Aviation Investigation Report A98H0003

1.19.4  Electrical Wire Arc Sites Analysis

Twenty segments of electrical wire from the occurrence aircraft exhibited areas of copper melt consistent with characteristics caused by electrical arcing events. Electrical wire arcing can occur when the insulation protecting a powered wire is damaged, exposing the conductor. The arc event creates sufficiently high temperatures to initiate a fire. Alternatively, a fire-in-progress can burn away the wire insulation, exposing the wire conductor. An arc will occur if the conductor makes contact with a conducting material, such as a metal structure or another exposed conductor of different electrical potential. Attempts were made to determine whether the SR 111 electrical wire arcing events were the result of a pre-existing fire or whether the arcing had provided the energy necessary to ignite nearby flammable material.

  1. - Auger Electron Spectroscopy
  2. - Focused Ion Beam and Transmission Electron Microscope Analysis
  3. - X-Ray Microtomography  Auger Electron Spectroscopy (STI1-123)

AES is a scientific technique that was used to attempt to differentiate between electrical wire arcs that could have caused a fire and arcs that resulted from being subjected to a fire-in-progress. The technique is based on the premise that combustion by-products are trapped in molten copper during an arcing event. A scanning Auger multiprobe, capable of high-resolution AES, is used to analyze the surface and near-surface chemistry of the copper arc melt sites and to detect the presence of combustion by-products in the resolidified copper. The absence of combustion by-products in the resolidified copper at an arc melt site is indicative of an arcing event that may have taken place in the absence of fire contaminants in the immediate area of the arc, and could indicate that the arc initiated a fire. This method also allows for a chemical examination of the resolidified copper melt surface without destroying the sample.  Focused Ion Beam and Transmission Electron Microscope Analysis

Auger spectra typically provide an indication of the presence and amount of specific elements on the surface of a copper arc melt. Knowledge of the vertical elemental profile, morphology, and porosity of selected arc sites was obtained by combining AES techniques with focused ion beam (FIB) etching and transmission electron microscopy (TEM) examination.

Arc sites of particular interest were identified during the AES surface analysis. Subsequently, thin vertical slices (lift-outs) were removed from these specific sites using a Gallium ion gun. A micro-manipulator was used to transfer the lift-outs from the FIB chamber to the TEM. High-resolution photomicrographs were then collected to determine the vertical homogeneity, porosity, grain size, and layering of each sample. Additional TEM analysis was then carried out to determine the elemental depth profile of selected lift-outs.  X-Ray Microtomography

X-rays were taken of the recovered wire segments that exhibited copper arc melts characteristic of electrical arcing. A transmission X-ray micro-scanner, equipped with a precision object manipulator, was used to produce two-dimensional X-ray images of the wire's internal micro-structure at various orientations. Tomographical reconstruction software was used to render a three-dimensional image of the wire's internal micro-structure, by combining the successive plane-view X-ray images. The resulting micro-tomographs provided a permanent three-dimensional record of the morphology of the original wire samples. Characteristics, such as porosities, extent of melting and solidification, single or multiple arcing events, and inclusions can be assessed. This information can be used to guide decisions pertaining to more intrusive analysis techniques.

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