Page 6, # 2

Remark: "Wire is wire", says McSweeny when he is questioned abour aircraft wiring. He presumably does not recognize his subordinates’ (Blake and Cahill) reports.

"Each of the hot wires were protected by a 10-amp aircraft type circuit breaker."

Remark: In the Cahill reports 7-amp circuit breakers were used for testing. A 7-amp circuit breaker trips quicker than 10-amp, with different test results.

Page 7

"The arcs ignited two of the three insulation batts and after burning for several minutes: all three batts fell down into the bottom of the check area and were completely consumed by the fire."

"The insulation blankets were contaminated with Holt Lloyd Corporation’s LPS-3 heavy duty rust inhibitor. This product is a spray used by some (?) airlines as a corrosion inhibitor in the bilge area."

Remark: Uncontaminated insulation blankets were found not to readily support combustion, but the definition "readily" clearly shows that such blankets are not safe under adjacent wire arc tracking conditions. There is a direct relation between the actual conditions (progression) of arc tracking and the specific (contaminated/uncontaminated) condition of the blankets at that specific time. I add that insulation blankets are contaminated by time due to maintenance and other causes: oil and lubricant, fuel and corrosion inhibitors.

Toxic effects

There is no mention of toxic effects.

Remark: The FAA has – at last – ordered replacement by a new type of insulation blanket. Kapton covered batts gave the best test results (in 1991!). As expected, because Kapton is a relatively excellent thermally resistent material. Its greater propensity for arc tracking does not apply to the insulation batts.

Test results

It is a pity that the overall test results are not described in detail. Figures 13, 14 and 15 do not suffice.

Halon extinguishers

Discharging these hand-held extinguishers did not always completely extinguish the fire.

My conclusion

Current insulation blankets contribute to growth of fire in inaccessible places. They do not spontaneously ignite, but adjacent arc tracking wires may cause the blankets to combust. Hand-held Halon extinguishers are of no use under in-flight conditions. Sprinkler units are not installed. The only hope is that a fire will self-extinguish quickly enough, before it can reach a (more progressive) stage. Arc tracking and insulation batts will easily turn out to be a deadly combination on itself.

Basic remarks on testing

The Blake and Cahill reports have strictly limited value. Wires are tested under optimum laboratory conditions. The entire setup shows the narrow-minded approach. All tests deny the practical circumstances of aircraft specifically in-flight. It is not recognized that dry and wet arc tests should be executed in an environment that closely resembles the conditions under which and by which aircraft and maintenance operate. What is substantially missing in these tests is the – more or less – gradual deterioration of aircraft wiring under service conditions. Aircraft vibrations leading to chafing, but also originally bent wires are left out of the test conditions.

60 Degrees testing

Cahill adopted this test, because the FAA was currently requiring this. It is, [as with the 7-amp circuit breakers]: "let’s not make our demands too high, because we need to be friendly to the aircraft makers." But it was good to see that Cahill also adopted the multi-directional pilot burner, instead of only the (called for) straight one.

Direct correlation

Cahill established a direct correlation between dry and wet arc tracking tests, but she also concludes that the halogenated polymers formed no conductive chars upon thermal decomposition and, therefore, no dry arc tracking. Aromatic polyimide samples, however, formed a conductive char upon thermal degradation, and severe arc tracking occurred. We all know that aromatic polyimide is widely used in aircraft. As expected, the TPT performed well, showing the same parameters compared to halogenated polymers, with no dry arc tracking evident. Unfortunately, hundreds of aircraft are flying each and every day without having TPT installed.

Smoke test

Cahill writes that in the confined environment of an aircraft cabin, it is imperative that smoke emission from all sources be minimized as the smoke may cause passenger panic and obscure exits and routes of escape. But she ‘forgets’ about the cockpit crew being blinded, and she omits mention of the toxic hazards involved.

It goes without saying tat the extruded radiation crosslinked EFTE samples produced large (and thick!) quantities of smoke.

Circuit breakers

For the tests 7,5-ampere Klixon aircraft circuit breakers were used. ‘Heavier’ circuit breakers were not considered. The reasons are not explained, but presumably because of the use of 20 AWG aircraft wiring, based on industry testing and computer modeling. For 20 AWG wiring 10- and 15-amp circuit breakers may not be used (FAA Wire Charts & Tables).

Wet or dry arcing

I don’t see much value in the – more favorable – dry arc testing. Because no one can tell if there are specific wet/moistured spots in aircraft, and at least in less accessible and inaccessible areas. The best test is always based on the worst conditions. The other way around may only ‘please’ those who only want to see a sunny world.

Severity

On TKT wire (Teflon outer wrap/polyimide middle wrap/Teflon inner wrap) testing initiated no dry arc tracking. The initial arcs struck were all found to be moderate in severity. There was the vaporization of the conductor, but with insulation material remaining, with no evidence of carbonization. The aromatic polyimide wires gave massive arcs, causing multiple circuit breakers to trip. Of course, because of carbonization, severe re-arcing occurred upon any resetting of circuit breakers.

Burn length

All test specimens with the exception of PVC-Nylon passed with average burn lengths within the 3-inch maximum and no flame time. Cahill produced a scientific report and, without repeating her tests, no-one in the world could dispute the results. However, one test concerning PVC-Nylon wire raises doubt. I quote her: "MIL-W-5086/1 marginally passed the 30-second flame time, and the average burn length was greater than the 3-inch maximum specified in the FAR." I remember Ed Block’s cute little try-out last week, when he held a lighter under PVC-Nylon (the same wire type that was also used in ValuJet and for this Cahill test sample). We saw after mere seconds the flames creeping up progressively, destroying the entire cable; if he had not extinguished it in the ashtray. Smoke was heavy, odor dreadful (toxic stuff). So???? PVC-Nylon does not spontaneously extinguish.

Cahill’s conclusions

She writes: "No dry arc tracking was seen for any of the specimens tested with the exception of MIL-W-81381/12 Kapton samples." On three halogenated polymers initial arcs were mild to moderate, showing that all circuit breakers either stayed in after resetting, retripped immediately or resulted in a small arc restrike followed by a circuit breaker retrip. The basic conclusion on these is that there was minimal insulation damage, but Cahill omits to mention that because of this the conditions have been created – later on – for arc tracking. Because this was beyond her test assignment there was no mention of this.