MarkBoston From United States of America, joined Oct 2001, 74 posts, RR: 0 Posted (11 years 3 months 4 weeks 19 hours ago) and read 3788 times:
In the Swissair 111 accident report issued yesterday, the Transportation Safety Board of Canada indicated that the fire started with an "arcing event" in wires located above the cockpit ceiling. I could not find anything in the report to explain why an arcing event occurred.
Does the report explain why an arcing event occurred on this flight?
Airplay From , joined Dec 1969, posts, RR:
Reply 1, posted (11 years 3 months 4 weeks 15 hours ago) and read 3719 times:
Arcing occurs when voltage differential between surfaces is elevated to a point where electron flow is possible.
The criteria for this to occur can be affected by the distance between the surfaces, and the insulation factor of the gas or other medium the two surfaces share.
Once conduction is started it is often aided by an ionized gas formed around the arc.
The result is burning of the conducting surfaces (wires) and the surrounding material.
In the aircraft industry, the common usage of the term "arcing" can also mean the sparks generated by conducting surfaces rubbing against one another when the insulation breaks down either by wear or damage. This is most likely the type of arcing the report was meant to describe because the voltages used in aircraft electrical distribution systems don't typically "arc" due to the relatively low voltages.
Kapton wire insulation used in the MD11 and other aircraft is made from a film which is wrapped around the wire rather than extruded. Water can sometimes contaminate the insulation by getting trapped between layers of the wrapping. This is called "hyrdorlyzation" among wire geeks! Tiny currents in the trapped water cause ionization of the Kapton and slowly break it down. Eventually the material turns to carbon and creates additional current paths and heat. This leads to further breakdown of the insulation until you get "arcing" between conductors.
Saab2000 From Switzerland, joined Jun 2001, 1610 posts, RR: 11
Reply 3, posted (11 years 3 months 3 weeks 6 days 7 hours ago) and read 3608 times:
Living and working in Switzerland, this is big news here. Additionally, I work for SWISS, which has big connections with the former Swissair.
Basically, the IFE system was blamed for faulty wiring, which led to the arcing so well described by Airplay above. Then the aircraft insulation started on fire and was basically out of control before the pilots could do anything.
This scenario is every pilot's worst nightmare. The cockpit was most probably a raging inferno at the end. The pilots tried to keep the plane flying but at the end could not because all systems to keep the wings level and the speed up were destroyed.
Interestingly, the report indicates that the pilots almost certainly could not have landed safely even if they had diverted to Halifax immediately upon smelling the smoke. This runs in the face of the "armchair" pilots here in Switzerland who claim that they should have been able to land if they were not so hung up on following their checklists and troubleshooting procedures.
The fact is that the pilots did their best in a doomed situation. A lot has been learned from this and hopefully MD-11 safety will be increased as a result. Unfortunately, the MD-11 got a bad rap from this when it was really the fault of the IFE system and not the MD-11 per se.
Avt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 7, posted (11 years 3 months 3 weeks 23 hours ago) and read 3363 times:
Last night I saw a good show on this by the CBC. The point that the TSB kept stressing was that the arcing, while the lead event, was not the biggest problem. Wires chafe and short and arc in a/c on a regular basis. Trust me, I've spent the last 17 years in avionics, repairing such problems. The biggest finding was that the insulation was previously certified as non flammable by a flawed test by the FAA. The show last night showed the tests done on this material, and it burned quite well. It was the covering that was burning, not the insulating material itself. Also, silicone plugs used to close off airconditioning ducts burned easily as well, allowing the smoke to be drawn into the ducting system. This combined with no fire detection equipment other than the human nose, and the hidden nature of the fire above the cabin ceiling, allowed it to progress for far too long before it became apparent ot the crew. As Saab2000 said, the crew appeared to have acted properly given what they knew, and even had they diverted immediately, would have been too high or too fast and heavy to land had they tried to do so.