On a balmy September evening in 1998, Swissair flight 111 was in big trouble. A fire in the cockpit ceiling had at first blinded the pilots with smoke, leaving them to rely on instruments to divert the plane, en route from New York to Geneva, to an emergency landing at Halifax Airport in the Canadian province of Nova Scotia. But the fire raging above and behind the pilots, intense enough to melt the aluminum of the flight deck, consumed wiring harness after wiring harness, cutting power to vital flight control systems. With no way to control the plane, the MD-11 hit the Atlantic ocean about six miles off the coast. All 229 souls were lost.
It would take months to recover and identify the victims. The 350-g crash broke the plane into two million pieces which would not reveal their secrets until much later. But eventually, the problem was traced to a cascade of failures caused by faulty wiring in the new in-flight entertainment system that spread into the cockpit and doomed the plane. A contributor to these failures was the type of insulation used on the plane’s wiring, blamed by some as the root cause of the issue: the space-age polymer Kapton.
No matter where we are, we’re surrounded by electrical wiring. Bundles of wires course with information and power, and the thing that protects us is the thin skin of insulation over the conductor. We trust these insulators, and in general our faith is rewarded. But like any other engineered system, failure is always an option. At the time, Kapton was still a relatively new wonder polymer, with an unfortunate Achilles’ heel that can turn the insulator into a conductor, and at least in the case of flight 111, set a fire that would bring a plane down out of the sky.
Space Age Stuff
Electronics hobbyists can recognize Kapton on sight. The familiar rich amber color shows up inside so many devices, from the flexible PCBs that link a laptop’s display to its motherboard to the seemingly miraculous adhesive tape that can withstand soldering temperatures or line a 3D printer’s bed for better adhesion. Kapton has become an indispensable material in electronics manufacturing.
Along with nylon, Teflon, Kevlar, Neoprene, and dozens of other polymers, Kapton was the product of the chemists at DuPont in Wilmington, Delaware. Kapton is an aromatic polyimide with outstanding thermal and dielectric properties. It can do things few other plastics can do, like withstand temperatures from nearly absolute zero to 400 °C. Formed into a film and aluminized on one side, Kapton made an early and dramatic public debut as the golden insulating blanket wrapping the Apollo lunar lander descent stage.
It didn’t take long for DuPont to discover myriad uses for Kapton, and by the early 1970s, the space age stuff was being used to create ultrathin, ultralight insulation for electrical wires. Aerospace companies latched onto the idea quickly; with hundreds of kilometers of wiring in a modern jetliner, shaving off a few grams of weight from each wire translates to huge weight reductions, leading to fuel savings, increased range, and increased payload. Driven by concern for the bottom line, Kapton made rapid inroads into passenger and military aircraft throughout the 1970s and 1980s, appearing in almost every major aircraft. Even the Space Shuttles were outfitted with the stuff.
Arcing and Charring
But the aerospace industry’s love affair with Kapton wiring would soon sour. As early as the 1980s, the US military was beginning to see aircraft fires and crashes related to electrical short circuits. Studies showed that Kapton was far from the ideal insulator everyone had hoped it would be — it tended to develop circumferential cracks from the slightest of nicks, exposing the conductors within. Kapton is also easily degraded by moisture, exacerbating the problem in humid environments or areas of an aircraft subject to moisture, like galleys and lavatories.
Once the insulation is compromised, arcing can occur, which leads to charring of the Kapton. This changes the insulation’s dielectric properties, turning it into a conductor. In some cases, this led to overloaded circuits that were not detected by circuit breakers, since the insulation was carrying the excess current rather than the conductor. Other times, the circuit breaker would trip to protect the circuit, but when the crew reset the breaker, the charred insulation would catch fire and burn like a fuse, flames traveling along the wires far from the original arc.
The US military mothballed many Kapton-equipped aircraft in the late 1980s. In 1999, a short-circuit during the launch of Columbia caused two of the engine control computers to fail, and NASA grounded the entire shuttle fleet to remediate Kapton insulation failures. Recommendations were made to develop methods to test all the wiring on the shuttles, but the fleet was retired with Kapton wiring still in place.
Boeing continued to manufacture planes with Kapton wiring until 1992; the Swissair MD-11 was one of the last Kapton-equipped planes to roll off the line, in fact. Planes built since then generally use TKT, which is Kapton with two thin layers of Teflon to provide better resistance to mechanical abrasion and water intrusion. Still, hundreds of planes take off and land every day with Kapton insulated wires routed through every nook and cranny, some almost impossible to reach for inspection. And even when wire bundles are accessible, inspection comes down to a flashlight and a Mark I eyeball, looking for that cracks that might be lurking.