There isn't much to basic engine wiring, but it's amazing how many guys have trouble in this area, since kit instructions often leave out these essentials.

There is no more important safety feature on any ultralight than a simple and reliable kill switch to turn off the engine in an emergency or at the end of a flight. Kill switch wiring depends on the type of engine in use.

Older Rotax engines use a point/magneto system for ignition and kill switch wiring is quite simple.

 The two BLACK leads from the engine wiring harness are connected to the primary side of the two spark coils. If these leads are connected, the engine cannot be started or, if running, it will stop. The kill switch has the simple job of making that connection! Although there are several kinds of switches that will do the job, one of the simplest and best approaches is to use a heavy-duty (industrial grade) toggle switch. An SPST switch (single pole/single throw, having just two contacts) will do the job, but SPDT (single pole, double throw, with three contacts as shown above) are more commonly available. Wired as shown above, in the RUN position the leads are open and the engine will operate. In the OFF position the switch shorts the two leads and the engine cannot run.

All current Rotax engines use Ducati (solid-state, breakerless) ignition systems that require a different type of kill switch hook-up:

The Ducati ignitions require an extra set of contacts, so we need a double-throw/double-pole (DPDT) switch. We could get by with an DPST switch, but the double-throw ones are easier to find. The drawing above shows the switch from the back side. The LT label on the yellow and yellow/black wires means these are light gauge wires. There is a heavier set of yellow wires that is used for the power circuit, but the ones we want here are smaller in diameter. The center switch contacts are both connected to ground (the engine case). Wired as shown, when the switch handle is UP the engine will RUN. When the switch handle is cycled to the DOWN position the ignition control leads are both grounded and the engine can neither be started or run.

For system s using an electric starter, there are some fancy ignition switches that include the starter function, ignition module testing, and the kill-switch function. Most ultralights are not anywhere near this complex and the simple toggle switch system shown above is more than adequare.

It is VERY important that you use the highest quality switch you can get for this application. Remember, if the switch fails to close when needed in an emergency or shorts out and kills your engine in-flight, you could have a very bad day.

• A suitable SPDT switch for the magneto/point kill switch is a #275-603A from Radio Shack. I would not use most Radio Shack or hardware store switches in this application, but this one is an exception. We used one on the original Gyrobee and it is still doing fine after over 10 years of service. California Power Systems (CPS) has a heavy-duty SPST toggle switch (#9207) which would also work.
• A DPDT toggle switch for the Ducati ignition can be obtained from a local hardware store if you shop carefully. Look for a solidly-built switch with a firm/solid snap action. If the switch seems wimpy, it probably is!

We need to satisfy two conflicting requirements here - the switch needs to be easy to get to for an emergency engine shut-down, but must never be located where you can accidentally trip it in flight. The Gyrobee's kill switch is located off the left side of the seat, just a bit to the rear.

Orientation of the switch is critical, especially if you have to activate it by feel. I would suggest that the switch be oriented horizontally with the toggle handle toward the front for run and to the rear for stop. You can do it any way you want, but the key is to be able to reach it easily and know, by feel, whether it is set to run or stop.

The generator coils on your Rotax engine can supply anywhere from 30 to 170 watts of 12V power. This can be great for powering instruments, radios, a GPS, etc. as long as it is possible to produce the voltage in a simple but reliable way.

There are several rectifier units designed for use with the Rotax generator output. However, producing a stable 12V with these units is anything but easy. The least expensive units (~$40 or so) must be used with a 12-volt battery in the 16-20 ampere-hour range. In electrical terms, the raw output from the generator coil is extremely "dirty", with the voltage varying from 6 to 60V or more, along with a high level of electrical noise and nasty voltage spikes. The battery serves as a load to stabilize the voltage output and also like a big capacitor to help clean up the output voltage. The reason the battery must be relatively large (expensive and heavy!) is that the raw output from the rectifier would overcharge and destroy a smaller unit. This type of a system is good where you need a good-sized battery for an electric starter.

Other rectifiers are designed to eliminate the battery requirement, but need a big capacitor on the output (filtering and stabilization) along with a light or other resistive load that will draw at least 1 ampere of current. Anything less and the rectifier doesn't regulate!

Both of these approaches are overly complex for an ultralight and I certainly wouldn't feel good about attaching an expensive piece of electronic equipment to either one. A really good alternative is now available from California Power Systems (CPS) in the form of a rectifier/regulator (Stock #9105) that will provide up to 10 amps at 12V directly from the generator coil output. The output is a precisely-regulated 12V with no noise or transients. What's really nice is that this unit uses no additional external components and the price is very reasonable ($60) compared with all the alternatives. It is also very easy to use:

In the case of a Rotax with a point/magneto ignition system, the engine leads are yellow and yellow with a black stripe. There is no polarity to worry about, so the two leads can be connected to the input of the unit in any order. In the case of engines with a Ducati ignition, the two engine generator leads are the heavy-gauge yellow wires. Again, polarity to the input is not an issue.

The negative (-) output should go the ground and the positive (+) output goes to the equipment items requiring power.