Electrical Harness Maintenance
By Dan Warchol
September 2000
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)
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.
Maintenance
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.
Harness Routing
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).
Connections
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.
Thermocouple Lugs
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.
Lacing Ties
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.
Cleanliness
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.
Electrical
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.
Connectors
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.
Backshells
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.
Acceptance Test
After all inspections and repairs are completed, perform the electrical acceptance checks as specified by the CMM.
Handling Protection
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.
Quality Assured
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.