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Monday, April 25, 2005

Compare and Contrast

Here's progress for you: "This is the first issue of this Advisory Circular to be written on aircraft wiring and bonding practices." The 389-page document was put out in draft form (for review and comment) by the Civil Aviation Safety Authority (CASA) of Australia. Draft AC 21-99(0) is titled "Aircraft Wiring and Bonding," and interested parties have until April 29 to submit comments. The document evidently draws heavily from parts of U.S. Federal Aviation Administration (FAA) guidance, specifically AC 43.13-1B, "Acceptable Methods, Techniques and Practices - Aircraft Inspection and Repair," and AC120-XX, "Guidance Pertaining to Aging Airplane Records Review and Inspections," as well as from AC 25.15, "Electrical Fault and Fire Prevention and Protection."

The Australian document incorporates the recommendations in AC 43.13-1B, specifically regarding "wire to wire abrasion," and states:

"Install wires and wire groups so that they are protected against chafing or abrasion in locations where contact with sharp surfaces or other wires would damage the insulation. Damage to the insulation may result in short circuits, malfunction, or inadvertent operation of equipment.

"Abrasion of wire insulation can also occur because of differences in 'hardness' between adjacent wires. Therefore, new wires, added during modification, which have significantly different insulation 'hardness' or abrasion characteristics to current aircraft wiring, should be routed in separate bundles. This is particularly important in areas of high vibration."

However, the Australian AC only makes reference to "new wires" and makes no reference to existing wire bundles comprised of wires with different insulation characteristics.

On the other hand, neither AC 43.13-1B nor AC 120-XX make specific reference to aromatic polyimide wire (Kapton), but the Australian document does. In fact, it reiterates statements made in AC 25.16, saying in the Australian AC:

"Due to the undesirable properties exhibited by polyimide (Kapton) insulated wiring [e.g., its tendency to flashover], its use in aircraft should be avoided wherever practicable."

In fact, the Australian AC makes specific reference to the arcing characteristics of polyimide. However, as in the U.S. documents, the Australian AC makes no mention of the dangers represented by contaminated insulation blankets being located in close conjunction to wiring bundles.

The CASA document may be viewed at http://rrp.casa.gov.au/casr/021.asp#2005, or it may be accessed directly at http://rrp.casa.gov.au/drafts/draftac021_99_v0.pdf.


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