Certified Flight Instructor - Flight Instructor Airplane Practice Exam

Under what condition is gaining altitude more likely to cause fouling of spark plugs?

1. If the mixture is adjusted properly

2. If there is a rich mixture

3. If there is a lean mixture

4. If the throttle is constant

The correct answer is: If there is a lean mixture

Gaining altitude often leads to lower air density, which can cause the fuel-air mixture to become overly lean if the mixture isn't adjusted appropriately. A lean mixture results in a higher combustion temperature that can lead to insufficient cooling of the spark plugs, causing them to operate at temperatures that may allow for deposits to build up. As an aircraft climbs, the engine also tends to compensate by leaning the mixture automatically, which means that, unless the pilot enriches the mixture accordingly, the spark plugs may not receive the optimal mixture they require for efficient operation. This insufficient mixture can result in incomplete combustion and the potential fouling of the spark plugs. High operating temperatures related to a lean mixture can also result in pre-ignition or other combustion anomalies that may further exacerbate the fouling issue. Therefore, the condition that is most likely to cause spark plug fouling during altitude gain is indeed related to a lean mixture.