraulics are also absent.


In operation, an electrical signal generated at the brake pedal is fed by wire to electro-mechanical actuators at the brake units themselves. Here, the signal commands mechanical movement, bringing the brake stators and rotors together to retard wheel rotation through friction. Each actuator consists of an electric motor and reduction gear which, typically via a screw drive, produces the mechanical movement that brings the discs together. The signal is mediated by an electronic control unit, which conditions the final electrical commands providing the required degree of braking. This system also offers additional functionality, such as ensuring maximum retardation without wheel lock-up, as well as various other sensing features.


Safran group company Messier-Bugatti Dowty (MBD) – one of two firms providing electric braking solutions for Boeing’s Dreamliner – cites specific maintenance advantages of its system over conventional hydraulic brakes. Electric motors and actuators, it explains, are configured as independent LRUs that can be removed and serviced separately. Sensors for brake pressures, wheel rotation and other variables can help minimise brake wear, thus prolonging the device’s life between shop visits. Sensors are also key to condition monitoring: by alerting MRO departments when attention becomes necessary, brakes can be maintained on condition rather than at manufacturer-defined intervals. Another cost saver is that electric brakes are purportedly less prone to FOD (Foreign Object Damage) than conventional hydraulic/mechanical types.


MBD’s competitor on Dreamliner, UTC Aerospace/Goodrich, claims that reliable electronic/electric controls enhance dispatch reliability and increase the ease of maintenance. Its electric brake actuators are each single piece and line replaceable. Because brake wear and system health are reported automatically, major components can be replaced on condition. Even the wire bundle assembly is similarly replaceable. 


Credibility is lent to these claims by the prior experience of Goodrich, which has supplied full-authority electromechanical brake systems for US combat aircraft for some 15 years. Other maintenance savers introduced by the company include improved carbon heat sink materials and installation of wheel torque bar bushings at the factory.


Interestingly ‘green’ manufacturing measures, becoming de rigueur among aerospace equipment suppliers, can themselves alter the details of MRO processes. MBD, for one, has adopted a whole battery of them – for example, technicians will encounter silicon bronze brake bushings rather than beryllium and chromium. The systems are also cadmium free, have asbestos-free insulation and are finished with paints that are water based and free of organic solvents, along with a low VOC primer. Additionally, no chlorofluorocarbons or halons are used in the manufacturing process.


Admittedly, not all is rosy on the electric braking front. A number of brake incidents and groundings that accompanied Dreamliner’s introduction into service were mainly due to warning indicator lights coming on in the cockpit. It must be noted, however, that reliability has since improved as technicians have addressed these sensitivity issues.


More locations

One effect of the electrification of brakes is likely to be further dispersion of MRO to more locations around the globe, with some of the work going to avionics shops. More airlines will bring at least a proportion of this work in house, since electrical and electronic capability is already well established among many of them. This will be a change from routinely sending all brake content back to the OEMs, or to shops that have the specialised hydraulic rigs and associated equipment.


As is often seen with a new technology, the OEM’s are initially likely to capture a large slice of the MRO cake, both as a result of acquisition contracts that include aftercare for a given period, and because it will take time for other shops to gear up to handle the business and win customer confidence. OEMs will exploit existing avionics repair networks in order to handle electronics maintenance. MBD, for example, will rely on its primary centres in Dallas for the Americas, Singapore for Asia-Pacific and Paris for Europe, the Middle East and Africa, plus partners elsewhere as necessary. UTC Aerospace/Goodrich can likewise use its global MRO presence to good effect. The company points out that component exchange programmes are fully supported by its global service network, adding that many components are replaceable on the flight line, thereby improving dispatch reliability. >>

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