Designing Aircraft Wire Harnesses 101

Nov 15, 2023

So, you’re a new engineer freshly out of college. You’ve been hired at a large OEM to design wire harnesses and wired box assemblies. Not having the proper training in college, you ask yourself, "Where do I start?" The possibilities are limitless.

No worries! We’re here to help.

Each aircraft typically has an air vehicle specification (AVS). This is a very large document that the OEM writes prior to designing the aircraft. It contains tons of information about every aspect of the aircraft, including wire harness design. Ask if there is an AVS or similar document. If there is, read all the sections that deal with electrical systems design.

The next document you need to review is the Society of Automotive Engineers (SAE) standard AS50881 Aerospace Vehicle Wiring. Formerly MIL-STD-5088 and last revised in 2015, this document covers information such as wire current-carrying capacity; how wires should be identified, marked, routed and supported in military aircraft; and hundreds of other guidelines and requirements. You can find it at www.sae.org.

There are thousands of part numbers to choose from when designing a wire harness. How do you decide which parts to select?

The first thing you need to understand is the difference between standard parts, nonstandard parts and commercial off-the-shelf (COTS) parts.

A standard part is a military part. It is controlled by the U.S. military. These parts are often called MIL SPEC parts (short for military specification).

Many of these part numbers start with "M," which is short for military. An example is M39029/4-110. This is the military part number for a metal contact that is inserted into electrical connectors. Other military part numbers start with "MS," which stands for military specification. One example is MS3154, which is a strain relief for wires that enter a connector. Other prefixes include "NAS," which stands for National Aerospace Standard. An example is NAS514, which is a flat head machine screw. There are other military part number prefixes, but these are the most common for wire harness manufacturing.

Each military part number has a specification associated with it. These specifications are available on-line at http://www.dscc.dla.mil/programs/MilSpec/DocSearch.aspx. Since the late 1990s, control over these specifications has been transferred from the U.S. Navy to the SAE.

One good thing about military parts is that they are common across many aerospace platforms. For example, the M83723/72W1212N connector might be used in the F-35, V-22, F-16, F-15, C-130 and C-17. It's a very common connector in the aerospace industry.

In most cases, the U.S. government only approves certain suppliers and manufacturers to make a particular part. These suppliers are listed on a qualified parts list (QPL) for that part number.

A nonstandard part is based on a military part, but is slightly changed from the military specification. For example, Lockheed Martin might want to use a military connector in a bulkhead assembly. Because the depth of a bulkhead is slightly thicker than the size of the military connector, Lockheed must design a new connector based on the military connector, increasing its size slightly so it can fit in the bulkhead. In this case, Lockheed writes and controls a specification and gives the connector a new part number.

Typically, the OEM will also designate which suppliers are approved to manufacture nonstandard parts. The specification is typically called a source control drawing or specification control drawing.

The good thing about nonstandard parts is that they fill a need for the OEM, and the OEM controls the specifications and the suppliers that can build them. The bad thing is that these parts are typically more expensive, since fewer are required throughout the industry and, in many cases, they are used on only one aircraft.

The third category of parts is commercial off-the-shelf parts (COTS). These parts are designed and controlled by a manufacturer. Based on marketing studies and the need for the items in commercial and military products, a manufacturer will design and make its own part numbers. The manufacturer controls revisions to the specification and when to release revisions. Neither the U.S. military nor the aircraft OEM has control of these parts.

The good thing about COTS parts is that they are readily available and less expensive than both standard and nonstandard parts. They are less expensive because they are not qualified by the U.S. military, SAE or OEMs. The bad thing about them is that the manufacturer can change its own specification at any time without approval from an outside source. The manufacturer is also the sole source for its COTS parts, so it controls the price and distribution.

When an aircraft's electrical system is designed, the starting point is at the system level. A system schematic shows the larger components and how they are connected. In the system schematic, they are simply connected together with a line, but the size and specification of the wire are not called out. Also, the system schematic does not show where there are breaks in the wire, such as when it passes through a bulkhead via two connectors. Even though it is a very basic design, the schematic will indicate the types of signals that the wire will carry (for example, AC, DC, data, or radio). Additionally, the schematic shows where shielded wires or cables are needed, as well as twisted pairs, triplets and quads.

