Introduction to Aircraft Harness Construction and Open Bundle Wire Harness Configurations.
Electrical wiring harnesses are an integral part of today’s’ aircraft. Used for signal, power and communication interconnects, harnesses can be found on gas turbine engines, nacelles, and airframe fuselages. With modern airliners relying on fly-by-wire technologies, the harnesses connecting the aircraft’s flight components are critical.
As new automated systems and technologies are integrated into today’s commercial airliners, the amount of wiring has increased dramatically. In fact, there can be literally miles of wiring harnesses on an aircraft. The Boeing 737 family can have between 40 and 45 miles of wiring (depending upon variant) whereas the 787 can have over 60 miles. Even more impressive is the Airbus A380 with 320 miles of electrical interconnects! (Rice, 2015)
Electrical Wire Harness Construction
So how are these harnesses manufactured? All these miles of wire are bundled together into assorted lengths and configurations consisting of jacketed wire cables, backshells, connectors, contacts, etc. Each with their own purpose, configuration, and the needed capability of performing in extreme environments. Wiring harnesses, whether in the airframe or on engine, must be able to perform in all types of conditions. They must withstand bending, cracking, intense heat, humidity, dust, foreign object damage (FOD), moisture, grease, corrosive fluids, vibration, abrasion, and EMI.
In general, harness fabrication consists of two primary construction options – open bundle and closed bundle. Types and styles of harnesses vary greatly and are dependent upon application, OEM preferences, and/or end-user requirements. The initial layup is similar for both.
Based upon the engineering/customer design, insulated conductors (individual, twisted pairs, or triplets, etc.) are cut to length and then laid up. This is typically done on a 1:1 or 3-D form boards. The jacketed cables are then grouped together into wire bundles. These may be a twisted or a parallel lay, depending upon the need for flexibility and repairability.
Harness lengths and layouts can vary widely – everything from a single branch to multi-branch configurations. Harness leg breakout s are determined and routed accordingly, and then the bundles and legs are typically secured with either tape or lacing ties. It is at this stage that open bundle and closed bundle manufacturing differs on the path to finalization.
Both designs are completed via the termination of branch ends (unless there is a kitting requirement). This entails the assembly of backshells, connectors and /or contact pins and sockets. Backshell configurations include potted, closed, or strain-relief styles (MIL Spec or Custom). It is important to note that quality control is critical in the wiring harness manufacturing process. This includes in-process inspections and testing along with a final inspection before shipment. Quality standards are determined by the OEM as well as conforming to recognized IPC/WHMA-A-620 standards.
The balance of this article will focus on open bundle wire harness assemblies. Closed bundle and inspection processes will be further discussed in future, follow-on articles.
Open Bundle Wiring Harnesses
The most obvious physical identifier of an open bundle harness is the lack of an over braid or protective covering (as with a closed bundle harness). The harnesses’ cable jackets are exposed and secured with lacing ties and or tape. Exceptions include areas where added protection is required for abrasion/chaffing protection and clamp areas where the harness is secured to the engine or airframe. While the wire’s cable jacket does provide a degree of protection, the downside of open bundle design is a lack of added protection and durability.
The advantages of the open bundle design, however, is that the harness is open and accessible, making this the preferred design when it comes to repairability. Because there is no external braiding, sheathing, or covering over the wire jacket bundles, backshells, or connectors they can be repaired and or replaced as needed.
Being fully repairable is very cost effective for operators. The ability to reuse serviceable components and only replace those parts that show damage is very efficient from the standpoint of repair and overhaul. This design helps prevent having to discard a damaged harness due to individual component or component’s failure. Backshells and connectors are readily accessible and damaged wire cables can be removed and replaced individually.
The open bundle design also offers significant weight savings on the aircraft. Not having the added braiding or protective covering minimizes weight.
Open bundle harnesses are most commonly found on gas turbine engines, airframe fuselages, nacelle, and landing gear applications.
References: Rice, Nick (2015) MRO Yearbook – Wired for Safety, p.42.
About the Company: kSARIA is an AS9100:D and ISO9001:2015 registered company with facilities in Methuen, MA and Fort Worth, TX. The company manufactures harsh environment fiber optic and electrical cable assemblies and harnesses. kSARIA also offers turnkey installation services of fiber optic and electrical cable for military and aerospace applications as well as Certified Fiber Optic Training. In addition to manufacturing, training, and services they also provide Part 145 repair services such as check & test, overhaul and repair, and S/B incorporation for many of the aircraft electrical wiring harnesses in service today (FAA No.: OI0R891N, EASA: EASA.145.5897, CAAC No.: F00100406).