Understanding Aircraft Electrical Systems with ordinary objects.

I was talking with a couple of students at a local flight school the other day. It reminded me of my own time trying to figure out the 727 electrical system as well as a brief time trying to teach aircraft systems at Florida Institute of Technology as a substitute teacher for a friend who was teaching advanced aircraft systems.

Electrical systems can quickly get confusing. There is a point that I reach a level of fog when it gets to apparent power, imaginary power, phase angles, etc. However, I think from a pilot's point of view, the basics are pretty easy to understand if you can find some useful analogies. The problem with trying to figure out an electrical system is that I, like I imagine most pilots to be, think in mechanical terms. There is nothing touchable or seeable about electricity. However, I find some analogies helpful.

With a couple of trips to a local Sports Town, Office Depot, and Home Depot, you can find all of the elements of an aircraft electrical system.

Bus

The above is a an AC bus for 110 Volt, 60 cycle, household loads. This really isn't that different from an aircraft bus in principle. It is a way to plug multiple components into one power source. Notice there is a switch to disconnect the entire bus as well as protection, in this case a surge protector or circuit breaker. You could connect this bus directly to the wall (think of the wall socket as a generator) or plug it into another bus. By plugging one bus into another, you can organize the loads. For example, you can plug all of the galleys into a bus and then plug the galley bus into the main bus. This allows you to separate and control all of the galley power at once.

TR (Transformer Rectifier)

You probably have more TR's around the house than you realize. Here is a picture of one:It is nothing more than an AC adapter. They are used for laptops, cell phones, radios, etc. The transformer changes the voltage from house current (110 volts in the U.S.) to whatever voltage is required for the device, usually something like 6, 9, or 12 volts. The rectifier changes AC to DC. This adapter transforms and rectifies, hence the name- Transformer Rectifier.

Most large aircraft have AC main electrical systems and the TR is used to power DC components.

Inverter

This is a picture of inverter which plugs into the cigarette lighter of your car. It takes the 12 Volt DC and converts it to 110 Volt, 60 cycle AC power.

A common use of inverters for backup power. In the event of losing all generators, the battery can be used to power critical AC electrical components using an inverter.

A very practical example of this is for backup power for computers, which integrates several of the components that we have discussed into one box. These are available at a variety of office and electronic stores. When plugged into the wall and the plug is powered, the TR takes some of the AC power and charges the battery. If the wall plug loses power, the system switches power sources from the AC wall plug to the battery through an inverter.

AC vs. DC electricity

AC power is alternating current verses DC which is direct current. It is easy to think of DC in hydraulic terms like a flow through a hose. AC is a little harder to imagine.

I find AC power easier to understand if I use the analogy of a leg peddling a bicycle or a piston pushing on a crank shaft. The constant turning of the shaft, the bicycle chain, or the tire is a DC type of motion. The back and forth of the leg or piston is the AC motion.

Many aircraft, such as the 727 and L-1011 have systems where the AC generators are normally synched together.

You can combine DC power sources with little problem. For example, remember the Flintstone's cartoon car. Everybody put their feet through the floor and started running to push the car. If you were riding in the car, you could simply drop your legs and start running. This is like DC power. You don't have to worry about synchronizing with the other power sources. Your all going the same direction. Other analogies which come to mind are a bunch of people tugging on a rope. If you can find a spot, grab and pull.

If AC power sources are to be combined, they have to be synchronized. If you suddenly decide to add your power, it is important that you synchronize when you bring your AC generator on line. Let's say that you are riding on the back of a tandem bicycle. If you suddenly start with a down stroke on the pedal as it comes up, then something bad is going to happen to your leg, the other generator (the other guy's leg), or the machinery of the cranks and chain.

A similar analogy would be one of those paddle boats with pedals on a shaft on either side of a crank with the paddle in the middle. This is an AC example similar to the tandem bicycle.

Many aircraft have AC systems where the generators are split. Think of a tandem kayak in which assuming that the cockpits are far enough apart so the paddles don't clank, one kayaker does not have to paddle in synch with the other kayaker. This is like a split power source. I can't think of a good analogy, but many aircraft such as a 737 have AC systems where the AC power is not connected together. If a generator on one side fails, then that side switches over to get power from the operating generator. However, at no time on these split bus aircraft are two sources of AC power providing power together. Switching, but no synchronizing in this case.

CSD (Constant Speed Drive)

All of the aircraft that I have flown with AC generators have had a CSD to provide power from the engine to the generator. The engine speed varies, but the AC power needs to be of constant frequency. Along the bicycle analogy, frequency is just the number of times per unit of time that your legs go up and down. This is related to the RPM of the crankshaft or engine in this case. If the RPM of the generator varies, then the frequency will vary. The CSD's function is to keep the rotational speed of the generator constant.

The CSD is a like the transmission of a top of the line lawn tractor that has a continuously variable ratio transmission. It uses hydraulics to change the gearing between the engine and the output. In the case of the of the lawn tractor case is the wheels and blades. In the case of the airplane, the output goes to the generator and there is an automatic controller to keep the speed of the output shaft constant, but the idea is the same.

Disclaimers and copyright

If you find any information that is wrong, please drop me a note at handheldgps@hotmail.com.