FAA Powerplant Written Practice Test

What is one cause of after-firing in an aircraft engine?

1. An excessively rich mixture

2. A lean fuel mixture

3. Faulty ignition timing

4. Overheating of the engine components

The correct answer is: An excessively rich mixture

The phenomenon of after-firing in an aircraft engine can occur due to an excessively rich fuel-air mixture. When the mixture is too rich, it means there is an abundance of fuel in relation to the air. In such cases, not all of the fuel burns completely during the combustion process within the engine cylinders. When the unburned fuel enters the relatively cooler exhaust system or other downstream components, it can ignite due to heat sources in those areas, resulting in a popping or backfiring sound. After-firing is associated with inefficient combustion, and while other factors like ignition timing and fuel mixture can also influence engine performance, the key aspect of an excessively rich mixture leading to after-firing is the surplus of unburned fuel that finds its way into the exhaust system. This understanding is essential for pilots and mechanics when diagnosing issues related to engine performance and ensuring optimal combustion efficiency.