Various types of braid material are also available. Material selection is primarily driven by environmental or cost requirements. A layer of metal braid is usually provided, for EMI protection. Unfortunately, metal braid is subject to chafing damage and is therefore typically overbraided with a more durable protection. Two overbraiding material choices are generally available:
• PEEK (Polyetheretherketone)
PEEK is the more durable type although it is also typically more expensive. Nomex, suitably coated with a pigmented material, can be provided in various colors. Various other new materials are now becoming available and are undergoing evaluation at cutting-edge new and overhauled harness manufacturers.
The advantages of Closed Bundle harnesses are greater durability, smaller size, and reduced weight. The primary disadvantage is that most repairs are limited to the factory.
Conduit is also sometimes utilized as the outer protective layer on a harness. It can best be described as tubing through which the wires are routed. Conduit systems usually include special "transitions" to accommodate the harness bundle branches. Such designs provide the best durability and also facilitate a high degree of repairability. However, conduit designs are disadvantaged by higher cost and weight.
Overmolded (Typical Applications: Gas Turbine Engines, Landing Gear)
Some applications utilize "Overmolded" harnesses. They are a subset of Closed Bundle harnesses and are distinguished by the addition of molding over the backshell. Viton® is a typical molding material and is applied using mold presses and curing ovens. It is a very durable design, but lacks repairability in the field. In fact, the harness manufacturer is also very limited in its repair capability due to the permanence of the overmolding. Such designs are losing popularity, although they do accommodate the need for a moisture seal in some severe applications.
Due to the limited user repairability of the Closed Bundle and Overmolded designs, maintenance actions outside of the harness factory are typically focused on Open Bundle designs. The following information will describe the most important aspects of a preventative and reactive Open Bundle maintenance program. For harnesses on wing, the line maintenance is the same as in a service center except for the extent of repairability. There are instances when time is a major factor and after lengthy periods of line maintenance troubleshooting, the harness should be removed, replaced and sent to a service center for more in-depth troubleshooting and repair.
In addition, if the recommended time between overhaul has been exceeded, it is strongly recommended that harnesses be removed from the aircraft. Some engine manufacturers recommend 15,000 hours between harness overhauls. Inspection and overhaul can be provided at a qualified repair facility or ideally at a harness manufacturer's service center holding FAA Repair Authorization.
On-Wing Maintenance for Open Bundle Harnesses Mounting Clamps
Check all mounting clamps to verify that they are secured properly and have the correct amount of cushion material. Ensure that the clamps are sized correctly to prevent harness axial movement. Replace any clamps with damaged cushion material or any clamps that are the incorrect size. If possible, clean any foreign material that has lodged between the clamp cushion material and the harness. Mounting clamps should be positioned to allow the harness to flow smoothly through the clamp.
Check the installation of the harness to ensure that no contact is possible with tubes, brackets, fasteners, clamps or some other external component on the aircraft. Adjust the mounting clamps to eliminate any interference. Where harnesses are mounted side by side, it may be permissible to lace the harnesses together to eliminate movement (Be sure to check the Aircraft Maintenance Manual).
Check all connectors for the correct tightness to mating components. If found loose to hand pressure, tighten the connector coupling nut with either soft-jaw pliers or a strap wrench. Also, check the backshell to connector interface for tightness. If found loose, tighten the backshell coupling nut using the same procedures.
Examine the area where the wiring exits the backshell. On most designs, the strain relief bar screws should be tight and compressed to obtain metal to metal contact. If necessary, tighten the strain relief bar screws. (Not all designs require tightening of the strain relief bar screws. Refer to the Component Maintenance Manual (CMM) for specific instructions.) The cushioning material between the strain relief bar and wiring should be in satisfactory condition. Damaged cushion material can result in chafing and should be replaced. If the harness includes a bonding clamp (hose clamp), make sure it's not loose. If necessary, tighten the hose clamp screw. All braid pigtails should be secured between the clamp and the backshell body by tightening the hose clamp.
Some older applications utilize aluminum backshells. These should be replaced with the more durable stainless steel backshells (if authorized by service bulletin or other document). Besides the backshell manufacturers and their distributors, full service harness manufacturers can also provide these components at a reasonable price and delivery.