Hundreds of thousands of feet of wiring - more than 200 miles - lace the insides of a large commercial jet, controlling its most vital functions.
Yet for decades this web of bundled threads constituted a blind spot in the usually cautious world of aviation safety. Wiring problems, it was thought, were self-evident: A fuel gauge, for instance, will register erratically, or lights will flicker if there's an electrical short. Such problems are fixed immediately.
But yesterday, the US Federal Aviation Administration (FAA) announced new, wide-ranging guidelines requiring a more intense scrutiny of airplanes' electrical systems, particularly in older aircraft. The sweeping program grew out of recommendations from the 1997 Gore Commission on Aviation Safety. But the reforms were sparked by a growing consensus that the 1996 crash of TWA Flight 800 was caused by barely visible corrosion on a wire in the plane's center-wing fuel tank.
Wiring problems are now also a prime suspect in the recent crash of Swiss Air Flight 111.
"It is critical that we act pro- actively today to ensure that tomorrow's passengers enjoy the highest level of safety," says Transportation secretary Rodney Slater.
The FAA will now require planes to undergo more regular, rigorous inspections of wiring, particularly in places that are corrosion-prone, close to flammable liquids or gases, and where high vibration can cause chafing or cracking. Electrical connectors, wiring harnesses, cables, fuel hydraulics, and pneumatic lines - anything that could cause a system failure - will also be examined more closely.
"In the past, wiring was not treated as a system, but as a commodity - you didn't really worry about it that much," says George Slenski of the electronic materials evaluation group at Wright Patterson Air Force Base in Dayton, Ohio. "It's taken time to change that culture, but it's now considered a system that does ... have to be maintained with some type of program."
Insulation is key
With the wiring, the goal is to ensure that the insulation remains intact. Even a microscopic crack, subjected to the right conditions, could cause electrical arcing that might produce a deadly spark.
Last May, that concern prompted the FAA to ground the oldest 737s - those with more than 30,000 flight hours - until all had been checked for damaged insulation on wires going to fuel boost pumps buried in the wing tanks. Because some damaged wiring was found on planes with only 29,000 hours, the FAA on Monday expanded its order to include 737s that have flown between 20,000 and 30,000 hours.
But that order was separate from the sweeping program announced yesterday. It is intended to ensure that electrical systems on older planes get the same attention as the structural frames do.
In 1983, concern about the aging of the American fleet prompted the Boeing Co. to start an "aging aircraft program" to give special attention to planes that had exceeded their "operational design life," usually about 20 years.
The FAA instituted its own guidelines and requirements for older planes after the Aloha Airlines accident in 1988, when part of the metal body ripped off in mid-air. But in both cases, the focus is on a plane's physical structure - its metal fuselage, rivets, and nuts and bolts. This new program will give electrical systems the same level of attention.
"While we may not be able to reverse the aging process, we can certainly make it safer," says FAA Administrator Jane Garvey.
Light touch on the wiring
The FAA currently requires planes to go through routine, rigorous inspections, which culminate in what's called the "D Check." At least once every four years, a plane is essentially taken apart and scoured from nose to tail.
But the wiring is usually left intact, unless inspectors see a problem that needs to be corrected. That's because tampering with wiring, even to check it, could create more problems than just leaving it alone.
"You don't want to do an inspection that in and of itself creates a hazard that is greater than the one you're trying to prevent," says Clinton Oster, an aviation expert at Indiana University in Bloomington.
One key to the controversy over wiring is Kapton, a kind of wiring insulation that was widely used throughout the aviation industry until recently. It is lightweight and has superior flame-resistance to gasoline fires, according to Armin Bruning of the Lectromechanical Design Co., a high-tech firm in Sterling, Va., that specializes in testing wiring characteristics.
"It also has good chafing resistance when it's in good condition, compared with some of the other insulation on aircraft. But it has poor resistance to arc degradation," says Dr. Armin.
Indeed, if the Kapton insulation on a high-voltage wire is cracked, and if it gets exposed to water, it can cause dramatic arc tracking - something that looks like a little explosion.
Both the Air Force and the Navy began phasing out the use of Kapton in the mid-1980s. Commercial airlines have also shifted to using other types of wiring insulation. But thousands of planes - in military and civil aviation - still contain significant amounts of Kapton wiring.
"We understand how it fails, but with proper installation and maintenance this wire will not fail," says Mr. Slenski.
How big a safety threat?
Indeed, in the overall context of aviation, serious accidents caused by wiring are extremely rare. While wiring is a suspect in two recent international crashes, there has not been a single major air accident tied directly to it in the last 100 million domestic flights - a number that covers the past two decades, according to Arnie Barnett, an aviation expert at the Massachusetts Institute of Technology, But Dr. Barnett also applauds the FAA for its decision to focus more sharply on wiring.
"It's not enough to say in the past that it hasn't been a problem, because as [planes] age new dangers can arise. We can't just say that in the past we find the prologue to the future."