Commercial Pilot License (CPL) Practice Exam 2026 - Free Commercial Pilot License Practice Questions and Study Guide

Spark plug fouling is primarily caused by an excessively rich fuel mixture. When the fuel-to-air ratio in the combustion chamber is too rich, it leads to incomplete combustion of the fuel. This results in carbon buildup on the spark plugs. When too much fuel is introduced, it does not allow for the spark plugs to reach the temperatures necessary to burn off the deposits. Over time, these deposits can accumulate, leading to misfires or engine performance issues.

In considering the other options, insufficient oil in the engine can lead to various mechanical issues but is not a primary cause of spark plug fouling. Cold weather conditions might impact engine performance and starting but typically do not result in spark plug fouling on their own. Pilot error during takeoff can include issues that affect the initial flight phase, but it does not directly relate to the specific phenomenon of spark plug fouling caused by fuel mixture dynamics.