On the night of July 17, 1996, a TWA 747, Flight 800, departed New York's JFK International Airport heading for Paris and Rome with 230 passengers and crew aboard. Shortly after takeoff and during initial climb out, the jumbo jet exploded, killing all aboard and initiating one of the most extensive aircraft investigations in history. The initial fear was that a terrorist bomb or missile had destroyed the aircraft and an enormous recovery effort was launched along with parallel investigations by the FBI and the NTSB, and supported by the FAA, U.S. Navy, Boeing and hundreds of contractors. Eventually, the remains of those aboard and over 95 percent of the aircraft were recovered from the Atlantic into which the aircraft had fallen.

To the continuing frustration of conspiracy theorists, investigators determined that the aircraft was not downed by a bomb or missile, but rather fell victim to a much more insidious enemy - faulty wiring. Hot or chafed wire is believed to have ignited the fuel vapors in the empty center wing tank, creating a massive explosion and inflight breakup of the aircraft.

Little more than two years after the TWA 800 tragedy, on Sept. 2, 1998, another aircraft departed JFK, this time a Swissair MD-11, Flight 111 bound for Geneva. While cruising at 33,000 feet near the coast of Nova Scotia, the crew noticed a foul smell in the cockpit and believed it to be from the air-conditioning system. Four minutes later smoke entered the cockpit; the crew declared an emergency and donned masks. Air traffic control diverted the aircraft to Halifax International (YHZ).

The crew began dumping fuel and was heading toward YHZ when flames erupted in the cockpit. The captain left his seat to fight the fire while the first officer stayed at the controls. Two minutes later the aircraft struck the water near Peggy's Cove and Bayswater, Nova Scotia, at approximately 300 knots airspeed, killing all 229 aboard. The ocean recovery that followed was much more difficult from the first due to the aircraft's high speed at impact. Eventually investigators were able to theorize that the inflight entertainment system wiring had overheated and had ignited materials in the cockpit. It is unknown why the circuit protection failed, but the resultant fire and smoke incapacitated the crew.

These two accidents serve to remind us that aircraft electrical system malfunctions can lead to tragic results. On a smaller scale, wiring problems often lead to troubleshooting nightmares, and canceled missions. With miles of wire bundles running through barely accessible compartments, inspecting and repairing wiring problems can eat up your maintennace budget and resources in a very short time. To help maintenance managers address these types of issues, we investigated the tools and techniques that can be used to mimimize down time and improve safety.

Attacks on wiring insulation can come in many types: Extreme heat and cold temperature swings can accelerate hardening (or softening), humidity and chemical spills or leaks can create brittleness, mechanical strains can fatigue the wire conductor creating a gap that can arc and generate heat. Over voltage can cut holes in insulation leading to flashover. Mechanical damage that occurs during the wire installation, often in the form of metal chips lodged in wire bundles, can go undetected for years. Tight bend radiuses can chafe the insulation and also fatigue the conductor.

Most electrical problems can be described in two simple terms - short or open. If a wire breaks cleanly, the circuit is broken and the result is an inoperative component. With a short circuit, the load is bypassed and the surge in current should trip the circuit breaker. The most dangerous problem with aircraft wiring occurs when there's a breakdown of the wire's insulator and current jumps outside of the conductor, creating a corona discharge or spark. Insulation does not have to completely disappear for this to happen. If the insulative properties weaken through the effects of age, ozone, ultraviolet and chemical attack, the insulator can actually become a resistor and generate heat. Heat just needs a fuel source and oxygen to create fire.

Typical aircraft wire insulation is made from polymers such as aromatic polymide, alkane imide and others. In the late 1980s, one famous wiring insulation type that received a bad reputation was DuPont Kapton. The material has great temperature stability characteristics from extreme hot to extreme cold but is much thinner and lighter than similar materials. This light weight made it highly desireable for aircraft manufacture and its use was widespread. Like all insulators, Kapton is susceptible to degradation, but its thin shell made it susceptible to chafing and wear modes as well.

There were aircraft incidents and for some reason the insulator received a disproportionate amount of criticism. Perhaps it was because of the famous name of its maker, or the aviation industry wanted to believe that blaming a particular type of wire would shift attention away from the growing realization that all wires need to be inspected and replaced when necessary.

As part of a larger effort to assess the problems associated with senior aircraft, the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) was created, and in part due to the TWA and Swissair accidents, scientists at the Aging Aircraft Laboratory at the National Institute for Aviation Research (NIAR), located at Wichita State University, conducted a comprehensive wiring assessment of two high-time commuter aircraft. Both were torn down and inspected using several non-destructive techniques such as general and intrusive visual inspections. Destructive testing was also performed to determine condition of the wiring. Circuit breakers were examined as well.