Aircraft Wiring Techniques
Originally published December 1997 and January 1998
The program on Aircraft Wiring was presented by the one and only, and our very own, Charlie Wagner. Although Charlie's RV-6A is his first total aircraft wiring project, Charlie is no amateur. Since age 13 he has rewired all of the automobiles he has ever owned; and for a great part of his working life as an electro-mechanical engineer at NASA-Dryden, was involved in the development of aircraft fly-by-wire controls.
Since this program broke all records for duration and complexity, space permits only covering the high points of Charlie's presentation. For more detailed information, or for tips and suggestions concerning your own wiring project, contact Charlie directly.
Charlie's objective was to design an electrical system for his aircraft to support a starter, alternator, and night VFR capability. Several good ideas put forth include:
- Comm jacks BEHIND the seat to avoid the tangle of wires in the occupants' laps.
- Push-to-talk switches on the control sticks so that communication and avigation may occur simultaneously.
- Panel mount RCA-type jacks to provide ship's power to ANR headsets and hand-held avionics.
- Jump-start connector in the panel to give the PIC complete control over the jump-start procedure.
- Automatic avionics protection circuit to ensure avionics shutdown during engine start.
- Battery, voltage regulator, and over-voltage protection circuitry mounted inside the cockpit to avoid the harsh environment forward of the firewall.
- "Idiot Lights" , including low oil pressure, over-voltage protection engaged, and engine overspeed.
Charlie next expounded upon several rules to keep in mind while laying out an aircraft electrical system. Following these rules to the extent practical will help to avoid noise and interference in the system.
- Do not create magnetic fields to affect the compass.
- Avoid ground current in the airframe.
- Keep wiring for noisy stuff away from wiring for sensitive stuff to the extent practical.
- Do not create power or ground noise in sensitive circuits.
In general, items 1, 2, and 4 can be accomplished by providing a return wire in close proximity to each supply wire and by isolating electrical devices from the airframe where possible. Especially make sure that comm jacks are isolated from the airframe and are grounded only at their respective device (radio, intercom, etc).
The "noisy stuff" referred to in item 3 includes ignition harness, P-leads, strobe power and high voltage cables, relay coils, and the alternator output. "Sensitive devices" include radios, EGT/CHT thermocouples, over-voltage protection circuit, and to some extent, the voltmeter.
Charlie also emphasized the importance of planning and designing the electrical system before jumping in and getting started with the actual wiring. The steps he uses are a follows:
- List all devices in the system.
- Make a map showing the approximate location of each device or group of devices in the airplane.
- Sketch lines showing the interconnects. Group lines together where it makes sense.
- Refine the interconnecting lines to make sure that the interference rules are being followed.
- Sketch in the desired disconnect points. Done properly, this step allows most of the wiring work to be accomplished on the bench instead of while standing on your head in a small airplane. It also makes future service easier, enabling devices to be unplugged for maintenance instead of removing individual wired connections.
- Make a list of the connectors and the required pins on each one. Assign names to each wire.
- Draw schematic diagrams of each subassembly. Include cables showing connector interfaces.
- Determine how much current will go through each device (switch, circuit breaker, connector, relay, contactor, resistor, diode, etc.) if it is significant. If manufacturers specifications for current draw are not available for a particular load, hook it up on the bench and measure it. From this information, select wire sizes and generate a fabrication drawing for each cable.
- Select Supporting Devices:
- Basic Style: pushbutton, rocker, toggle, etc.
- Current Rating: Take into account surge current at turn-on, and kickback at turn-off. Beware of high current capacity switches in low current applications. The contact material may not conduct properly below a threshold current.
- Termination type: Screw, push-on, solder.
- Physical Size
- Current rating/impedance: Compute voltage drop.
- Basic Style: Rocker, toggle, push/pull, pop-out.
- Termination type: Screw, push-on, solder.
- Wire gauge.
- Current and voltage rating.
- Ease of termination ( there may be several hundred to do).
- Contact current rating (same as switches).
- Coil current requirements.
- Physical packaging and mounting considerations
- Mounting orientation for minimum sensitivity to G-loading
- Termination style
Resistive Devices (dropping resistors, potentiometers)
- Power ratings:
P = V*I = I2*R = V2/R
P = Power (watts)
I = Current (amperes)
V = Voltage (volts)
R = Resistance (ohms)
- Current Rating
- Mounting considerations (may require circuit board)
- Current rating - surge and continuous
- Voltage rating
- Speed rating (kickback suppression diodes need to be fast)
- Mounting considerations (most require circuit board)
- Insulation: breakdown voltage, temperature, fuel resistance, availability of colors.
- Stranding: Finer stranding equals greater flexibility.
- Tin Coating: Lack of coating is not a problem with proper installation.
- Gauge (size) selection:
22AWG - gauges, senders, other very low power applications.
Note: In some applications (especially those under the cowling), mechanical strength for vibration survival may dictate a larger wire size.
18 AWG - lights, radios, etc
14 AWG - Alternator output, ground strap
4 AWG - Battery cables, start feed, etc.
2 AWG or larger if battery is located more than a few feet from the engine
- Update schematics with data such as wire gauge, color, connector types, breaker ratings, etc.
- Obtain parts you don't have.
- Build Subassemblies.
Total power consumptions for Charlie's RV-6A electrical system:
Day VFR - 11.4 A
Night VFR - 21.9 A
Night VFR while transmitting - 34.4 A
(All values assume an alternator field draw of 4 A.)
(Copies of Charlie's slides are available in the Chapter Library--see Russ Erb)
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The viewpoints expressed are not necessarily those of Chapter 1000 or the
Experimental Aircraft Association.
Revised -- 25 July 1998