Lead fouling is a problem unique to aircraft engines and probably the most prevalent operating challenge for spark plugs. Tetraethyl lead is a material added to aviation fuel to raise the octane rating of the fuel. A side benefit of tetraethyl lead is that it promotes valve life. When aviation fuel is burned, the lead undergoes a chemical reaction and is transformed into a salt. In a perfect world, the lead deposits are scavenged in the maelstrom of gasses that leave the combustion chamber on the exhaust stroke of the combustion cycle.
In the real world, the lead salt deposits can collect on the insulator and cause the plug to malfunction. In some extreme cases, lead deposits have been found to completely cover the spark plug electrodes, preventing ignition from taking place. In addition, the lead salts can be extremely corrosive to the plug electrode material, which shortens the overall service life of the plug. Lead deposits take a number of chemical forms, both conductive and non-conductive in nature. The most common form of lead fouling has a metallic appearance, and is manifested as deposits in the well of the spark plug and on the surface of the insulator, itself.
One method to combat the effects of lead fouling is the use of extended insulator plugs. In these plugs, the electrode extends deeper into the combustion chamber, exposing the insulator to the swirling combustion gasses with the intent to burn or whisk away the lead salt particles. The extended nose also benefits from greater exposure to the cooling blast of incoming air on the intake stroke. As a word of caution, be careful about the arbitrary use of extended nose plugs in an engine subject to lead fouling.
Sometimes the upper limit of piston travel may be great enough that the piston crown can contact the plug electrodes, resulting in severe engine damage. Always check the engine or spark plug manufacturer's recommendations before using different spark plugs in an engine.
Despite the incredible reliability of spark plugs, periodic maintenance is required as a preventative measure. Spark plugs also serve as an important diagnostic tool to help gauge engine condition and performance.
As with any maintenance task, having the proper tools on hand always makes the job easier. The most useful special purpose tools will be a deep well spark plug socket, spark plug tray, electrode erosion gauge, and abrasive cleaner and gapping tool. These special purpose tools can easily be obtained from your aircraft parts distributor.
The spark plug tray, such as the one manufactured by Unison Industries illustrated in the accompanying photos, not only provides a convenient means to hold the spark plugs, but can prevent inadvertent damage. Too often, mechanics remove a spark plug and place it on the engine or on their toolbox, only to have the plug roll off and fall on the shop floor. If a spark plug is dropped, it must be replaced as unseen damage can occur to the insulator.
Another use of the tray is to keep track of the order in which the spark plugs were removed from the engine. Each plug is a window into the cylinder in which it is installed and can be used as a diagnostic tool. Also, due to the reversing polarity of many types of aircraft magnetos, the electrodes of the spark plug may wear unevenly, so spark plugs should be reinstalled in the opposite position (top or bottom) in the same cylinder.
The condition of the spark plug electrode end provides important clues about engine operation. The mechanic's guide, FAA Advisory Circular AC43.13-1A, provides a detailed description of spark plug analysis. Use a spark abrasive grit cleaner to gently clean the deposits from plug electrodes, but be careful not to use excessive air pressure or expose the electrodes to the grit blast any longer than necessary to remove the deposits. Excessive exposure to the abrasive blast will erode the electrodes and reduce spark plug service life.
Keep the gap of the spark plug within specifications. If the gap is too wide, components of the ignition system can be over-stressed due to higher voltage demand. Magneto or ignition harness service life may also be shortened if the plug gap is incorrect. Spark plug go/no go erosion gauges should be used to gauge plug wear as opposed to an "eyeball" interpretation of wear. The gauge has a calibrated hole that is placed over the spark plug electrode. If the electrode enters the hole, then scrap the plug.
Keeping the Fire Going Some spark plug maintenance tips By Joe Escobar October 2001 Spark plugs are an integral part of every reciprocating engine. Many of us know enough about them...
Turbine Ignition Maintenance By Jennifer Sparks and Brad Mottier November 1999 Most aircraft engines rely on an electrical ignition system to create a spark, which in turn initiates...
The Pratt & Whitney PT6 is a widely used turbine engine in the aviation market.