A United Airlines B767-300 on a Jan. 9, 1998 flight, from Zurich to Washington DC, was forced to execute an emergency landing at London’s Heathrow airport because of a food chiller unit improperly installed in the electronic and equipment bay. A jagged edge on the chiller scraped wire insulation as the chiller was pushed into position. The result was wholesale arcing damage. This is just one of several events that highlighted the need for more attention to be paid to the maintenance of aircraft EWIS analysis and wiring systems on an aircraft and illustrated that inattention to these details can affect the airworthiness of an entire airplane.
A Risk Analysis Tool (RAT™) has been developed for the Federal Aviation Administration (FAA) to aid aircraft manufactures and maintenance personnel in the continued airworthiness of their aircraft. This tool introduces a fresh perspective to the maintenance and upkeep of Electrical Wire Interconnect System (EWIS). Problems and malfunctions of the wiring systems are broken down into categories, each identifying specific levels on which malfunctions can occur and to what degree of severity. Implementation of the RAT™ requires us to introduce three types of EWIS failures: wire level, bundle level, and zonal level.
At the wire level, insulation breaches can have effects ranging from degraded function to eroded safety margins. Because most wires are routed in groups, or bundles, adjacent wires in a bundle are often subjected to similar types of environmental and mechanical strains. Consequently, these adjacent breaks may increase the likelihood of cascading damage.
At the bundle level, an insulation break can lead to arcing, and failure of many wires in the bundle. Depending on the number of systems to which the wires are connected, this type of spreading damage can wipe out functions and force the aircrew into emergency procedures. If not already tripped, circuit breakers may have to be pulled, smoke evacuation and fire fighting equipment put into action. The repair effort involves time, expertise and cost to return the airplane to airworthy condition.
At the zonal level, or damage that extends across multiple bundles, the problems are worse. Failures at the zonal level can completely negate fail-safe design and built in redundancy features. The electronics and equipment bay, typically located below the cockpit, is a notable case in point. The sheer concentration of power feeder cables, signal wires and the end-use components to which they are connected makes the E&E bay particularly vulnerable.
The RAT™ facilitates evaluation of a particular wiring layout. Evaluation can be done during original design to assure adequate separation and segregation of aircraft wiring, thereby minimizing the potential for single point failure. The tool can also be used for assessing the safety of modifications to the original layout, as might be the case in the supplemental type certificate (STC) process.
By identifying the function, length and type of every wire on the airplane, its proximity to potentially volatile fixtures (fuel tanks and hydraulic lines), and its exposure to the elements, the RAT™ can calculate the failure rate per foot of wire per flight hour. This metric will prove extremely useful in choosing the wire installation procedure that will satisfy the planned service life of an aircraft.
By identifying EWIS failures at the wire, bundle and zonal level, we can change the way we think about the continued airworthiness of an aircraft. Implementation of the RAT™ tool requires that all personnel understand the importance of wiring maintenance and the circumstances that contribute to EWIS failure.