At which speed is the maximum surplus of power observed?

The maximum surplus of power is observed at the maximum rate of climb because this speed optimizes the available excess power a pilot can use to achieve the best performance in gaining altitude. At this point, the aircraft is in a condition where the power available exceeds the power required for level flight, allowing it to convert that excess power into vertical climb.

In practical terms, at the maximum rate of climb, the aircraft is operating at a specific speed where drag is minimized, and thrust is maximized relative to the aerodynamic forces acting on the aircraft. This combination results in the highest possible altitude gain within a given time frame, leading to the effective use of the engine's power.

Choosing this speed ensures that the aircraft maintains a climb with the most efficient use of its engine power, effectively utilizing the horsepower available to ascend rapidly. This is crucial during various flight operations, particularly in emergency situations or when a pilot seeks to achieve a safe altitude as quickly as possible.