More Separation Needed In Design of Aircraft Wiring Systems

Air Safety Week, May 13, 2002

'The Lord didn't design the human body then throw the veins in afterwards'

Electrical wires connecting flight-critical electrical systems need to be more separated for protection against the effects of arcing. When arcing occurs in a packed bundle containing both power wires and signal data wires, the damage can lead to a cascading sequence of failures that can vastly compound the hazard created by the original arcing event.

"There is no substitute for distance" when it comes to wiring connecting flight-critical systems, maintained Jim Shaw in his recent presentation on wire separation from the pilot's perspective to the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC). Shaw is head of the In-Flight Fire Team for the Air Line Pilots Association (ALPA).

"Do we need a hard and fast number?" Shaw asked, with respect to a minimum wire separation standard. "Probably not," he said, going on to explain, "Whatever is used should have the dielectric equivalent of two inches of air." Moreover, Shaw suggested, any wire separation standard should address the potential for severe electrical arcing to create damage. In this respect, even a two-inch dielectric equivalent distance may be optimistic. The temperature at the center of an electrical arc is about 5,000? C (9,032? F), which is more than sufficient to melt most aeronautical metals.

That kind of intense heat will make short work of any stainless steel high-pressure oxygen lines anywhere in the vicinity. And as soon as such a highpressure oxygen line is pierced, the oxygen rushing out of the pinhole creates a blowtorch-like effect than can spread fire. A similar concern relates to highpressure hydraulic lines. The jet of mist emerging from a pinhole opening in a steel tube carrying hydraulic fluid can have an effect similar to that of escaping oxygen - an impressive increase in the fierceness of a fire.

Documentation during design

In calling for the establishment of minimum separation standards for aircraft wiring, Shaw may be well ahead of efforts in the Federal Aviation Administration (FAA). Its latest policy statement talks about the need simply to have a better understanding of where all wires in an aircraft originate and where they terminate. The FAA's February 2002 "Notice of Final Policy" said there have been too many instances where the wiring data on which officials must approve aircraft designs, modifications thereto, or post-production installations, such as in-flight entertainment systems (IFE), "did not adequately and clearly define the configuration for the model to be certificated." The lack of documentation "left an inordinate portion of the installation to the discretion of the installer," the policy notice said.

In other words, the FAA wants more than silhouette drawings. The new policy says installation drawings should depict the exact path a wire takes from starting point to termination, around spars, through bulkheads, its proximity to other systems, and so forth. Under the new policy, the documentation must show if a wire is routed close to an oxygen line, or in the same bundle as highcurrent-carrying cables.

The new policy also touches on the safety aspect that is Shaw's primary concern as a pilot. When applicants submit their design packages for FAA approval, a system safety assessment should be included. "The analysis should consider the possible effects wire system failures would have on systems for safe flight and landing due to damage in collocated wiring bundles and the possibility of smoke and/or fire events," the policy intoned.

However, the call to do better may have been undercut by the new policy's own closing remarks. The actions called for are "not intended to establish a binding norm," the policy said. "It does not constitute a new regulation." The FAA seems to be trying to improve substandard practices short of issuing new regulations. In a way, this course may reflect a sense of urgency. Regulations can take years to put into effect. Policy can be put in place by pronouncement.

Furthermore, regulatory teeth may be coming from the work of a government-industry committee under the guise of the Enhanced Airworthiness Program for Airplane Systems (EAPAS). This effort is a direct outgrowth of ATSRAC activity on wiring over the past three years. recommendations for rulemaking are expected this coming August.

"These [new] regulations and associated advisory circulars (ACs) are a giant leap forward," Shaw said. Present regulations, he added, are "woefully inadequate," and fall under the heading of "Thou shalt build safe aircraft."

Indeed, Shaw does not appear to be overstating his case. The FAA recently requested bids for a three-year, $18 million research project, titled "Evaluation of Aircraft Wiring Separation and Segregation Requirements and Practices." The April 30, 2002, announcement said with revealing candor, "Today, there are no available criteria for what is a safe separation from a wiring fault perspective ... Should the wiring for two redundant channels be 4 inches apart or 5 feet apart?".