Electrical Wiring

Overview

In this section the design and installation of the aircraft’s electrical and electronic wiring is discussed. Due to the complex nature of wiring installations, it cannot be overemphasized that coordination between the system designers and the electrical designers must begin at an early phase.

Accessibility

Due to the increasing role that electrical and electronic equipment play in today’s commercial aircraft, wiring installations continue to become increasingly complex. Because of this complexity, one of the first design considerations must be given to accessibility.

While the length of a wire run should be as short as possible, it should always remain accessible for the following reasons:

• • ease of wire assembly, installation, or removal
• • ease of equipment installation and maintenance, i.e., placement of terminal strips, mod blocks, grounds, and electrical connectors
• • troubleshooting
• • wire assembly inspection and repair

7.5.2.3 Safety

New designs must be coordinated with other design groups (structures, hydraulics, interiors, etc.) in order to assure wire accessibility. An issue related to accessibility is personnel and passenger safety. Wiring routing and components must be maintained by personnel, and not always under ideal conditions. Hazards must be eliminated and the wiring and personnel must be protected. When a wire installation can be used as a step, hand hold, or seat, chances are it will be. To eliminate this possibility, routing should be moved to prevent such an occurrence, or adequate protection provided by use of conduits or covers.

Wire separation categories

In Section 7.7, coupling is discussed, along with some ways in which electromagnetic interference (EMI) can be eliminated. One of the most obvious ways to control EMI is through isolation of wire runs. All wiring separation requirements must meet the aircraft wiring installation specification detailed in WZZ7002. In summary, this spec groups wire categories into: (1) distribution and interconnecting wiring, and (2) primary generator power, control, and regulation. These wire categories are summarized in Table 7.5.2.

Table 7.5.2

Category	Distribution and interconnecting wiring	Separation
	Description
I	Electrical loads (motors, heaters, lighting)	6″
II	Instrumentation and electronic loads	3″
III	Not used	Not used
IV	Sensitive (audio, video, sensor, signals, etc.)	3″
V	Extremely susceptible (antenna coax, cable, flight functions)	3″
VI	System	3″
Primary generator power, control, and regulation
V	Primary power feeders, generator to load center	12″
VI	Primary generator control and regulation	6″

The wiring separation requirements are also specified in the following FARs:

Figure A-1. 25.1431 (c) Electronic equipment

1. (c) Radio and electronic equipment, controls, and wiring must be installed so that operation of any one unit or system of units will not adversely affect the simultaneous operation of any other radio or electronic unit, or system of units, required by this chapter.

Figure A-2. 25.1353 (a) Electrical equipment and installations

1. (a) Electrical equipment, controls, and wiring must be installed so that operation of any one unit or system of units will not adversely affect the simultaneous operation of any other electrical unit or system essential to the safe operation. Most wire assemblies are not limited to one specific area of the aircraft. Therefore, to help prevent wire runs from crisscrossing as they enter other areas of design responsibility, an attempt should be made to run the wiring per the design diagram.

In certain structural areas of the aircraft it may be impossible or impractical to maintain the required wire separation distances. Such cases are also addressed by WZZ7002. A typical example of this is where wires must pass through a lightening hole. Here, the wire bundles must maintain the minimum spacing until they immediately enter the lightening hole and shall break away as soon as possible. (A lightening hole is a hole in a structural member to reduce weight.)

System wiring separation

In addition to wire separation based on the magnitude or sensitivity of the loads they carry, separating redundant systems in different wire bundles must also be considered. The objectives of redundant systems can be negated if wiring routed to the systems are contained with the same bundle—both systems can be taken out by damage to the single wire bundle. Generally, in redundant systems the wiring for System #1 is routed on the left side, and System #2 on the right side of the aircraft. This left/right separation should be maintained from the system’s originating point, to its final termination. FADEC (full authority digital engine electronics control) systems are such an example. Each engine has a FADEC with channel A and channel B wiring. The wiring for each channel is routed through different wire bundles and disconnects. These bundles are routed through the left and right side of the fuselage, as well as the leading and trailing edges of the wings. In some areas of the aircraft, redundant system wiring must be routed together, as in the case of the Main Avionics Rack. Here, physical separation is provided by using shielded wires for each of the redundant systems.

Section 7.6 discusses how the power generating channels must be separated during autoland. The same autoland separation principles must be applied to the critical systems in the aircraft, such as Flight Control Computer (FCC), Display Electronics Unit (DEU), Inertial Reference System (IRS), and Flight Management Computer (FMC). For these systems, the Avionics group must coordinate with the Wiring Installation group to determine what circuits are considered critical and need to be isolated. System designers must also coordinate with outside vendors to ensure that separation requirements are designed into the LRU. This includes channel separation through separate inserts in the electrical connector of the LRU. The design separation must also satisfy the following FAR:

Figure A-3. 25.1309 Equipment, systems, and installations

1. (a) The equipment, systems, and installations whose functioning is required by this subchapter, must be designed to ensure that they perform their intended functions under any foreseeable operating condition.
2. (b) The airplane systems and associated components, considered separately and in relation to other systems, must be designed so that-
   1. (1) The occurrence of any failure condition which would prevent the continued safe flight and landing of the airplane is extremely improbable, and
   2. (2) The occurrence of any other failure condition which would reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions is improbable.

Coax cables for antennas, a category V wire, are developed and routed by the Wire Installation engineers. Coax cables may be routed by themselves or as part of a wire assembly.

