If an airplane is loaded towards the rear of its CG range, it will be unstable about its:

When an airplane is loaded towards the rear of its center of gravity (CG) range, it will exhibit instability about its lateral axis. This instability occurs because the aircraft's moment of inertia changes as the CG shifts rearward. In general aviation, the lateral axis runs from wingtip to wingtip, and the stability around this axis is characterized by the pitch behavior of the aircraft.

A rearward CG reduces the effectiveness of the tail's stabilizing force, making it more difficult for the aircraft to return to level flight after a pitch change. As a result, the aircraft may become more sensitive to pitch disturbances, leading to a tendency to pitch up or down more readily and increasing the risk of stalling or losing control.

Understanding this relationship between CG location and flight stability is crucial for safe aircraft operation, as the aircraft's design and loading must account for maintaining stability within prescribed limits.