FLYING Magazine

Pilots and A&P mechanics can bond over setting spark plug gaps or tossing them if they fail to meet muster. 

Diving deeper into the world of aviation spark plugs, we will pull back the cowling and affix our inspection mirror to discuss the types commonly used in different aircraft models, insights into their maintenance, and recommendations for their replacement. 

Understanding the Basics

At their core, spark plugs are devices that deliver electric current from an ignition system to the combustion chamber of an engine, igniting the compressed fuel/air mixture by an electric spark. Properly functioning spark plugs are essential for smooth engine operation and optimal performance.

Types of Aviation Spark Plugs

“The two major types of electrodes in today’s spark plugs include the dual nickel alloy massive electrode and the single Iridium fine-wire electrode,” saidAlan Woods, sales manager for piston and power at Champion Aerospace in Liberty, South Carolina. “The nickel alloy electrode design allows for a long-lasting spark plug [300 to 500 hours] at an affordable price. The Iridium fine-wire electrode design offers TBO life [2,000 hours plus] but at a higher cost due to the high cost of Iridium [$4,000 per ounce].”

Massive Electrode Spark Plugs

Massive electrode spark plugs are the most commonly used type in general aviation. They feature large electrodes designed for durability and extended use.

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Massive electrode plugs are critical features in terms of durability. They can withstand significant wear and tear, making them ideal for aircraft that undergo frequent and long flights. Massive electrode plugs are also cost-effective. They are generally more affordable than their counterparts, the fine-wire spark plugs. Another attribute is their ease of maintenance. Due to their stout construction, massive electrode plugs are easier to clean and maintain.

There are a few downsides to massive electrode plugs. Over time, massive electrode spark plugs can suffer from performance issues due to electrode wear and increased gap size, leading to less efficient combustion. They are also heavier as the larger electrodes add to the weight, which can be a minor concern in aircraft performance calculations.

Fine-Wire Spark Plugs

Fine-wire spark plugs are designed with thinner electrodes, often made of precious metals such as platinum or Iridium, to provide superior performance and longevity.

The fine-wire plug offers improved ignition over massive electrodes, giving the fine-wire electrodes a more concentrated spark and leading to better combustion and engine performance. They also last longer because they are constructed using durable materials, such as platinum and Iridium, reducing the frequency of replacements. Fine-wire plugs are also lighter than massive electrode plugs, contributing to overall aircraft efficiency.

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These enhanced attributes come with a cost. Aircraft fine-wire spark plugs are substantially more expensive than massive electrode spark plugs. They also require careful handling during maintenance to avoid damaging the fine electrodes.

Choosing the Right Spark Plug 

The choice between massive electrode and fine-wire spark plugs often depends on the specific requirements of your aircraft and your flying activity. Massive electrode spark plugs might be more suitable if you fly frequently and cover long distances due to their durability and cost-effectiveness. Fine-wire spark plugs could be the better choice if you prioritize engine performance and are willing to invest in premium parts due to their enhanced ignition efficiency and longevity.

Fine-wire plugs provide a more efficient burn rate and last longer at a much higher purchase price, according to Vince Bechtel, director of aftermarket sales at Tempest Aero Group, which entered the aviation spark plug market in 2010 by acquiring the Autolite brand. A relatively small niche market, the company represents about 10 to 15 percent of the aviation aftermarket. Turbocharged aircraft flying at higher altitudes favor fine-wire plugs, according to Bechtel.

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Maintenance and Replacement Recommendations

Proper maintenance and timely replacement of spark plugs are crucial to avoid engine misfires and ensure smooth operation. Some tips:

●      Regular inspections: Conduct routine inspections every 100 hours of flight time or as your aircraft’s manufacturer recommends. Check for signs of wear, fouling, or damage. Common issues include carbon buildup, oil fouling, and electrode erosion.

●      Cleaning: Use an approved spark plug cleaner to remove carbon deposits and debris. Be cautious with fine-wire spark plugs to avoid damaging the delicate electrodes.

●      Gap checking: Ensure the spark plug gap meets the manufacturer’s specifications. A correct gap is crucial for optimal spark plug performance. Adjust the gap if necessary using appropriate tools.

●      Replacement: Replace spark plugs at the manufacturer’s recommended intervals or if significant wear or damage is observed during inspections. Always use spark plugs that meet the specifications of your aircraft’s engine model.

“Honestly, the biggest issue I see is over-cleaning,” Bechtel said. “Individuals and shops tend to clean plugs until they look brand new out of the packaging. The only thing this does is wear out your electrodes and insulator faster, preventing you from getting the full life out of a set of plugs.”

Troubleshooting Common Spark Plug Issues

Even with regular maintenance, spark plug issues can occur. Some common problems and their potential causes include:

Engine Misfire

Caused by worn electrodes, incorrect gap, or fouled plugs.

Solution: Inspect, clean, or replace the spark plugs as needed.

Hard Starting

Often due to spark plug fouling or improper gap.

Solution: Check and clean the spark plugs and correct the gap.

Poor Engine Performance

Can result from degraded spark plugs or incorrect heat range.

Solution: Verify that you are using the correct type and heat range of spark plugs for your engine.

The introduction of fired-in suppressor seal technology, or FISS, is a recent advancement in aircraft engine spark plugs.

“This technology eliminates the high-voltage silicon resistor, which is prone to resistance value increases over time,” Woods said. “The FISS technology incorporates fired-in conducting and suppressor glasses that establish the resistance value of the spark plug. This means that the end user has a stable resistance value over the entire life of the spark plug. With the introduction of electronic ignition, spark plug designs will evolve with wider gaps to handle the increased energy being produced.”

Understanding the various types of aviation spark plugs and their benefits and limitations can help you make informed decisions about aircraft maintenance. Whether you choose massive electrode spark plugs for their durability and cost-effectiveness or fine-wire spark plugs for their superior performance and longevity, regular maintenance and timely replacements are critical to engine operation. 

Please consult your aircraft’s technical publications and an A&P mechanic to ensure your spark plugs are in an airworthy condition.

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