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Checking and maintaining spark plugs is an essential part of aircraft engine maintenance.
We always start by securing the engine and turning off the magnetos to ensure safety.
Each cylinder has two spark plugs—one on top and one on the bottom—with their own connecting wires clearly marked to identify their position (like “B2” for bottom cylinder number two).
When inspecting spark plugs, we look for undamaged threads, complete electrodes, and crack-free insulators.
The cleaning process involves removing old gaskets, using a vibrating tool to clear deposits, and cleaning the threads with a wire wheel.
We check the gap between electrodes using a special tool and test each plug’s spark capability before reinstalling them with new copper gaskets and the proper torque setting.
Key Takeaways
- Regular inspection ensures spark plugs are free from damage and carbon deposits for proper engine performance.
- Proper cleaning, gapping, and testing are crucial steps before reinstalling spark plugs in aircraft engines.
- Always replace the copper gaskets and apply thread lubricant correctly to prevent leakage and hot spots.
Safety Precautions
When working with aircraft spark plugs, we must follow several important safety steps.
First, always make sure the engine is secured and the magnetos are turned off before starting any work. This prevents the propeller from spinning unexpectedly, which could cause serious injury.
Never drop spark plugs on the floor! The ceramic insulator inside can break, even if the damage isn’t visible. A damaged spark plug is unsafe and must be discarded.
Always use the proper tools for the job:
- Special spark plug socket
- Calibrated torque wrench
- Proper gapping tool
- Clean testing equipment
When testing spark plugs, be cautious with the high voltage equipment and compressed air connections. Always verify the spark visually both with and without pressure to ensure proper function before reinstallation.
Taking Out Spark Plugs
Finding Your Spark Plugs
Identifying spark plugs on an aircraft engine is pretty simple once you know what to look for.
Each cylinder has two spark plugs – one on top and one on the bottom. Look for markings on the spark plug leads that tell you which cylinder and position they connect to. For example, “B2” means “bottom spark plug for cylinder number two.” These markings help us keep track of where each plug belongs when we’re working with multiple spark plugs.
Using the Right Tools for Removal
We always use a special socket designed specifically for spark plugs. This special tool helps us remove the plugs without causing damage to the surrounding parts or to the plug itself.
Here’s our basic removal process:
- Select the correct spark plug socket
- Carefully loosen the spark plug
- Remove it completely by hand
- Place the removed plug in a marked tray to keep track of its position
Checking Condition of Aircraft Spark Plugs
When examining spark plugs, we first need to evaluate their physical state. After removing the spark plug, we check if the threads are intact and undamaged. We look for any chipped pieces that might have fallen into the cylinder.
Next, we inspect the electrodes to ensure they’re complete and undamaged. Then, we need to check the ceramic insulator inside the spark plug for any cracks or chips.
Cleaning and Testing Airplane Spark Plugs
Using a Vibration Cleaner
We use a special spark plug buzzer tool that vibrates to clean the inside of the spark plug.
This tool helps remove carbon deposits and lead buildup that comes from aviation fuel. Simply insert the tool into the spark plug and let it vibrate away the debris. After cleaning, check for any visible damage to the spark plug.
Cleaning the Threads
A wire wheel works great for cleaning the threads of spark plugs. This step removes any buildup or residue on the threads, ensuring a proper fit when reinstalling.
Clean threads also help prevent leaks and make torquing the plug to the right specification easier.
Checking the Gap
The gap between spark plug electrodes must meet manufacturer specifications.
For our plugs, we aim for 16,000 to 21,000 of an inch.
To check this:
- Use a gap tool
- Place the tool between the electrodes
- Gently squeeze the electrodes together if needed
- Test that the tool fits snugly
Be careful not to drop the spark plug during this process, as the ceramic insulator inside can crack, making the plug unusable.
Sandblasting the Inside
We use a sandblasting cleaner to remove stubborn deposits from inside the spark plug.
This process helps restore the plug to near-new condition by removing carbon buildup that simple vibration cleaning might miss. The sandblaster reaches areas that are otherwise difficult to clean.
Air Flow Cleaning
After sandblasting, we blow air through the spark plug to remove any sand particles left inside.
This step is vital because leftover sand could cause damage to the engine. The air flow ensures the plug is completely clean and ready for testing or reinstallation.
Testing Spark Performance
To test the spark plug, we:
- Place it in a testing port that simulates cylinder conditions
- Connect the high voltage lead
- Press the test button to check for a visible spark
- Test with and without pressure
A good spark plug will show a strong, consistent spark. We can also add pressure to simulate engine operating conditions. If the spark remains strong even under pressure, the plug is ready to be reinstalled with a new gasket and thread lubricant.
Installing Aircraft Spark Plugs
Using Thread Protection
When reinstalling spark plugs, we need to apply a special thread lubricant.
This helps prevent the plugs from seizing in the engine.
Be careful to only apply the lubricant to the middle threads of the spark plug. Keep the first two threads clean!
This is important because the lubricant contains carbon, which can create dangerous hot spots if it gets deep inside the cylinder. A small amount on the middle threads is all you need for proper installation.
Getting the Tightness Right
After applying thread lubricant, hand-tighten the spark plug until it’s snug.
Then use a calibrated torque wrench to achieve the correct tightness.
For our engine, we need to tighten each spark plug to 35 foot-pounds (or 420 inch-pounds). Always check your specific engine manual for the correct torque specification.
When the wrench clicks, you’ve reached the proper torque. This prevents over-tightening which can damage the threads or under-tightening which can lead to leaks.
Attaching the Wires
The final step is connecting the spark plug leads. Each lead is marked to match its correct cylinder position. For example, “B2” is for the bottom spark plug on cylinder 2.
Carefully place the lead onto the spark plug terminal. Then, use the appropriate wrench to tighten the connector nut. Make sure it’s secure but not overtightened. Proper connection ensures good electrical contact for reliable engine operation.
