How Do Spark Plugs Work in Aviation?
Spark plugs are used in a variety of vehicles to deliver electric current to the combustion chamber to ignite the fuel/air mixture during the appropriate phase of the engine cycle. While most recognize spark plugs through their association with automobiles, these components play a crucial role in many aircraft to facilitate continuous engine operation. When it comes to aviation, the spark plug is exposed to unique challenges and is therefore explicitly designed to perform its task. In this blog, we will discuss everything you need to know about spark plugs in aircraft, including their design, operation, and maintenance.
Unlike automotive spark plugs, which are usually small and less complex, aircraft spark plugs exist as large barrels that must be tightly mounted to the engine and a particular housing. While there is slight variability between the different types of spark plugs, they all contain an electrode, insulator, and outer shell. The outer shell makes up the visible portion of the mentioned barrel and is commonly made from durable, corrosion-resistant machined steel. Although the threading on the outer shell is designed with narrow tolerances, there is still a small risk that some pressure might escape. To combat this, an insulating layer is installed between the electrode and outer shell. This layer provides heat protection and electrical insulation while also preventing damaging EMI from reaching other areas of the plane.
The primary difference between the two most commonly used spark plugs in aviation is the electrode design. Spark plugs actually contain two electrodes, one of which is ground and the other being a core. It is the gap between these two electrodes that determines the voltage created, as well as the component's lifespan. As the electrodes send and receive sparks during routine operation, a small amount of metal from each is ionized during each cycle. Eventually, the electrodes reach a point where they can no longer communicate effectively, leading to their replacement. The most common spark plug is the "massive electrode," containing two large nickel-alloy electrodes. This affordable option has a lifespan of 300-500 hours, after which time it must be replaced during a maintenance window. This contrasts with iridium spark plugs, which use an incredibly durable wire to provide up to 1500 hours of service life.
While spark plugs are generally reliable and require little maintenance, they should be regularly checked to ensure their proper function and condition. When inspecting a spark plug, one should use a specially made tray that prevents the cylindrical component from rolling off. If the device were to fall, the outer shell would likely prevent severe damage, but the insulation might easily come undone, leading to suboptimal protection. Once removed, visible debris should be removed from the electrodes with an abrasive cleaner. However, the user should be careful not to damage the actual electrodes, as this will lead to a decreased lifespan. It is also necessary to check the distance between the electrodes with a gauge and compare it to the associated spark plug manual, which quantifies the allowable distance between the two.
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