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
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.
If the harness contains thermocouple circuitry, inspect the thermocouple lugs for any damage. Also check the wires for damaged insulation. Confirm that ground pigtails and ground lugs are acceptable. If necessary, replace the harness assembly.
Wire Cable Condition
Examine the harness for evidence of heat distress that could cause the harness to be discolored. Heat could also cause the harness insulation to melt, bubble and crack. If heat distress is noted, locate the heat source and take appropriate actions to correct the problem. It may be necessary to adjust the harness routing (by eliminating excess slack), to move portions of the harness that are too near hot tubes or components.
Check for any evidence of chafing of the wire bundle. If metal braid is exposed, take the appropriate actions to alleviate the problem. Chafing may result from a worn or loose mounting clamp, or rubbing against a bracket, tube, fastener or some type of external component.
Check lacing ties for tightness. Ties should be tight and not able to be move along the wire bundle. Discolored ties are normal and it is not necessary to replace them unless they are loose. If ties are loose, remove and replace in accordance with the CMM.
Harnesses are typically designed to withstand exposure to various fluids that are expected in the given application. However, they should be kept clean if possible. Wipe excess fluids and other materials from the harness. If a fluid leak has occurred, check for fluid in the connector backshells. Remove the fluid and clean the backshells if necessary. Make sure the connector insert is clean and free of any fluids. Try to eliminate the leak or source of fluids so the harness won't be exposed to the same harmful fluids in the future.
Examine connectors to ensure the contacts are in good condition. Check pin contacts for bending, corrosion and other damage. Check socket contacts for corrosion and damage. Replace all contacts that exhibit these undesirable conditions. Using the appropriate tools per the CMM (not probes or paper clips), make sure that contacts are locked in position and have not backed out or pushed into the connector grommet. Replace the connector if any of the contacts can not be locked in position.
If worn contacts or intermittent circuits have been identified, check the contacts for engagement force with a new mating pin or socket. A slight force should be required to insert or remove a mating contact.
In-Shop Maintenance for Open Bundle Harnesses
Lay each harness out on a bench and examine for any evidence of damage that needs to be corrected. Look for chafing, nicks, thermal distress, damaged connectors, backshells, terminal lugs, etc. Tag the damaged areas for repair.
Clean the accessible portions of the harness to remove any fluids or other contaminates that have accumulated. Be careful to protect the front face of all connectors. Connector faces and contacts should be cleaned using the procedures defined by the CMM.
Perform a complete electrical check of the harness using the CMM procedures. During the electrical testing, move the harness on the inspection table to expose any intermittent short or open circuits. Repair any damage using the CMM procedures.
All connectors should be completely disassembled so an adequate inspection can be performed. The coupling ring should be relatively free turning and should have indication that the self-locking mechanism is working. Some plug style connectors have an internal metallic EMI band on the barrel of the body (below the alignment keys). The EMI band should be undamaged. The connector face should be undamaged with no chips if it is a hard-faced connector. For soft-faced connectors, no tears, bubbles, puncture marks or swelling of the connector grommet are permitted. If any of the above damage exists, replace the connector per the CMM procedures.
Check the rear grommet of the electrical connector. If excessive distortion of the wire entry holes is noted, replace the connector per CMM procedure. Unwired positions should have a contact sealing-plug installed.
Even if worn contacts or intermittent circuits have not been identified, it is still a good idea to check the contacts for engagement force with a new mating pin or socket. A slight force should be required to insert or remove a mating contact. Replace the pin or socket if they fail this test per the CMM procedures.
All backshells should be completely disassembled so an adequate inspection can be performed. Check for wear/chafing of conductor wires inside the connector backshell. Evidence of wear should result in addition of protective sleeving over the wires or replacement of the harness cable.
After all inspections and repairs are completed, perform the electrical acceptance checks as specified by the CMM.
Install protective caps on all connectors. Protective caps must fit on the outer diameter of the connector coupling nuts to eliminate possible damage to the internal EMI bands. Some form of protection (i.e., bubble wrap) is recommended around the entire connector/backshell or lug termination. This can reduce damage to the harness caused by contact with sharp edges.
For in-shop maintenance and complete overhauls, always use a properly qualified and approved harness repair facility that is capable of fully interpreting and implementing the approved maintenance procedures. The FAA, JAA, and CAAC will approve a shop only after it passes a stringent on-site shop audit. In addition for overhaul and repair, it is often more convenient and cost effective to have harnesses overhauled or exchanged by a harness manufacturer's service center. Harness manufacturers' and their service centers, like Unison's Service Center and Harness Express program, offer factory quality through extensive upgrade and exchange programs that are not described in the CMM. Programs like these increase component reliability and decrease operating costs for the operator.
Sustained Revenue Service
We all know that uncomfortable feeling when the passengers need to be removed from an aircraft due to a problem that can not be corrected quickly. Electrical harness problems can be particularly difficult to isolate. Periodic and diligent performance of the maintenance practices described in this article in concert with the recommendations of the aircraft, engine and/or harness manufacturer's recommended practices can help to keep the aircraft flying.