Electrical Harness Maintenance
By Dan Warchol
Author's note: Unison Industries and specifically the author of this article would like to acknowledge the contributions of Pratt & Whitney to the completion of this article. With Pratt & Whitney's permission, portions of the "On-Wing Maintenance for Open Bundle Harnesses" section of this article were taken from a Pratt & Whitney PW2000 Service Information Report dated May 2000 (of which Unison was a contributor). All other sections were developed and written by Unison Industries.
All aircraft utilize electrical wiring harnesses to interconnect various sub-systems and components located throughout the aircraft. The type of harnessing varies widely depending on the intended user environment, OEM preference and end user requirements. Most of the miles of wire found in use on aircraft today perform a critical function in the proper operation of the aircraft. The majority of the remaining harnessing is key to ensuring and delivering high quality and reliable passenger comfort, safety, entertainment, and/or information. Loss of function in either of these roles will be noticed immediately so implementing a thorough harness inspection, maintenance, repair, and overhaul maintenance program for all aircraft types should be a key goal of all aviation maintenance departments.
Before a maintenance program can be properly established, it is important to understand the types of gas turbine engine, airframe, nacelle, and landing gear harnesses flying today. The different types are described as Open Bundle, Closed Bundle, and Overmolded. The design differences between the harness types have a direct effect on the field repairability of a harness.
Open Bundle (Typical Applications: Gas Turbine Engine, Nacelle, Airframe)
The term "Open Bundle" refers to a harness that is not sheathed with an overall protective covering. Durability is sacrificed to some degree, although the individual wire cable insulation does provide some level of protection. Additional protection can be added to specific locations on the harness that are potentially susceptible to chafing from the surrounding environment. The chief advantage of an Open Bundle harness is 100 percent repairability. Any one or more of the individual components that make up the harness can be replaced without having to discard other portions of the harness.
To simplify replacement of the individual wire cables in an Open Bundle harness, the cables are routed in a "straight lay." This means the cables are not twisted or rolled around the harness bundle axis, thus making it easier to re-string a replacement cable. The drawback to a straight lay is the limited flexibility of the overall harness bundle, which necessitates extra care when routing the harness, especially in the tight confines of a gas turbine engine.
To address the harness routing requirements, 3-D tooling is often employed by harness manufacturers offering high quality, great fit, and superior performing products. Such tooling simulates the routing of the application and thus avoids overstressing the wires while also simplifying installation.
The advantage of an Open Bundle harness to an operator is that it is more repairable. The disadvantages are that Open Bundle harnesses tend to be bulkier and heavier.
Closed Bundle (Typical Applications: Gas Turbine Engine, Landing Gear, Fuel Quantity)
Harnesses protected by an outer sheathing are referred to as "Closed Bundle." Closed Bundle harnesses are typically not repairable in the field except to some small degree. The application of an appropriate protective sheathing dramatically improves the durability thus minimizing the need for repairability. If required, the original harness manufacturer or another FAA-approved harness Repair Facility can usually provide more extensive repair in its service center.
The outer protective sheathing on a Closed Bundle harness can take many forms including:
• Heat Shrink Sleeving
• Spi Rap
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