Repercussions from MEA on Electrical Wiring Interconnection Systems

Auteurs : Serge Roques
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Repercussions from MEA on Electrical Wiring Interconnection Systems


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	    <date dateType="Created">Sun 1 Oct 2017</date>
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Repercussions from MEA on Electrical Wiring Interconnection Systems Serge Roques, Safran LPS The TWA 800 and SWR 111 flight accidents led the NTSB to re-consider the design of electrical installation and technology to avoid such accidents. New airplanes will be More Electrical Aircraft ( MEA) with the implementation of Electro Hydrostatic Actuators (EHA) and Electro Mechanical Actuators (EMA). An International Working Group named ATSRAC was set up to define the new rules for EWIS (Electrical Wiring Interconnect System). EWIS is a new and stronger regulation released by September 2008, applicable to aircrafts weighing more than 5700kg powered per turbine, defining rules for electrical components and installation. EWIS involves all wiring related components making the interconnection between electrical centers and equipments but also protecting against specific threats of this wiring (Electrical Arc, Electromagnetic Interference, Partial discharge, Lightning strike, …) On previous conventional airplanes, servo controls were powered by hydraulic pressure, this hydraulic pressure was delivered trough a distributor actuated by flexible mechanical cables linked to the flight control commands inside the cockpit: Pitch, Roll and Yaw axis. It was the basic design of AIRBUS A300, BOEING 747, BOEING 737 and other smaller commercial airplanes manufactured by FOKKER , BOMBARDIER , EMBRAER and other manufacturers. Starting on the AIRBUS A310, some Secondary Flight controls (Flaps, Slats, Airbrakes, Spoilers) were electrically controlled but the actuation was still powered by the hydraulic system trough hydraulic motors or hydraulic jacks. CONCORDE was introducing for the first time Electrical Flight Controls with a full Mechanical Back Up on all axis to avoid losing the airplane in case of total electrical failure (which occurred once above the Atlantic Ocean for 20 minutes !) AIRBUS A320 introduced more deeply the “Flight by Wire” concept with a “Ministick” and most of back up with flexible mechanical cable disappeared .On this aircraft, only the rudder and the horizontal tailplane still have mechanical back-up. BOEING 777 is also Flight by Wire controlled but also has mechanical back-up similar to the ones installed on Concorde Then a new family of aircrafts appeared with the coming of the A380, B787 and A350 introducing Power Electronics for the first time on EHA, ETRAS, Electrical Braking Green Taxiing. These recent aircrafts are fully electrically controlled with no mechanical back up, that means that the total loss of electrical power must be extremely improbable. If not the risk of losing the airplane is quite obvious. So Airworthiness Authorities reconsidered the wiring , no more as a single link between several electrical components, but as a system comparable to all other systems of the aircraft, with its different failure modes specific to this associated technology : chafing, arc tracking, short circuit, EMI, lightning strike, overheating, voltage drop, partial discharge, Corona effect, etc… Evolution of design rules was mandatory and to prepare these new rules a new International Committee was set up in US named ATSRAC. This Committee was in charge to determine the origin of wiring ageing and to prepare the new rules for future fully electrically controlled aircraft. EWIS regulation is the result of the strong recommendations of this Committee. EWIS is defined by the FAR25/CS25 – Part 25 – Subpart H. We can say that the major EWIS directives are segregation of the electrical bundles, protection against arc tracking , protection against particular risks like engine burst , tyre burst , flammability of components …quality of electrical installation and consistency of components with the environmental conditions where they are located ( EMI for instance). Two major accidents: TWA 800 and SWR 111 were pushing the Authorities to reinforce their requirements with SFAR 88 for the fuel quantity electrical line and with the separation between Essential Circuits and Non Essential circuits inside the aircraft. The first commercial aircraft meeting EWIS requirements is AIRBUS A350 delivered for the first time to QATAR by the end of 2014. Systems are becoming more and more complex and in fact any new airplane includes two electrical network : one for Power Distribution and one for Data Exchange and they must be totally independent physically not only to avoid any risk of EMI between Power and Data but also to prevent against any damage occurring from an arc tracking for instance . These constraints led to define new Tools and Processes linked to digital mock up to evaluate the consequences of location of cables inside bundles . We can evaluate for instance the induced voltage coming from High Intensity Radiated Field (HIR) along a longitudinal wire inside a CFRP structure which is no more protecting as a “FARADAY cage” and so to define the technology to protect it : braiding , raceways … Others tools are defined to evaluate the heating produced by Joule Effect inside a bundle and to determine the limit value of the density of current inside the wire taking into consideration the environmental temperature in the bundle area. Digital Mock-Up is now connected to new Tools Process (e-CENTRIC in Labinal Power Systems) to define the best route to locate a dedicated cable taking in consideration the strong configuration requirements from the airplane manufacturer (PLM), but also all the new stringent regulation coming from EWIS. Such tool is able to propose the best optimized location of a wire coming from the distribution center to the electrical equipment it will supply. In the same time this tool will compute automatically the weight and the voltage drop associated with this line and will give guidance to improve weight and best routing. All interfaces from section to section or from bundle to equipment are mastered in real time using this tool and avoid discovering any mismatch on the assembly line. Finally such Tools an Processes allow the aircraft manufacturer to reach the objective of performance and quality in the assembly of the aircraft but also to answer the request of easy maintainability required by the Support Division and finally by the customers .