What is the main action taken by pilots during the descent phase of a flight?

During the descent phase of a flight, pilots primarily focus on reducing engine power. This action is consistent with the overall goal of gradually lowering the aircraft's altitude in a controlled manner. By reducing engine power, pilots allow the aircraft to descend naturally, utilizing the lift-to-drag ratio effectively while managing speed and altitude.

This reduction in power helps to ensure a gradual and safe descent, which is critical for stabilizing the flight path and preparing for landing procedures. A controlled descent also aids in managing fuel consumption, ensuring that the aircraft remains efficient and safe as it approaches its destination.

In the context of the other options, increasing engine power would typically be associated with climbing rather than descending, while maintaining constant speed would not accurately reflect the necessary adjustment of power during the descent. Engaging autopilot, while a common practice, is more about maintaining control rather than specifically addressing the power management needed for descent. Thus, the focus on reducing engine power during descent is crucial for flight safety and efficiency.