The primary effect of overloading an aircraft is:

The primary effect of overloading an aircraft is an increase in stall speed. When an aircraft is overloaded, it means that it has exceeded its maximum weight. This excess weight impacts several aspects of the aircraft's performance, particularly stall speed. As weight increases, the required lift also increases to maintain level flight. Because lift must counteract the weight of the aircraft, the stall speed—the speed at which the wing can no longer generate sufficient lift—also rises. This means pilots will need to fly at higher speeds to avoid entering a stall, which can be critical during takeoff and landing phases.

The stall speed increases because, according to the lift equation, lift generated by the wing is proportional to the square of the airspeed. Therefore, a heavier aircraft must reach a higher speed to produce the necessary lift to support its weight.

In this context, the other options do not accurately describe the primary effect of overloading. Decrease in thrust and greater fuel efficiency are not direct outcomes of overloading, as the engines will still provide the same thrust unless there are specific mechanical issues. However, overloading can negatively affect fuel efficiency because of increased drag and the need for more power to maintain performance at higher weight. The reduction in lift capability could be misleading;