Certified Flight Instructor - Flight Instructor Airplane Practice Exam

Fouling of spark plugs is more likely to occur if the aircraft:

1. Gains altitude with no mixture adjustment

2. Descends from altitude with no mixture adjustment

3. Throttle is advanced very abruptly

4. Idles for a long period of time

The correct answer is: Gains altitude with no mixture adjustment

Fouling of spark plugs in an aircraft engine typically occurs when the fuel-air mixture is too rich, which leads to excess fuel condensing on the spark plugs and forming deposits. This can hinder proper ignition and engine performance. When an aircraft gains altitude, the density of the air decreases, which means that the mixture should ideally become leaner to compensate for the reduced oxygen availability. If the mixture control is not adjusted accordingly while climbing, the engine continues to receive a richer mixture than optimal. This rich condition can result in unburned fuel making its way to the spark plugs, ultimately leading to fouling. In contrast, during a descent, the air density increases, and generally, the engine's mixture becomes closer to optimal without adjustments, which reduces the likelihood of fouling. Abruptly advancing the throttle can momentarily lead to a rich mixture, but this is typically not prolonged enough to cause significant fouling. Idling for extended periods can also contribute to fouling due to low engine temperatures and incomplete combustion, but in the context of altitude gain without mixture adjustment, the direct and immediate impact of a rich mixture makes it more likely to lead to fouling.