You’ll want to choose a wire based on the type of signal it will transmit and how much current it will carry. Generally, you should pick the smallest gauge wire that can successfully carry the required current. SAE AS50881 lists the current-carrying capacity for most common wire gauges. In addition, good CAD software will automatically calculate the current-carrying capacity of each gauge size.

In the 1970s, the most common wire for aircraft was Kapton-insulated wire. Kapton is a DuPont trade name. Today, aircraft wiring is typically insulated with Teflon, Tefzel, Cross Link Tefzel or TKT (Teflon-Kapton-Teflon). Typically, the AVS for an aircraft lists the type of wire insulation that will be used throughout the aircraft. For example, the OEM might decide that TKT will be used, with some exceptions such as radio cables and Teflon-insulated wire in boxed assemblies.

Now that you know the schematic, the type of signals to be carried, the amount of current in the circuit, and the recommended insulation, you can choose which wire part number to use.

Next, using a 3D CAD system, wire paths can be traced throughout the aircraft. In addition, wire paths typically start and stop at a connector when passing through a bulkhead. Each wire in a wire segment can then be uniquely identified and a detailed wire diagram can be developed. The wire diagram shows all wire segments, identification numbers and disconnects.

Every wire segment on an aircraft should have its own unique identifier. Even the shield of shielded wire has its own unique identifier. Put simply, every conductor of current must be uniquely identified. For example, a twisted pair that is shielded and jacketed contains three conductors: the twisted pair and the outside shield.

SAE AS50881 describes how a wire should be identified and marked. A wire identifier contains the circuit function letter, the wire number, the segment letter and the wire gauge. For example, P215A4 is a single-conductor wire.

There is an alternative method for identifying wire. It consists of the letter "W," followed by the wire harness number, wire identification number and wire gauge. For example, a single-conductor wire might be called W192-06-22. A shielded, jacketed twisted pair might be called W192-020-22.

InterConnect has been in business for more than 25 years. After such a long time, you might think we would have entered every military connector part number ever developed into our enterprise resource planning system. Unfortunately, this is not true. Almost every week, we enter new connector numbers into our system. To date, we have identified more than 3,200 part numbers for military connectors. With so many possible part numbers, how do you decide which part number to use when designing a wire harness?

The good news is that there are only a few common series of military connectors. Based on our experience in the military aerospace industry, the most common series of military connectors are:

Keep in mind, there are other series of connectors, as well as nonstandard connectors and COTS connectors.

In general, it's best to select a connector series that has as many contact cavities as possible. A favorite in the military industry is the MIL-DTL-38999 connector series, since these typically have many contact cavities. The next step is to analyze the following information and decide which systems (i.e. wires) you want to go through a connector:

Once this information is known, you can review different insertion arrangements (especially the size of the contact cavities for a connector). Then, based on wire and contact sizes, you can choose a connector. It's best to start with normal polarization for the connector. If the same basic connector part number is used in the vicinity of another connector, then a different polarization should be selected.

A reference designator identifies a component in a system schematic and wiring diagram. Reference designators for connectors typically start with:

For example, a reference designator might be 3315P706. It will be connected to 3315J706.

A jack is a connector that is mounted to the airframe. It can be fastened to the airframe with screws and nuts or with nut rings. A plug connector mates to a jack. It is not physically mounted to the airframe except by being connected to the jack.

There are a few other components that may be found in aircraft wire harnesses. Splices (also called wire ties) have a reference designator notation of "WT." Other components (and their notations) include resistors (R), capacitors (C) ground blocks (GD), diodes (CR), switches (S), relays (K) and circuit breakers (CB).

Shield termination refers to how the shield of a shielded wire is terminated to another component. There are a wide variety of termination methods. These include:

The purpose of a shield in a shielded wire is to eliminate electromagnetic interference (EMI) caused by the electrical current in a wire. Likewise, a shield limits EMI caused by other nearby wires. Shields eliminate crosstalk between wires so you have a clear signal.

