If the centre of gravity is behind the aft limit, how does this affect longitudinal stability?

When the center of gravity is located behind the aft limit, it has a significant impact on the aircraft's longitudinal stability. A center of gravity situated too far aft leads to a condition known as "nose-up" stability, which actually decreases overall stability. This means that any slight disturbance or change in the aircraft's pitch can result in an increase in the nose-up attitude, making it more challenging to return to a stable flight attitude. Consequently, the aircraft becomes more susceptible to pitch changes and more difficult to control.

As for controllability, while it may seem intuitive that decreased stability could enhance control, this is not the case in this scenario. The lack of stability means that pilots must exert more effort to manage the aircraft’s attitude, which can lead to increased workload and decrease effective control, particularly in turbulent conditions.

Therefore, when the center of gravity is behind the aft limit, it leads to decreased stability and decreased controllability, aligning with the correct answer provided.