FAA Ground School Practice Test

What effect does using fuel that has a lower-than-specified fuel rating have on engine temperature gauges?

1. It can help maintain normal operating temperatures

2. It will most likely cause the gauges to exceed their normal operating ranges

3. It has no effect on engine temperature gauges

4. It may cool the engine more effectively

The correct answer is: It will most likely cause the gauges to exceed their normal operating ranges

Using fuel that has a lower-than-specified fuel rating can lead to engine knocking or detonation because it does not ignite in the same controlled manner as higher-rated fuels. This uncontrolled combustion event can result in higher temperatures in the combustion chamber, which translates to increased overall engine temperatures. As a consequence, the temperature gauges are likely to show readings that exceed their normal operating ranges. It is critical to use fuel that meets or exceeds the required octane rating specified by the engine manufacturer to prevent overheating and maintain engine integrity. Lower-rated fuel essentially alters the combustion process negatively, thereby compromising engine performance and cooling, which is not only detrimental but may also lead to engine damage over time.