Commercial Pilot License (CPL) 2026 – 400 Free Practice Questions to Pass the Exam

Fouled spark plugs in an aircraft engine are primarily caused by excessively rich mixture settings. When the fuel-air mixture is too rich, it contains too much fuel relative to the amount of air, which can lead to incomplete combustion. This incomplete combustion results in carbon deposits accumulating on the spark plugs. These deposits can disrupt the spark plugs’ ability to generate a spark, leading to engine performance issues, misfiring, or even engine failure.

Operating at high altitudes can affect engine performance and the fuel-air mixture, but it is not a direct cause of fouled spark plugs in the same way a rich mixture would be. Although using the incorrect fuel type can negatively impact engine performance and could potentially lead to fouling, it is not the most common cause associated with spark plug fouling. Similarly, incorrect engine timing might cause poor combustion and performance issues but isn’t a primary contributor to the buildup of deposits on spark plugs like a rich mixture is.