Cloaking the Arc with a Coat of Many Colors
Despite all the attention being paid to tank flammability reduction via inerting, we shouldn’t forget that TWA800 was a wiring initiated fuel-tank explosion. Could there be a simpler and more cost effective method of preventing aircraft electrical fires and fuel-tank explosions?
TWA800 and Swissair have both focused the attention of aviation authorities, airlines, crew and passengers on serious electrical malfunctions emanating from wiring with a record of in-service deterioration. In the case of the earlier ValuJet Flight 592 crash, the NTSB found that “jostled oxygen canisters” ignited. There’s also been persuasive anecdotal evidence that the fire was caused by faulty wiring. If VJ592 had happened after TWA800, would a finding against wiring have been more likely?
It all begs the question: why this abiding concern over aircraft wiring? What are the causes of the fires, and how do they propagate to cause such horrific accidents? Without delving too deeply into the technical specifications of different aircraft wiring types, because it is a very intricate and technical field, we can generalize. There are complex criteria that dictate the optimum wiring insulation for different applications–e.g., insulation resistance, dielectric strength, vibration resistance, fluid resistance, chemical inertness, smoke generation , weight, manufacturing and installation requirements, ease of handling, min. radius, topcoat flaking, etc.). Environmental conditions differ in certain areas of an aircraft and so wire- types with differing characteristics are required. At this stage of wiring technology no unique wire type meets all the necessary criteria. However, the high frequency vibration of flight can cause wiring to “strum” like a guitar string and so chafing is a major concern for all types. This aspect and insulation cracking are of paramount importance.
After TWA800, all parties agreed that high time aircraft, especially pre- 1992 constructs, were more likely to have a fire due to their aging wire (most 737s and 757s, post-1992, are fitted with Boeing’s designedly safer TKT wiring). Aging wire is susceptible to nicks, cracks and the chafing of its insulations, all of which generate daily smoke emergencies. Aromatic polyimide (DuPont’s Kapton, BMS13-51) and X-linked ETFE Polymers (Tefzel BMS 13-48, Raychem’s wire) were two wire-types that had exhibited age degradation. Cuts, nicks, cracks and chafing could cause the insulation to be breached (cracked into the conductor), raising the possibility of electrical arcs (short circuits) and arc-tracking.
Arcs And Arc-Tracking
An arc will occur when a powered conductor contacts a ground potential. This happens if the insulation has been compromised by rubbing against the airframe (chafing), or by wire-to-wire abrasion. Arcing will result in a high current flow which should actuate the applicable circuit-breaker (CB) or fuse. However, arc-tracking is a phenomenon whereby an electrical arc between two or more damaged wires sustains itself through a conductive path provided by degradation of the insulation for a measurable length. It is produced by leakage current, and the associated heat effect of the arc, which locally decomposes the wire insulation material into carbon residues and gasses. If simultaneous conditions are present , the low resistance carbon path may allow the current to flow between the conductors, sustaining the arcing along the wire . Because the current is flowing and not going to ground, the CB may not trip.
Conditions For Arc-Tracking
To initiate arc-tracking:
* At least two wires separated by a small distance with insulation at least cracked to the bare metal.
* An initial short circuit-condition.
* A high voltage through the wire.
* A current sufficient to sustain an arc. The presence of an electrolyte or conductive fluid on or in the immediate vicinity of the damaged wires can accelerate the carbonization and cause the more explosive flashover of wet arc- tracking.
Arc-Tracking Limitations
A laboratory can easily reproduce the phenomenon. When concurrent conditions are met along a measurable length, the arc will move along the concerned wires towards the power source, until it is stopped–i.e., when one of the concurrent conditions disappears. Or,
*The current value sustaining the arc reaches circuit-breaker tripping threshold.
* The conductors (core) of the wires come into direct contact.
*It encounters a connection ; a bundle attachment; or a bulkhead pressure seal.
* There is a divergence of the affected wires within the bundle layout.
It is not just the actual electrical arc and the possibility of arc- tracking that is the only risk. The insulation can also ignite and while still burning, drip onto thermal-acoustic blankets, leaked hydraulic fluid, dust, etc., and set them alight. This is also true in the case of the actual conductor. Beads of molten metal are an indication that arcing has occurred. In 1999, Tim Dobbyn of Reuters wrote, “All aircraft wiring ages, and it is not uncommon to find five to 10 insulation cracks per 1,000 feet of wire in active aircraft, a congressional subcommittee heard Wednesday”. A Boeing 747 features approximately 140 miles of wiring, which translates into 7,390 wire cracks.