Aging Aircraft Conference Focuses on Electrical Systems

Air Safety Week, Sept 27, 1999

"There appears to be a near linear relationship between aircraft age and the degradation of wiring due to chafing."

-- Mark Brown, GRC International Inc.

ALBUQUERQUE, N.M. -- Aircraft wire is no longer an "on condition" item. That is, the traditional approach to leaving aircraft wire alone unless a clear electrical fault required repair has given way to a new realization that wire ages in service and requires a program of diligent husbandry to assure the safety of flight, particularly in older aircraft.

At a conference on aging aircraft held here Sept. 20-23, hundreds of industry experts gathered to address what was clearly recognized as a new reality. "All of a sudden, wiring has popped up equal to structure," declared Jim Anderson, director of flight safety for Delta Air Lines [DAL].

If attendance at the standing-room-only sessions on aircraft wiring was any indication, there is a clear apprehension of a major wiring problem in the fleet. In the crowd of more than 600 were representatives of the Federal Aviation Administration (FAA), National Transportation Safety Board (NTSB), National Aeronautics and Space Administration (NASA), major airlines, the military and supporting contractors. The smattering of empty seats at sessions on aircraft structures and engines reflected the intense concern over wiring, and the likelihood that the industry is staring at hundreds of millions of dollars in added costs to repair, replace and monitor the condition of aircraft wiring.

The choice, though, could be between a modest program of prevention, or allowing the problem to fester, running the risk of losing aircraft to wiring faults and in-flight electrical fires, and facing the resulting furor over the safety of air travel. The problems revealed last week ranged from the sinister to the startling.

Donald Eaton, a retired Navy rear admiral, presented the challenge in stark terms: "Do you want to use the Casino Royale approach, or do you want to spend a little money to avoid paying a whole lot later?"

For the second time in 10 years, almost to the month, the industry has been faced with a major aging aircraft problem. Steward Miller, aging systems program manager for the FAA, recalled the catastrophic April 1988 failure of the overhead fuselage of a highcycle Aloha Airlines 737. That event, he said, "was the origin of the aging structures program."

"The event took place because the industry and authorities did not understand that at the time we were overlooking a significant problem," he recalled.

History virtually repeated itself in May of 1998, when earlymodel 737s were grounded after it was discovered that some electrical wiring routed through conduits in the wing fuel tanks was found chafed all the way through the insulation to the conductor. The startling finding presented the specter of an another fuel tank explosion of the type that blew up TWA Flight 800 in 1996.

"We were watching the fleet, but we did not understand that maintenance programs were not as robust as we thought," Miller said. As an example, the wiring in fuel tank conduits was "in an area we were not inspecting."

Nor is the airline industry unique. "We've got an aging wiring problem ourselves," said Dr. Dan Mulville, NASA's chief engineer. The Space Shuttle fleet sits grounded while each of the four shuttles undergoes a thorough wiring inspection, and the U.S. Air Force, following the Navy's lead, is removing Kapton wiring from its fleet of B-1 bombers. Kapton wiring, a trade name for aromatic polyimide, is known for its tendency to arc explosively. It is found in anywhere from 50-80 percent of the commercial airline fleet (estimates vary, a point which underscores the uncertainty about the kind and condition of wiring in the commercial fleet).

Indeed, the growing evidence of a widespread wiring problem prompted Major General Francis Gideon, chief of safety for the Air Force, to ask, "Is the aerospace industry capable of identifying emerging problems?"

The physics of wire aging

The preliminary evidence from the fleetwide inspections of the 737s fuel tank wiring suggests an almost linear relationship between aircraft age and the degradation of wire due to chafing. Indeed, the data imply there is a one in six chance that an aircraft with more than 70,000 flight hours is carrying exposed wire somewhere, and with it the potential for dangerous electrical arcing.

Wire, it is now realized, ages at a different rate than aluminum structure, which is to say perhaps at a faster rate. Wire, it seems, may not last as long as the 20 or 30-year design service objective of the airplane (DSO). That objective is defined for structure as the years in which the airframe essentially is free of fatigue cracks, thereby establishing period of minimal structural-related maintenance costs.

For wire, service life is influenced by a variety of factors:

* As temperature increases, age decreases.

* As humidity increases, age decreases.

* Vibration accelerates aging.

* Poor installation can accelerate aging. Excessively tight radius bends put the insulation under strain. Improperly installed clamping and bundling devices that can impart chafing, crushing and cutting forces on thin insulation.