In what situation will an aircraft fly below the climb-out path due to wind shear?

In the context of wind shear and its impact on an aircraft's climb-out path, the situation where an aircraft may fly below the optimal climb-out trajectory is when there is a decreasing headwind after take-off.

When an aircraft takes off, it relies on the headwind to generate lift and achieve the necessary climb performance. If the headwind decreases shortly after take-off, the aircraft experiences a reduction in lift. This sudden change can result in an inability to maintain the desired climb rate, potentially causing the aircraft to descend or fly lower than the planned climb-out path. The effect of reduced lift combined with the aircraft's need for a certain airspeed to maintain effective climb performance underscores the critical role of wind conditions during this crucial phase of flight.

Conversely, the other options describe scenarios where the influence of wind would not lead to a scenario where the aircraft flies below the climb-out path. For instance, an increasing headwind before landing generally provides additional lift, while a consistent headwind during cruise would maintain the aircraft's performance without introducing descent risks. A tailwind during climb-out might result in an increased groundspeed, but it does not directly decrease the climb performance in the same critical context as a decreasing headwind shortly after take-off.