Boeing 727s, as well as early 737s, 747s, 757s and 767s. Boeing stopped using Kapton in 1993.
French manufacturer Airbus confirms that it uses Kapton in the pressurized areas of its aircraft (such as the passenger cabin) and a Teflon-coated Kapton in the unpressurized areas.
Kapton's advantages, as summarized by the GAO, are that it is "lightweight, resistant to abrasion and cuts, is able to withstand high temperatures, and is flame and environmentally resistant."
But government reviewers noted that it has two significant weaknesses.
"First, water alters the chemical composition of this insulation and diminishes its integrity. A second weakness occurs when two cracks in the insulation occur close together, enabling the current to arc between the cracks (arcing events) at high temperatures. Exposure to this heat causes the insulation to 'carbonize' and actually become a conductor rather than an insulator."
"Momentary short-circuit arcs between a defective polyimide insulated wire and another conductor may thermally char (pyrolize) the insulating material. The charred polyimide, being conductive, is capable of sustaining the short-circuit arc. The sustained arc may propagate along the wire through continuous pyrolization of the polyimide insulation (arc tracking). If the arcing wire is part of a multiple wire bundle, the polyimide insulation of other wires within the bundle may become thermally charred and start to arc track (flash over). Therefore, arc tracking may lead to complete failure of an entire wire bundle or harness."
In 1988, the FAA's researchers acknowledged the role of the insulation, saying that "the ability of an aircraft wire to resist wet arc tracking and possible flashover is highly dependent on the composition of the wire insulation."
The next year, another FAA report found that Kapton arcs were "massive in severity and caused multiple circuit breakers to trip." The report notes that "carbonization 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 summarizes the FAA's findings that 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."
In fact, Kapton's arcs can produce temperatures of 1,000 degrees Centigrade, or almost 2,000 degrees Fahrenheit, according to the 1988 report. A more-recent FAA study found that when polyimide wire arced against a titanium hydraulic line, it produced temperatures of almost 4,000 degrees Fahrenheit - hot enough to melt the titanium.
"In December 1985, the Navy decided that aromatic polyimide would no longer be its wiring insulator of choice. Subsequently, the Navy selectively removed this wire insulation from parts of aircraft where it was most problematic, such as fore and aft flaps, wheel wells, and around unsecured seals that could leak... The Coast Guard lagged behind the Navy in taking action to address problems with this wire insulation; however, it took the most extensive action by stripping it from its largest fleet of helicopters as a precautionary measure after occurrences of in-flight fires and cockpit smoke and fumes between 1993 and 1996.... The Army did have durability concerns, however; it found the degree to which aromatic polyimide chafes in Apache and Blackhawk helicopters is unacceptable over time and decided to remove it gradually as it refurbished older aircraft."
Still, a 2002 Naval Safety Center article described continuing problems with "The Silent Menace" of Kapton on some helicopters. "Despite our maintainers' successes in dealing with Kapton wiring, in time, the failures will increase and, unfortunately, could lead to a catastrophic failure or a mishap," it says.
NASA also had reason for concern. Five seconds after the July 1999 launch of the Space Shuttle Columbia, Kapton wiring short-circuited five seconds. It's a situation that, NASA says, "could have caused one or two of the three main engines to shut down." Shuttle launches were put on hold so that engineers could develop ways to better protect Kapton wiring from damage.
Still, Federal Aviation Administration officials have claimed that such comparisons are irrelevant since commercial aircraft are not subjected to the same stressors as military or space aircraft.
"FAA wiring experts reported that they were aware of only one incident on a commercial aircraft linked to aromatic polyimide wire insulation," the GAO reported in 2002.
"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."
But some sort of Kapton arcing event is believed to have ignited flammable insulation blankets on an MD-11 in 1998, according to the Transportation Safety Board of Canada. The aircraft, known as SwissAir Flight 111, crashed into the Atlantic Ocean just off Nova Scotia, killing all 229 people on board.
A 2000 FAA report lists seven incidents in which flammable, metallized Mylar insulation blankets had been ignited by arcing wires. While the report does not specifically mention it, all seven aircraft (two MD-11s, an MD-82, an MD-87, an MD-88, and two Boeing 737-300s) are among those apparently wired with Kapton.
Service difficulty reports filed by the airliners, as well as investigations by the U.S. National Transportation Safety Board (NTSB) and Britain's Air Accidents Investigation Branch (AAIB), also provide examples of other Kapton-related incidents.
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."
In 1999, an Alaska Airlines 737 was forced to make an emergency landing after receiving warnings of low fuel pressure. "Inspection revealed heavily sooted and compromised wires (with melted insulation) in wiring bundles ... in the cargo hold," the NTSB wrote. In all, 113 conductors were found to have been involved. Boeing told investigators that "the extent of the arc damage ... appeared to be consistent" with the arc tracking of Kapton.
In 2000, a Delta Airlines L-1011 experienced an electrical fire over the Pacific Ocean, two hours out of Honolulu. The crew was able to contain the fire using fire extinguishers. The investigation revealed that the fire was caused by electrical arcing of Kapton wires near the cockpit, according to the NTSB. "Numerous wires were melted and resolidified. Some wires had been fused into solid copper for up to 0.6 inches in length, while others displayed resolidified beads of molten wire metal at their burned tips."
In 2003, a British Airways 737 had to be evacuated prior to takeoff after the crew smelled an electrical burning odor and saw smoke. An 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."
"self-resetting circuit breakers installed that had been and were continuing to reset and allowing the Kapton wire insulation and spiral wrap to burn. One of the areas that burned was only three inches below a carbon flight control rod. This happened on a very long flight. The outcome could have been tragic."
An unidentified FAA official told the GAO that "the rules that admitted aromatic polyimide 20 years ago would prohibit it today because of the failure modes that have been identified."
But Airbus still uses Kapton, telling NewsChannel 5 Investigates that it "continues to test and retest all of its wiring types, and we have yet to find anything better than Kapton."