Sparking Respect: Unique facts about the aviation spark plug

Sept. 1, 1999

Sparking Respect

Unique facts about the aviation spark plug

By Harry Fenton and Dick Podiak

September 1999

Imagine this engineering project: Design an electromechanical device for the aircraft engine that will spark between 90 to 150 thousand times per hour and average between 500,000 to 750,000 spark events at 6,000 to 20,000 volts during a routine cross county flight. The device will mount into the combustion chamber and extend into the ambient air of the engine compartment, resulting in a temperature differential over the two-inch length of the device that may be as high as 1,500 degrees on the engine side and as low as 180 degrees on the engine compartment side. The end of the device that will install into the combustion chamber must survive a hellish environment of flame, heat, pressure and corrosive chemicals. The device must be long-lived as it is expected to operate for hundreds of hours spanning several years, even decades of calendar time, and must fire reliably, every time for about 53 million times during the device's service life. Most challenging of all, the device must demonstrate flawless reliability with minimal maintenance.

Sound impossible? The fact is, this device has already been designed and has been part of all internal combustion engines for almost 150 years — it is called the spark plug. Yet, despite all of the hidden science inherent in the spark plug, it is among the least glamorous parts of the engine, relegated to obscurity by virtue of its near perfect reliability.

Aviation plug development
Although automotive and aviation spark plug technology were developed concurrently, the paths of evolution for the two products diverged, resulting in two significantly different products. As with most aviation components, the features of the aviation spark plug are a result of the specialized needs of the aviation application, primarily reliability and redundancy, so, as they say, form follows function.

The design of the aviation plug shell is a feature that immediately distinguishes itself from its automotive counterpart. The aviation plug shell is typically a large, metallic barrel, threaded on one end for mounting into the engine and threaded at the other to accept the unique mounting hardware of the aviation ignition harness. The robust design is no accident as it is intended to protect the insulator against breakage, seal the connector well and offer repeated serviceability.

World War I spark plug — 85 years old but strikingly similar to modern plugs. Note the stack-up of mica insulators in upper area of plug.

The basic plug shell designs are usually referred to in a number of different ways, depending upon design and application: Big barrel, small barrel, short reach and long reach. Small and Big barrel refers to the diameter and thread pitch of the ignition harness connecting hardware as either a 5/8-24 connection (small) or a 3/4-20 connection (big). Automotive ignition harnesses use a push-on connector. This design would not work in aviation as it is prone to being blown off by the cooling air rushing over the engine, or may even pop off as air pressure trapped in the plug boot becomes greater than the ambient air pressure as the aircraft ascends in altitude. As reliability and safety are the driving forces in aviation, aircraft ignition harness connections are literally screwed onto the top of the spark plug to prevent them from loosening.

Another benefit of the screw-on connection is environmental sealing of the inner spark plug well. Because the aircraft engine uses ambient ram air for cooling, moisture contained within the ambient air must be sealed out of the spark plug to prevent electrical shorting of the ignition wire conductor. The 3/4-20 connector of the Slick ignition harness, for example, is specifically designed to provide an airtight, waterproof seal to keep moisture out and trap ambient air within the plug to limit the chances of misfire at high altitudes.

Finally, the metal to metal connection between the ignition harness shielding, the ignition harness connector, and the spark plug provides a reliable electrical ground to suppress electromagnetic interference (EMI) generated by the ignition spark at the firing tip of the spark plug. Suppressing EMI is important. If unchecked, it can cause interference or "noise" in the aircraft's communication and navigation radios.

As a rule of thumb, the 5/8-24 connector is used on most normally aspirated engines that operate at less than 10,000 feet of altitude. The 3/4-20 harness, commonly referred to as a high altitude or all-weather harness due to its superior sealing characteristics, is used on virtually all turbocharged engines or normally aspirated engines that routinely operate at altitudes of 10,000 feet or higher. The 5/8-24 harness nut is installed using a 3/4-in. wrench, and the 3/4-20 harness nut is installed with a 7/8-in. wrench.

Another signature characteristic of aviation spark plugs is the electrode configuration. For the most part, automotive spark plugs use a single electrode design, while most aviation spark plugs favor a more complex, multi-electrode configuration.

C A. Antique four-prong electrode B. Antique three-prong electrode C. Electrode view of WWI era plug. Note lack of adjustment for gap wear.

Contemporary electrode design features either a two-prong massive electrode configuration or a single, fine wire iridium electrode design. The fine wire, iridium plugs use a single electrode for a number of reasons. First, iridium is an expensive material, which dictates conservative use of the material. Second, the durability and resistance to lead corrosion of the iridium is excellent, which offsets the need for the second electrode to guard against fouling and to promote long plug life.

For the most part, aviation spark plugs feature an 18mm mounting thread diameter, although some vintage and low volume production engines can use a 14mm plug. The base thread size is a result of the need to have a large surface area at the electrode end to achieve scavenging, provide adequate surface area for heat transfer to cool the plug, and to accommodate the multi-electrode design.

The length of the mounting threads that engage the plug into the cylinder head define the reach of the plug. The reach of the plug is carefully engineered to optimally place the electrodes in the combustion chamber, to provide adequate cooling and scavenging. Dimensionally, an 18mm aviation spark plug with 13/16-in. of mounting thread is defined as a long reach plug, and an 18mm plug with 3/4-in. of mounting thread is considered a short reach.

