Monday, April 19, 2004
Redundancy is lacking
In a new paper on safety and aircraft electrical systems, former airline pilot Alex Paterson argues that redundant design has yet to be applied to the wiring and bundles of wire. Below, extracts from the Federal Aviation Regulations (FARs), Part 25, which deals with airworthiness standards:
- FAR Sec. 25.1351. General (subparagraph numbers in parentheses)
"(2) No failure or malfunction of any power source can create a hazard or impair the ability of the remaining sources to supply essential loads."
Paterson: "While system redundancy was designed into the sources of electricity in airline aircraft, proper thought was not given to redundancy of the actual wiring system itself. Thus, hundreds of wires pertaining to a diverse group of unrelated aircraft systems, with different power sources and varying voltages, are all wrapped together in massive wiring looms. An explosive arc-tracking event invariably severely damages surrounding wires, taking out unrelated systems. TWA Flight 800, which exploded off New York City July 17, 1996, is thought to have been caused by low voltage fuel tank quantity wires shorting out onto (or inducing a current in) adjacent high voltage wires triggering an explosion in the center fuel tank.
"As a result of this design deficiency, no airline aircraft flying today meet FAR 25 and their European counterpart (JAR 25) pertaining to system redundancy."
- FAR Sec. 25.1351. General
"(6)(d) Operation without normal electrical power - Parts of the electrical system may remain on if,
"(1) A single malfunction, including a wire bundle or junction box fire, cannot result in loss of both the part turned off and the part turned on, and;
"(2) The parts turned on are electrically and mechanically isolated from the parts turned off."
Paterson: "As above, this requirement cannot be met due to the fact that the wires from various system (including emergency power items) are usually bundled together."
- FAR Sec. 25.1357 Circuit Protective Devices
"(a) Automatic protective devices must be used to minimize distress to the electrical system and hazard to the airplane in the event of wiring faults or serious malfunction of the system or connected equipment."
Paterson: "The current generation of bi-metallic circuit breakers used in aircraft are inadequate for the task of 'circuit protective devices' because they cannot detect the micro-discharges of electrical current through hairline cracks in the insulation material of aircraft wire. Such 'ticking faults' can eventually lead to an arc-tracking event or even a short circuit. The situation can be further exacerbated by the fact that bi-metallic circuit breakers can actually 'weld' themselves closed, due to the very high heat generated in the circuit breaker at flash-over. Arc fault circuit interrupter (AFCI), ground fault circuit interrupter (GFCI) and universal fault interrupter (UFI) technologies may be the answer to these problems." >> Paterson, e-mail firstname.lastname@example.org . See also www.vision.net.au/~apaterson/aviation/wire_virgin.htm<<