What could cause a gradual decrease in power during flight?

A gradual decrease in power during flight can be attributed to carburetor icing or improper leaning of the mixture.

When an aircraft operates in conditions of high humidity and low temperatures, ice can form in the carburetor, obstructing airflow and leading to a loss of power. This situation is particularly common when flying at lower power settings or during descents where temperature and humidity may change rapidly. Carburetor icing can significantly reduce engine performance and must be monitored by the pilot, who may need to apply carburetor heat to mitigate this issue.

Improper leaning of the fuel mixture can also cause power loss, particularly at altitude, where the air is less dense. If the mixture is too rich (too much fuel compared to air), the engine may not operate efficiently, resulting in reduced power output. Pilots must carefully lean the mixture according to altitude and power setting to optimize engine performance.

While the other options present scenarios that could affect flight performance, they do not specifically lead to a gradual decrease in power in the same way that carb icing or improper leaning does.