A REVIEW OF IN-FLIGHT FIRE INVESTIGATIONS SINCE 1987AND THEIR RELEVANCE TO A REVIEW OF THE HELDERBERG ACCIDENT
PART 2: IF YOU NEED AN AIRCRAFT FIRE IGNITION SOURCE - KAPTON DID THE JOB WELL
Kapton wire was adopted in the aviation industry in about 1966. According to a report by the U.S. Government Accountability Office (GAO), it was used used on the following aircraft:
- Lockheed L-1011s
- Douglas MD-80s and MD-11s
- Boeing 727s, as well as early 737s, 747s, 757s and 767s.
Kapton's advantages, as summarised by the GAO document at the time, were that it is "lightweight, resistant to abrasion and cuts, is able to withstand high temperatures and is flame and environmentally resistant."
It was introduced when wiring bundles became increasingly complex in aircraft. It was believed that the insulation would permit the reduction of wire diameter without needing to reduce current flows. This was advantageous as the lower diameter meant a considerable saving in weight. However, if you reduce the diameter of a wire but retain the same current through it, it is more prone to heating up. It was believed that Kapton, with good heat dissapation, would retain the status-quo.
Boeing and other manufacturers have since replaced it in all new build airliners with another wire known as TKT.
I have to ask: If it was such a wonderful material, then why change it?
The reason is that Kapton wiring has repeatedly proven to be highly dangerous and the source of ignition of many aircraft fires.
There are multiple reports that reveal its inherent dangers.
The higher internal heat carried by the wires aged the material faster than expected. This would lead to the formation of hairline cracks in the insulation. Special tools were needed to handle it without cracking of the insulation. One technician I interviewed on this subject stated he was scared to sneeze near the wiring "It was that fragile...".
Where the wiring has been in use for some time and has chafed or developed cracks in the covering - even slightly - and where it is used within a humid environment such as the interior of a passenger aircraft like the Helderberg during a long-haul flight - the easily cracked or damaged insulation permits the absorption of droplets of water. This results in electrical arcing with extremely high temperatures and energy releases.
If the cracks are near each other, the entire sheath surrounding the wire can become carbonised and this then becomes a highly efficient conductive path for the current. This, in turn leads to neighbouring wires being compromised and more high energy arcing that soon ignites nearby flammable materials.
That latter situation is known as "arc tracking."
It was such a significant danger to flight that NASA threw it off the space programme very early in the 1980s.
While the Helderberg investigation was still underway, a FAA report found that Kapton arcing was almost an everyday event and should be suspected whenever circuit breakers apparently tripped for no reason on aircraft.. The report notes that "carbonisation of the polyimide due to the temperature of the initial arc occurred each time." Then, "upon resetting of the circuit breakers, severe re-arcing took place resulting in more insulation degradation."
A 1991 FAA report summarised the then known problem as follows: "(Kapton) has a greater propensity for arc tracking than other types of wiring insulation, and it produces the most intense arc should arc tracking occur."
Temperatures of 2,000 degrees Centigrade and more are known to arise where tracking takes place.
However, it can be even hotter. One FAA study found that when a Kapton wire arced against a hydraulic line, it produced temperatures of almost 4,000 degrees Fahrenheit - hot enough to melt the titanium hydraulic line. The aircraft aluminum used for the Helderberg's stringers and formers would not have stood a chance in such heat as aluminium melts at around 650-700 degrees.
The most incredible aspect is that the US armed forces had already begun acting on this information in the mid 1980s. This information came to light after one particular accident where a fighter had crashed and the pilot was blamed.
His wife refused to accept the official version and went about digging up information on similar accidents. She found that arcing of wiring in wheel wells was a known problem. This could result in un-commanded control effects. She found that Kapton's propensity to arc had been identified in the mid-80s by engineers at the U.S. Naval Research Laboratory.
Federal Aviation Administration officials have said that it is not a valid comparison to civil aviation as the operational environments between military aircraft and passenger airliners are so different.
This is disingenuous as a hand grenade is still a hand grenade whether you put it in a mini-bus taxi or a sports car!
Furthermore, the FAA then either exhibited their total ignorance of statistics or they willfully tried to down-play the issue. The GAO reported in 2002 that "FAA wiring experts reported that they were aware of only one incident on a commercial aircraft linked to aromatic polyimide wire insulation," In this they were referring to the loss of Swissair flight 111.
