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General Aviation Electrical Systems
Jim Davidson, President
Davidson Engineering Resources, Inc., Tucson, AZ
Phone (520) 977-9824
Fax (520) 232-3660

-------------------------------June 2006-------------------------------

Part 1: Introduction and General Stuff to Know

This write-up focuses on single and twin piston Part 23 type aircraft electrical systems. The basic single piston engine electrical system is basically duplicated for the twin piston engine electrical system, however, there are changes.

The basic electrical system can be broken into categories for better understanding of function and inter-system relationships.






In the image above several components are shown: VB is the Aircraft Mainship Battery, Battery Contactor is a device that operates to close the blue contacts (thus connecting the battery with the Battery Bus), and finally a line of circuit breakers. Taking one at a time:

The Main Ship Battery comes in many sizes, type, construction, and a couple of standard voltages. Note in the image above the positive side is indicated by the ( + ) sign and the negative side of the battery is indicated by the ( - ). Grounds in schematics are shown by what appears to be an upside-down triangle fashioned from decreasing length lines. Battery information can be found by performing a web search for such topics as; "Sealed Lead Acid", "Lithium Ion", "Nickel Cadmium". and for companies such as "Gill", "Marathon", "Saft". These sites have considerable information on their battery products and all are great reference sources.

I'll go over Battery Contactor and electrical relays a bit later. Suffice it to say that these devices are based on a flowing current through a series of smaller gauge wire windings. The current through the smaller gauge wire windings, seaking the electrical ground attached to the other side of the switch contact, produces a magnetic field that pulls the iron slug (not really shown in this figure) attached to the spring-loaded "OPEN" contact assembly stem. In this figure the reaction of the contact assebly is to be pulled downward until the two sets of blue contacts close. When the blue contacts close the battery is connected to the Battery Bus (Bus Bar). A bus bar is, in most cases a rectangular rod of copper, fairly thick in all dimensions. All of the electrical systems that rely battery power (as is the case in this figure), are connected to the battery bus through a circuit breaker. Circuit breakers are devices that come in varying current ratings, or amount of current they'll pass before opening up. Most circuit breakers are of the resettable type, that is to say, if the circuit breaker "button" indicates open, the flight crew can (and this is a topic for further discussions) MAY push the circuit reaker button back in to re-set the breaker. More on circuit breakers later. Remember on thing first, the circuit breakers protect the "wiring" between the circuit breaker and the component - they are not to protect the component.

What is NOT shown in this figure is the design that must occur prior to developing the layout and component selection. What is the criticality of the systems and are they adequately protected by labled circuit breakers? Are all non-essential aircraft electrical loads connected to a non-critical "accessory bus" - that can be switched of if necessary? Is the wire correctly sized for the maximum expected current draw for a given cmponent? Are the circuit breakers sized correctly with respect the wire size and current draw? How long are the wire runs and is the wire gauge selected to assure a minimum of voltage drop across the wire run? Correct design is critial for the safe operation of the aircraft....

The above figure can be summarized as having a power source, a method consisting of relays and switches, of connecting and controlling the power source to power consumers via bus bars and wires that are protected by circuit current limiting devices.

Ok, now on to the symbology you'll see on electrical schematics.

In the figure above, note that the bus bar is shown as a rectangle. If the complete bus bar is shown, then it will appear with straight ends. If the bus bar shown is only a portion of the total bus bar, it will be shown as in the figure with ragged ends. this indicates there are additional circuits connected to the bus bar, but not shown.

The battery is shown as a seried of long and short lines that represent the cells in the battery, each cell having an anode (positive terminal) and a cathode (negative terminal). When placed in series, a number of cells make up a battery that is either 12VDC or 24VDC (other voltages are possible, but 12VDC and 24VDC are the one used in aviation applications).

Another symbol you will see alot of is the "resistor" symbol.

In the figure above, you can see that the resistor (load) is indicated by the zig-zag line. The load in this case would be a Direct-Current (DC) type load, like a light bulb, or DC motor, since the battery is a DC battery. The resistor symbol is the same for Alternating-Current (AC). However AC loads are not purely resistive, they have inductance or capacitance effects depending on the type of load.

In addition to the load, the interfacing copper wiring also has finite resistance per foot levels. The selection of the correct diameter (gauge or AWG) for a given wire length, could be a significant contributor to a circuit's voltage drop if not properly analyzed. Copper wire resistance tables can be found in FAA Advisory Circular AC 43.13-1B. In fact, AC 43.13-1B is an excellent source for the engineering and analysis of wire selection for a given aircraft system installation (we'll address the AC in greater detail later, but I'd recommend getting a copy and giving it a good read).