Certified Flight Instructor - Flight Instructor Airplane Practice Exam

What is a possible consequence of an excessively rich mixture at high altitudes?

1. The engine may overheat

2. Fouling of spark plugs may occur

3. The engine operates smoothly with increased fuel consumption

4. The aircraft may experience increased climb performance

The correct answer is: Fouling of spark plugs may occur

An excessively rich mixture at high altitudes can lead to fouling of the spark plugs due to the increased amount of fuel relative to air. At high elevations, the air density decreases, and if the mixture remains excessively rich, there may not be sufficient oxygen for complete combustion. This incomplete combustion can result in carbon deposits that collect on the spark plugs, leading to fouling. When spark plugs become fouled, they may not ignite the fuel-air mixture effectively, causing engine performance issues, rough running, or even engine failure. While other outcomes, such as overheating or increased fuel consumption, may be points of consideration in various contexts, they do not occur directly from running a rich mixture at altitude in the same manner as spark plug fouling does. Understanding this relationship helps pilots manage fuel mixtures appropriately to maintain engine performance and reliability, particularly in scenarios with lower atmospheric pressure.