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GENER AL AVIATION 30 JANUARY/FEBRUARY 2017 AIRCRAFT MAINTENANCE TECHNOLOGY This is a typical example of the new technological possibili- ties when it comes to parts shortage. It can be applied to any other complex part and the whole process follows this path: Digitization of the used part. Using a high-precision optical scan- ner, the visible surfaces of the existing engine block are scanned creating a cluster of points with an accuracy of 0.02 millimeters creating a very good start point for a complete CAD reconstruc- tion. To capture hidden details, like internal cavities or ducts, tomography technique may be used — the same complex and costly technique used in a medical scanner. Dedicated software allows to interpolate missing surfaces and to re-create the details using original design principles and geometry. When the high- est accuracy is needed, a 3-D measuring robot is used for the most sensitive areas. 3-D MODEL, MANUFACTURING DRAWINGS SETUP Most of the time, the data acquired by the optical scanner cannot directly be used, and a thorough functional analysis should be per- formed, what often requires careful measurement of adjacent parts. A major step in this process is the CAD model creation — made by using all data acquired in the digitization step. This is a real reverse engineering step where dimensions and tolerances are determined to allow the final assembly, taking into the consider- ation the bearing alignment, gear axis matching, etc. If drawings/ blueprints are available, they are extensively used, adding very useful information in this step. In a case where no part and/or drawing is available, a new design can be established by using all available clues like pictures, location of adjacent parts, and a functional analysis. Usually, when the material has been in service for many years, users will know the weakness areas of the part. While staying close to the original design, improvements may be introduced by using better suited alloys or slightly changing the geometry. This improvement process likely requires an approval of local aviation authorities through a minor or major modification request. As Stampe belongs to EASA Annex II aircraft list, this is done under French Civil Aviation Authority (DGAC) and the mod approval is granted to each aircraft owner. MATERIAL ANALYSIS In a case that the original material/alloy is not known a mass spec- trometer is used. Yield samples are used to assess the mechani- cal resistance of the material. This analysis allows the choice of the closest alloy amongst referenced materials. For this Renault crankcase — as no exact material match was found — a high resistance aluminum alloy was chosen. This is a real improve- ment but only possible for vintage planes under EASA Annex II rules, and under the owner's responsibility. CASTING, SOLIDIFICATION SIMULATION The crankcase is a sand casting made product, so the following step is performed in a foundry. Once validated, the CAD model is directly used to design the casting mold. In this process a new, powerful simulation software is used, which helps to predict molten metal flow in the mold, temperature gradients, and metal shrinkage. This software simu- lation allows to avoid real-life testing and dramatically reduces development costs and time. This phase ends with the sand casting mold design, where extensive foundry know-how is necessary, specially for parts with thin walls or relevant variations in wall thicknesses. 3D-PRINTING OF SAND CORES The cores which constitute the mold are then 3-D printed with a 3-D, S15 sand-printing machine. One layer of sand — to which one component of resin has been added - after another is applied WHEN DRAWINGS aren't available, a new design can be established by using all available clues like pictures, location of adjacent parts, and a functional analysis.