However, some plugs suffer an identity crisis. For instance, a plug common to Lycoming four-cylinder engines features an extended nose insulator with long electrodes designed to resist fouling and improve the overall lead scavenging qualities of the plug while having the 3/4-in. or short reach threads. This design places the discharge end of the plug closer to the flame front to improve the anti-fouling and lead scavenging qualities of the plug. This extended insulator plug is sometimes incorrectly referred to as long reach, when in fact, the mounting thread is short reach.

The spark plug electrode end faces the harshest operating conditions in the engine. Each spark event displaces a minute amount of electrode material, while the combustion process generates a massive release of heat in excess of 3,000 F, and releases a cloud of harsh chemical byproducts such as carbon and lead salt deposits. Once the combustion process has been completed, the tip of the plug must cool, so that the entire process can continue. The discharge, chemical exposure, and cooling cycle all occur within an instant — 20 to 30 times each second — and each event has a degrading effect on the spark plug electrode. Accordingly, this hostile environment drives the design of the electrode and insulator.

The center electrode material typically is a composite of copper and nickel alloy materials, which provides excellent electrical conductivity, as well as good heat transfer qualities and resistance to wear. The side, or ground, electrodes are usually a high percentage nickel l alloy material.

Typical spark plug gap for the aviation plug is .016-in to .021-in., as opposed to as much as .050-in. for automotive type plugs. Why the big difference? The tighter gap of the aviation spark plug requires less voltage to arc across the gap, reducing the chance of misfire, and lower voltage means less electrode wear due to material transfer loss during the arc event. If operating stress is reduced, reliability is improved.

The heart of the spark plug is the aluminum oxide, ceramic insulator material that extends from the firing end of the plug in the combustion chamber up to the top of the plug where the ignition harness is connected. The composition, glazing, and manufacturing control of the insulator are critical in determining the reliability of the spark plug.

The percentage of aluminum oxide in the insulator affects the mechanical strength of the material, and is a determining factor in the ability of the insulator to accept and reject heat from the combustion process. If the composition is incorrect, the ceramic material may crack, resulting in damage to engine components. A poor quality ceramic insulator may cause the electrodes to remain at too high of a temperature, which can result in pre-igntion of the fuel/air mixture. The glaze applied to the firing tip of the insulator ceramic improves the mechanical strength of the ceramic surface. The glaze applied to the ceramic at the ignition lead end of the spark plug provides a smooth surface for easy cleaning.

Inside the insulator and inline with the electrode is a resistor. This resistor serves to limit the peak electrical discharge current when the plug fires, thereby reducing electrode wear. The resistor of the plug can be manufactured in a number of different ways: either as a multi-part assembly or in a monolithic fashion — "doped" into the glass seal. The monolithic resistor is a more modern technology and offers improved reliability, longer life, and is more economical to manufacture.

Sparking Respect

Unique facts about the aviation spark plug

By Harry Fenton and Dick Podiak

September 1999

Selecting the proper plug
Plug selection is typically straightforward due to extensive documentation and application information provided by the spark plug and engine manufacturers. But, for any given application, there may be more than one spark plug approved to allow for alternatives to optimize performance.

3/4-20 vs 5/8-24 spark plugs. Note the difference in the thread and barrel sizes. Short reach vs. long reach plug

Some plugs offer identical fits, but different heat ratings, so don't be fooled into thinking that "as long as it fits, it's ok." A spark plug with a heat rating incorrect for a particular engine can cause significant, if not catastrophic damage to an engine. Always use the spark plugs found in the spark plug or engine manufacturer's FAA-PMA approved application listing.

Spark plug manufacturers use a part numbering system that describes the qualities of the plug. For instance, within the spark plug part number, there is a code for the heat rating of the plug. Intrinsically, the value of the heat rating numbers reads like a thermometer. High numbers denote "hot" plugs and low numbers denote "cold" plugs. A hot plug features a longer insulator nose, and transfers heat more slowly back to the engine, therefore remaining physically hotter. Inversely, a colder plug has a shorter insulator nose and transfers heat more rapidly, operating at a lower overall temperature than a hotter plug. As a rule, hot plugs are used in cold running engines and cold plugs are used in higher performance running engines.

Use a plug tray to hold, organize and as an aid in alternating/rotating plugs during installation

The differences in heat ratings offer distinct advantages in terms of anti-fouling and lead scavenging. Generally speaking, the plug needs to be hot enough to burn off carbon deposits from the tip, but not so hot that the plug tip retains enough heat to initiate pre-ignition. If the plug is too cold, carbon deposits will not be burned off the tip. If the plug is too hot, lead salts that accumulate on the insulator nose will cause a lead fouling.

ead 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.

Maintenance required
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.

A spark plug gauge, such as Unison's T-242 makes gap checking convenient.

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.

Reinstall the plugs using a new seat gasket and coat the threads of the plug using a special purpose anti-seize compound. Do not use motor oil or other non-approved lubricants because they can coke up under heat, causing the plug to be difficult to remove from the cylinder. Always torque the plug to the engine manufacturer's specifications. Lycoming recommends 35 ft/lb of torque and Continental specifies 25-30 ft/lb.

Undoubtedly, there is more development ahead for the spark plug, as the internal combustion piston engine remains the powerplant of choice for the conceivable future.