What does the fuel drain do in the Cessna 172?

The function of the fuel drain in the Cessna 172 is primarily to allow for the removal of any sediment, water, or contaminants that may accumulate in the fuel system. The fuel drain does this by providing a means for the pilot or maintenance personnel to sample the fuel from the fuel system, typically from the fuel sumps located on the wings and near the engine.

As a result, the correct answer is not about pumping air into the tank. Instead, the fuel drain serves a critical purpose in ensuring fuel quality by allowing for the inspection and removal of harmful substances that could affect engine performance. This maintenance step is essential for safety, as contaminated fuel could lead to engine failure during flight. By regularly checking and draining the fuel, pilots can help ensure that the fuel that enters the engine is clean and suitable for combustion.