The most common practice in the aerospace industry is to terminate the shields to a jumper wire, and then connecting the jumper wire to a ground point. In many cases, the ground point is the backshell of a connector and, ultimately, the airframe.

A common question in the aerospace industry is, "What's the best way to hold a bundle of wires together?" There are five main methods used in the aerospace industry: string ties, tie wraps, expandable braided sleeving, heat-shrink tubing and braiding.

Each has advantages and disadvantages. The least costly (and lightest weight) method is string ties. The best-looking wire harnesses are braided (especially with a variety of colors). The OEM will typically specify which method to use.

Also known as cable lacing, string ties consist of a thin cord, traditionally made of waxed linen, that binds together a group of wires using a series of running lockstitches. This method is inexpensive, easy to modify and repair, lightweight and stays tied over decades. On the down side, it does not protect the wire bundle. Some wires can accidently move out of the bundle during installation or retrofit programs. Wire bundles can potentially lose their shape. And, installation is time-consuming.

Tie wraps are available in a variety of materials, styles, lengths, widths and colors. They are inexpensive, easy to modify and repair, lightweight and quickly installed. But, like string ties, they do not protect the wire bundle. In addition, they can break off (creating a foreign object debris issue), and their use conflicts with the requirements of SAE AS50881.

Expandable braided sleeves offer some protection to the wire bundle. However, they can be expensive, and they add weight. In most cases, they do not provide a close fit around a wire bundle, and it takes time to push a wire bundle through the sleeve.

Heat-shrink tubing offers very good protection to the wire bundle, and it lasts a long time. But, like sleeving, it can be expensive, and it does add weight. Similarly, it takes time to push a wire bundle through the tubing, and it takes time to shrink the tubing over the bundle. The biggest disadvantage is that you cannot access the wires under the tubing after it has been shrunk.

Braiding has many advantages. It offers excellent protection to the wire bundle. It increases the physical appearance of the wire harness. It lasts a long time, and it compacts the wire bundle to aid in installation.

Color-coded braiding helps to easily identify a wire harness, and metal braiding provides EMI protection.

On the down side, braiding adds weight, and it's expensive to hand braid each wire harness. If additional wires are needed in a wire harness, they must be "piggy-backed." That is, they must be individually braided and run outside of the braid of the existing bundle assembly.

Testing

Aircraft wire harnesses are tested using automatic equipment commonly called wiring analyzers. Many companies make wiring analyzers. Unique programs tell the analyzer what tests to run, how much current and voltage to use, and how long to apply current. A technician connects the wiring analyzer to the wire harness using adapter cables.

The two most common tests are continuity and insulation resistance. But, wiring analyzers can do other tests as well, including empty contact cavity tests, AC dielectric tests, light lights, energize relays, measure capacitance, measure resistance, and ensure that diodes are working correctly.

Continuity tests ensure that each wire is connected in accordance with the design. For example, if a wire is supposed to be connected from connector 1 at pin 13 to connector 2 at socket 34, the continuity test will verify that the wire is there. Continuity tests are performed at 0.5 amp with a constant voltage and a minimum dwell time of 0.2 second.

The insulation resistance test (also called a megohm test) checks for short circuits. As an example, if two wires have nicks in the insulation and are close to one another, the insulation resistance test will show a short circuit. If a connector was not manufactured correctly and there is not enough material to separate two contacts, the insulation resistance test will show this short circuit as well.

Insulation resistance tests are run at 1,500 VDC with a constant current and a minimum dwell time of 0.15 second. The insulation resistance test does what the name suggests: It ensures there is enough resistance between two or more conductors so that a short circuit will not result. If the resistance is greater than 100 megohms, then it passes the test.

Most OEMs will specify how to test wire harnesses. The common military test standard is MIL-STD-202.

Myriad manufacturing standards are applicable to aerospace wire harnesses. The following are the most common:

For more information about designing and assembling wire harnesses for the aerospace industry, call 817-377-9473 or visitwww.interconnect-wiring.com.