FAA Powerplant Written Practice Test

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

1. A lean fuel mixture

2. An excessively rich mixture

3. Proper ignition timing

4. Cracked exhaust valves

The correct answer is: An excessively rich mixture

After-firing, also known as backfiring or afterburning, occurs when unburned fuel ignites in the exhaust system of an aircraft engine. An excessively rich fuel mixture contributes to this phenomenon because it leads to an abundance of fuel that may not completely combust in the combustion chamber. When this unburned fuel enters the exhaust system, it can ignite due to the heat present in the exhaust, resulting in a noticeable popping or banging sound. In contrast, a lean fuel mixture would generally lead to detonation or misfiring rather than after-firing, as there isn’t enough fuel to ignite properly. Proper ignition timing ensures that the fuel-air mixture is ignited at the optimal moment for efficient combustion, thereby preventing unburned fuel from making its way into the exhaust. Cracked exhaust valves may cause other issues like decreased engine performance or excessive exhaust emissions, but they are not a direct cause of after-firing. Thus, an excessively rich mixture is the primary cause because it allows for the conditions conducive to after-firing to occur.