Aviation Investigation Report A98H0003

1.16.1  AES Examination of the Recovered Arced Beads

  1. - Wire Inspection of In-Service MD-11 Aircraft

All of the copper wire melt beads from SR 111 were examined by Auger electron spectroscopy (AES)[73] (see Section AES provided a method to determine quantitatively, the surface chemistry or elemental composition of the melt beads as a function of depth below the surface. AES examination had the added benefit of being essentially non-destructive, as the depth profiling did not normally go below 5 000 Angstroms.[74] The elemental chemical data collected could be used to provide a comparative analysis between samples. Review of the available literature indicated that it may be possible to use this comparative analysis to differentiate between a copper bead arced in a clean environment (pre-fire) and one arced during a fire in a smoke-filled environment. Wires that arced in a clean environment could be identified as possible initiating events.

To assist in developing a protocol for the examination and comparative analysis of copper beads by AES, a number of beads were formed under known arcing and fire conditions. Twenty-four of these exemplar beads were examined and analyzed in a blind test. The results of these tests were that 7 of 14 beads formed in a clean environment were correctly identified as such. Of the 10 beads formed in a fire environment, 1 was incorrectly characterized as pre-fire, and 9 were characterized as inconclusive. This latter characterization highlighted a problem in using the AES methodology to assess arc beads that had been subjected to an ongoing fire, regardless of the environment present at the time of the arc. The fire subjected the copper melt surfaces to heavy oxidization that formed a crust, or environmental cap. When this cap was present, it was not possible to make any determination about the environment at the time the bead was formed.

Several other difficulties were encountered using the AES technique as a means to collect data for characterizing the environment surrounding the bead when it was formed. The irregular surface shapes of the copper melt sites on the SR 111 beads made it difficult to find a suitable flat surface on which to conduct the examination. Many of the sites were contaminated with the remnants of charred wire insulation and whitish- and greenish-coloured precipitates or deposits caused by exposure to sea water. This contamination resulted in a static charge build-up during the testing process, and made it difficult to locate sites that had been pre-defined for examination. Although each of the beads was ultrasonically rinsed in distilled water before being examined, this procedure did not remove all of these artifacts. Nor could the artifacts be readily removed by any other method; that is, given the small depths being examined, any physical distortion of the surface caused it to be potentially unusable for AES examination.

During the testing, there was a lack of repeatability, even in the data that was collected from sites that were just microns[75] apart on the same bead. In many cases, this lack of repeatability led to different interpretations of the environment present at the time the bead was formed. As the AES test results did not yield repeatable data that could be consistently interpreted, the comparative analysis using the AES methodology was not used in assessing the involvement of individual beads in the lead arcing event.  Wire Inspection of In-Service MD-11 Aircraft

Aircraft wiring in several in-service MD-11s was examined for sources of potential arcing and for any other sources of inappropriate heat generation. TSB investigators visited two maintenance facilities and examined several MD-11 aircraft in the areas of known heat damage in the SR 111 aircraft. The following anomalies were discovered on one or more of the aircraft examined:

  • Chafed and cut wires in the forward cabin drop-ceiling area above both the L1 and R1 doors;
  • Light chafing of the wire insulation topcoat on several wires behind the cockpit overhead CB panel, in the vicinity of where wire bundles enter the overhead switch panel housing;
  • Damaged, cracked, or chafed wires in several other areas;
  • Broken bonding wires,[76] and wires exhibiting bend radii that were smaller than manufacturers' specification;
  • Wire terminal connections with insufficient torque;
  • Inconsistencies in the routing of wires and wire bundles;
  • Physical openings in the smoke barrier installed above the cockpit wall between the cockpit and cabin; and
  • Inconsistencies in the installation location of the emergency lights battery pack above the cockpit door entrance.

No direct relationship between the wiring discrepancies discovered during these inspections and the damaged wires from the Swissair 111 wreckage was established.

[73] Auger electron spectroscopy (AES) is a surface characterization and depth profiling technique based on the determination of the energy of Auger electrons ejected from a solid surface bombarded with high-energy ions. AES detects all elements except H and He.

[74] One Angstrom is 10-10 metre.

[75] One micron or micrometre is 10-6 metre.

[76] Bonding wire or straps are used to connect two or more metal objects so that they have the same electrical potential, thereby preventing the build-up of a static charge.

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