U.S. regulators, concerned that applicants who request certification of wiring-related modifications don't always submit required data, have clarified what they expect applicants to provide as part of their requests - including detailed maintenance information related to the changed systems.

The Federal Aviation Administration (FAA) policy statement reminds applicants seeking "type design, amended design, and supplemental design" changes affecting wiring in transport-category aircraft that they are required by the Federal Aviation Regulations (FARs) to submit packages that "completely define the certification configuration." This includes, but is not limited to, complete wiring diagrams with routing details, mounting details, and a system safety analysis of the proposed changes.

The policy stresses that applicants also are required to submit instructions for continued airworthiness, or the information about the system that maintainers of the system would need during repairs.

"A review of past certification projects indicates that the maintenance aspects of system wire external to the installed equipment is not being adequately addressed," FAA wrote. "The integrity of the wiring is typically left to those doing general airplane maintenance that relies on visual inspections.

"However, visual inspections may not be adequate for wiring routed in metal or opaque conduits, wire in high vibration areas, or wire located in difficult to inspect areas," the agency continued. "Equipment installers need to address any special maintenance requirements for the airplane wiring associated with equipment installation."

The document, generated by the agency's Renton, Wash., aircraft certification office, is considered "advisory material," which does not carry the same weight as a regulation. FAA is seeking comments on the statement through Aug. 1.

Airliner wiring issues have gained visibility due to their role in several accidents, most notably the 1996 in-flight explosion of a TWA 747-100 and the September 1998 crash of a Swissair MD-11 cause by an electrical fire. Those two accidents led to numerous studies - and some new rules - on wiring integrity, maintenance practices, and design issues.

FAA's certification process has also come under fire during the last five years. The U.S. National Transportation Safety Board, pointing to evidence uncovered in the TWA crash and a few others, called for a review of FAA certification for several years. Earlier this year, NTSB Chairman Carol Carmody said doing the study would become a top priority for the board, even though the FAA planned to do its own probe.