FAA Powerplant Written Practice Test

What is a common cause of spark plug fouling in aircraft engines?

1. High cylinder head temperatures

2. Low cylinder head temperatures

3. Excessive engine RPM

4. Using incorrect fuel type

The correct answer is: Low cylinder head temperatures

Spark plug fouling in aircraft engines is commonly associated with low cylinder head temperatures. When engine temperatures are low, the combustion process may not burn fuel completely, leading to the accumulation of unburned fuel and byproducts, such as carbon deposits, on the spark plugs. These deposits can insulate the plug, making it difficult for it to produce a spark, which can eventually lead to engine misfires. Low temperatures can also result from inadequate operation of the engine, such as prolonged periods of taxiing or operation at low power settings, which fail to bring temperatures up to optimal levels. This scenario is particularly common in piston engines that operate at lower power settings for extended periods, thereby contributing significantly to spark plug fouling. The other choices, while they can affect engine performance, do not directly relate to the common issue of spark plug fouling in the same way that low cylinder head temperatures do. For instance, high cylinder head temperatures typically indicate a good combustion process, while excessive engine RPM is more likely to create conditions that prevent fouling. Using the incorrect fuel type can lead to other issues, but low temperatures remain a primary factor in this specific context of fouling.