Mechanic Support

A distributor is a necessary part of the ignition system because it performs the function of switching the magneto output in sequence to the various cylinders which are to be fired.

When an airplane equipped with such a distributor climbs to a high altitude, trouble begins to occur due to the air in the distributor housing becoming thinner. Air is used as an insulator keeping the high voltage from flashing over to the grounded housing instead of traveling down the spark plug wire and firing the spark plug.

One solution is to make the magneto distributor larger so that the length of the flashover path is lengthened. Another solution is to pressurize the magneto. Both techniques use Paschen's Law where the voltage required to bridge a gap depends on gap distance and gas pressure; V=f(D*P).

Making the magneto distributor larger adds weight, cost, and requires more room to mount the magneto onto the engine. The trick is to make the distributor as large as possible, while keeping the overall magneto size as small as possible.

Pressurizing the magneto is the other method of preventing flashover. However, the magneto cannot be completely sealed because the high voltage ionizes the air and makes it conductive. Fresh air is necessary to prevent flashover. The trick here is to pressurize the magneto but also provide a orifice to allow some of the air to excape. Slick factory pressurized magnetos have an established air flow at pressure requirement. Aftermarket pressurization schemes often do not.

Another potential problem with magneto pressurizing from the engine's upper deck is that any water, moisture, oil, and dirt in the engine's induction system has a path into the magneto. Pressurized magnetos require more frequent maintenance and internal cleaning.

The carbon brush transfers the high voltage from the magneto coil's secondary circuit to the distributor. As the brush wears small particles of carbon collect on the magneto surfaces. Routine magneto maintenance should include cleaning this carbon dust before it forms a conductive path inside the magneto. A routine magneto check might not detect arching from carbon dust.

The new style Slick distributor block uses ribs to contain the carbon dust and increase the arc distance to ground. The Slick magneto system intended for high altitude operation should be updated to the newer distributor block. The change was made in late 2009.

One common repair for a rough running engine is to replace the magneto points and capacitor. I know from our sales of magneto parts that a very low percentage of distributor blocks are sold along with the points and capacitor. If you believe that the rough running is caused by the magneto, it may be caused by carbon arc tracking between the distributor block electrodes and have nothing to do with the condition of the points or capacitor. Carbon arc tracking creates a high-voltage short circuit causing the wrong, or multiple spark plugs to fire. In the late stages of tracking, burnt carbon tracks appear going from one terminal to another; at earlier stages, carbon tracking is not visible so the block looks OK.

Inside the magneto, conducting carbon dust from the distributor gear brush combines with moisture (or oil from a leaking oil seal) to form a wet conducting contaminate (carbon conducts electricity). This conducting film, along with the large electrical potential difference between distributor block electrodes, causes a current of a few milliamps to flow through the moist layer causing slight heating. This heating leads to the formation of an occasional very narrow "dry gap" in the conducting film. Concentrating most of the voltage across the tiny dry gap resulting in tiny arcs. Such dry gaps and arcs occurs randomly over the surface in an effect known as scintillation (the original Bendix magnetos were called "Bendix Scintilla"). According to research done by The Electrical Distribution Industry on Carbon Arc.

Tracking:
These tiny arcs have a temperature around 1000 degrees Celsius and so cause intense heating of the insulation surface on a micro area basis sufficient to pyrolyse (chemically decompose by the action of heat) any organic polymer. Each tiny arc deposits a small spot of carbon. Over time a complete "carbon track" path forms to enable flashover. By the time you can see the "carbon track" your engine has been firing the wrong plugs at the wrong time; a situation that could result in preignition and severe engine damage. The best preventative is to make sure you have the latest design distributor block, replace the magneto distributor block regardless of its appearance, and perform regular magneto preventative maintenance.

Terminal sleeves should be handled only with clean, dry hands. Before installation, wipe off the connector with a clean, lint-free cloth moistened in methylethylketone (MEK), acetone, wood alcohol, naptha. Make certain that the inside of the spark plug shielding barrel is clean and dry. Then, without touching the connector or spring with the fingers, insert the assembly in a straight line with the spark plug. Screw the connector nut into place finger tight then tighten an additional 1/8 turn with the proper wrench. Damaged threads or cracked shielding barrels may result if the connector nuts are tightened excessively. Avoid excessive side load while tightening.

Three spark plug considerations for high altitude operation:
1. Electrode gap must not be allowed to get too wide. Fine wire plugs by wearing less, maintain the gap better.

2. The spark plug electrode well (shown here to the left) must be clean and the contact spring in good condition.

3. The Slick/Champion "3/4" inch "All Weather" harness with the larger barrel spark plug pressurizes the electrode well and keeps water out which gives it superior high altitude capabilities.

Aircraft engines can use either "5/8" or "3/4" spark plugs. In my opinion, spark plugs with 5/8 inch threaded terminals should have been thrown into the dust-bin of history 30 years ago. The "3/4" also called "all weather" are designed to prevent moisture and heavy rain from entering the spark plug, and advantage that comes with no extra cost. The size of the nuts on the ignition harness must match the spark plugs so the best time to change is at engine overhaul. Slap all those engine overhaul shops who don't upgrade the ignition system when there is no extra cost!

The Slick 3/4 harness end shown here has another design feature that comes out of their high-altitude government sponsered research, the red insulator is cone shapped. and just not straight. When you push the cone shaped insulator into the spark plug terminal air cannot escape and is pressurized and locked into the terminal. No matter how high you fly, the air pressure in the spark plug terminal is under pressure. Using Paschen's Law where the voltage required to bridge a gap depends on gap distance and gas pressure; V=f(D*P), the pressurized air offers superior flash-over protection at high altitude when compared with other ignition harness designs.