What is most likely to cause fouling of spark plugs in an aircraft?

The most likely cause of fouling of spark plugs in an aircraft is related to the use of a rich fuel mixture. When the fuel mixture is too rich, it means that there is an excess of fuel relative to air in the combustion process. This situation can lead to incomplete combustion, where not all the fuel is burned off, resulting in fuel residues that can accumulate on the spark plugs. This build-up can cause the spark plugs to become fouled, leading to poor ignition and engine performance issues.

While gaining altitude without adjusting the mixture can affect engine performance, the direct influence on spark plug fouling in this context is predominantly linked to the richness of the fuel mixture rather than just altitude changes. It is essential for pilots to adjust the mixture appropriately during climbs to maintain optimal combustion and avoid fouling.

Other factors, such as the ignition system's adequacy, can contribute to performance problems but are not typically the primary cause of spark plug fouling. High humidity levels could also potentially have an effect on engine performance, but they are less directly associated with the fouling of spark plugs compared to the richness of the fuel mixture. Therefore, the condition of using a rich fuel mixture remains the most significant cause of spark plug fouling in an aircraft.