Installation diversity

Wiring installation has to accommodate anything from wire assemblies of several thousands of wires, down to a single wire. The single wire can range from 24 gauge to 000 gauge. This translates into a wire 0.045 in. outside diameter (weighing 0.002 lbs/ft) to a wire of 0.660 in. outside diameter (weighing 0.7 lbs/ft). Wire comes in single conductors, twisted cables, shielded and jacketed cables, and coaxial cables. The conductors are usually copper, with the exception of the 24-gauge wire, which is a high-strength alloy. The other exception is the power feeder cables, which are usually aluminum. The wires also come with a variety of jackets or insulation, depending on the temperature range of the environment in which it will be installed. Most of the aircraft wiring is general purpose, 200°C temperature rating. A special “fire wire” is used in the engine pylons, which have to withstand 1500°F.

Power feeders

Power feeders are very rigid and large in diameter (000 gauge). Power feeder installations require special considerations. Since they are a Category V wire requiring a 12″ separation from other wiring, they must be routed separately. And because the power feeders are 3-phase, 400 Hz, parallelism between the phases and the structural ground plane is essential. The three power feeder wires must form a triangle in the wire bundle and be touching each other. The wire bundle for the Auxiliary Power Unit (APU) must include the engine’s starter cable.

Power feeders from the engines must be installed so that for the entire run of the wire, the cable twists no more 60 degrees either direction. The APU’s power feeder can twist no more that a total of 90 degrees in one direction. The design and installation of power feeders must also satisfy the following FARs:

Figure A-4. FAR 25.1353 Electrical equipment and installation

1. (b) Cables must be grouped, routed, and spaced so that damage to essential circuits will be minimized if there are faults in heavy current-carrying cables.
2. (c) Main power cables (including generator cables) in the fuselage must be designed to allow a reasonable degree of deformation and stretching without failure and must-
   1. (1) Be isolated from flammable fluid lines; or
   2. (2) Be shrouded by means of electrically insulated flexible conduit, or equivalent, which is in addition to the normal cable insulation.

Environmental conditions

The environment of the area within which the wiring is installed must be determined at the onset. Items that must be considered include cleaning procedures, humidity, shock, vibration, temperature, and lightning. While many of these topics are addressed in other sections of this chapter, we will look at those that particularly relate to wiring installations.

Lightning protection

The electrical/electronic system wire bundles are vulnerable to the effects of lightning strikes, both in the form of direct effects (structural melt-through, shock waves, etc.), and the indirect effects of transients and coupling. Wire lightning protection in high-exposure areas may include:

• • addition of wire braiding or shielding
• • installation of aluminum shields
• • installation of peripheral grounding brackets

Hazardous wiring installation areas

Wiring installations within the following areas require extra protection.

1. 1. Control cables
   A design objective requires all wiring to be routed a minimum of 1.5 in. from cables.
2. 2. Hot-air ducts
   Any wiring routed within the area of a hot-air duct must be protected by conduit, high-temp sleeving, or by heat deflectors.
3. 3. Fuel and hydraulic piping
   The wiring should be routed a minimum of 6 in. from this type of piping. The primary purpose is to prevent chafing that can result in arcing and eventually provide the ignition source for a fire.
   Incorrect implementation of design requirements for wiring in the vicinity of hydraulic piping can be disastrous. Incorrectly routed and clamped wire use to power the heated galley carts can cause fire if not installed correctly. The installation of the wire may chafe and abrade against a hydraulic line. The electric wire can cause small leaks due to abrasion in the high-pressure hydraulic line. The electric wire may be arced causing a fire fed by the hydraulic fluid.
4. 4. Fuel tank installations
   Current fuel tank design allows open wire runs and open terminations for capacitance-type fuel probes within fuel tanks. The interconnecting fuel probe wiring for this system conducts extremely low signals and, as such, is not considered to be a source of ignition. Wiring within fuel tanks conducting AC and DC power (i.e., fuel pumps and float switches) are required to be routed within rigid metal conduits.
   Development: Several tools are very useful in wiring design and installation.
   Mock-up: During the conceptual stage of design, the question of feasibility usually arises. When other engineering groups are involved, a mock-up (usually of wood) is advisable.
   Development fixture: A development fixture (DF) is a full-scale representation of all, or part of, a section of the aircraft. It is particularly suited for the installation development of such parts as piping, wiring, cables, and blankets.
   The DF is dimensionally accurate at all points that involve the integrity of the developed parts. Some of the parts or components may be simulated to determine proper installation, routing, clearance, or similar criteria pertaining to articles under development.
   Fabrication of a DF is authorized by Work Release Order (WRO), documented by a Tool Order (TO), and constructed as a result of Fabrication Orders (FOs) and Advance Assembly Outlines (AAOs) issued by the Planning group. All phases of DF fabrication and assembly are monitored and approved by Quality Assurance.
   The uses for a DF include:
   • • wire assembly development
   • • system development
   • • assembly sequence visibility
   • • maintainability
   • • installation tooling
   • • Proof for Production (PFP)
   • • Electronic design integration (EDI)

Wire assembly development

All the engineering work for wire assembly development cycle time is wasted through lack of coordination when late design changes are made that impact electrical design engineering.

When a major impact occurs, the development process is repeated at additional cost to the company.

The wire assembly development cycle details the following:

• • The wire installation and wire assembly engineer prepares an Advance Development Engineering Order (ADEO), and then releases drawing(s) with ADEO to the system.
• • Development Planning group issues Design Signature Approval/FAB Outline (DSA/FO) to develop wire assembly.
• • Development Planning group issues ADO Pink Development Master reflecting wire routing.
• • Upon completion of a developed wire assembly, Engineering, Development and Inspection sign and date approval. The wire assembly is then removed from the DF for Jig Board/Tool.
• • Upon completion of development, the ADEO Pink Drawings are returned to Engineering group to prepare the production drawing for release.

Electronic design integration (EDI)

The computer-aided design (CAD) electronic data file (EDF) is a three-dimensional electronically stored math model. The EDF provides all the capabilities necessary for electronic wire development.