Unique facts about the aviation spark plug
By Harry Fenton and Dick Podiak
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.
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.
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