This position was either a bald-faced lie or they really have not read their copies of the report into the 1993 in flight fire in Denmark, the UAL complaints about it in the 80s, nor I would suggest, the Helderberg report. Maybe they never read the NASA comissioned report from Harvard in December 1984 that highlighted the issue? Yes, the problem has been known about for that length of time! (You can read the Harvard summary here.)
Even if the circuit breakers trip (in most events they do not and if they do standard practice at the time was to wait a few moments then re-set the circuit breaker - a practice both Boeing and Airbus now actively discourage on aircraft fitted with Kapton wiring - Why?) Kapton can still be dangerous.
An FAA report on an incident with a Monarch Airlines 757 states:"In 1985, a Boeing 757, operated by Monarch Airlines in the United Kingdom, experienced an arc-tracking event in aromatic polyimide wire insulation after circuits were tripped, and smoke appeared in flight."
A wiring event ignited flammable insulation blankets on SwissAir Flight 111. It too crashed into the Ocean shortly after reporting a fire. Luckily for investigators all the wreckage could be retrieved so the evidence was readily to hand to confirm the link between the arcing and the catastrophic fire. Boeing sought to float the idea in public thatt his was something that only affected the out of production MD11. Another bald-faced lie as the literature on the subject shows (See links below).
There are multiple accident and incident reports involving Kapton arcing prior to its replacement by newer wire less prone to arcing. The tragedy, however, is that there appears to be an industry-wide reluctance to point it out as the ignition source of the fire. Most reports regarding ignition are recorded under other headings such as - catering equipment malfunction or Toilet water leak or similar. In all cases, when one drills down into the reports, Kapton arcing had resulted in ignition of materials.
So we have one ingredient for the fire aboard the Helderberg conclusively proven to be present. It is the Kapton wire proven to be the culprit for numerous fires and that was so dangerous it has now been removed from all older airliners.
All we need to find now is something that it could have ignited. Any position within the fuselage where Kapton wiring was near to a known flammable substance in an area mentioned in any of the official reports as having been exposed to high temperatures, could have been a potential seat of the fire.
The record in the investigation has evidence that a ready source of flammable material was on hand and thatt his material was affected by the fire.
A comprehensive document on the ageing of aircraft wiring looms and the effects of routine inspection handling commissioned by the FAA was published in early 2008. Test results in that document showed that Kapton wire insulation could fail in as little as 390 hours of operation in humid environments! The report also contains the following statement: "(Kapton) should not be used in airborne applications." (Read the full report by clicking here.)
The interior of the Helderberg was subject to high levels of humidity for many years. Condensation from exhaled breath, steam from galley kettles and humidity from the atmosphere coupled to high ambient daytime temperatures whent he airliner was parked would ahve aged the insulation rapidly.
CLICK HERE TO READ PART 3: THE INITIAL FUEL SOURCE OF THE FIRE
FOR FURTHER READING:
If you wish to read about some of the Kapton incidents and the role played by the wiring (there are many more you can search for on the web), click the links for the report file PDFs below the summary of each incident:In 2003, a British Airways 737 was evacuated after the crew smelled an electrical burning odour and saw smoke. The AAIB report says, "A short duration flash fire had apparently occurred; there was evidence of fire having consumed some of the aircraft insulation material. The majority of the wiring bundles from this area were insulated with polyimide (Kapton). Some of these wires had separated and examination showed evidence of damage from heat and arcing."
In 1996 a Delta Airlines 767 experienced an uncommanded roll to the left shortly after takeoff. After an emergency landing, crews discovered that an aileron cable had been severed, the NTSB reported. "Evidence of arcing was discovered on the severed halves of the cable."A 1/8-inch hole was discovered in the sheath over a generator power feed cable, but there was "no evidence of direct contact."
Boeing told investigators that "the extent of the arc damage ... appeared to be consistent" with the arc tracking of Kapton. They should know - they had been testing the material since 1985.
Given a reading of the Burgoyne report, it would appear that the arced wires recovered from the Helderberg may have looked similar to these hence the author's reference to the amount of copper that could have fallen away from the wire.