An aircraft's longitudinal (pitch) stability depends entirely on where its Center of Gravity (CG) is located. It is necessary to balance between stability and controllability, because as stability goes up, the aircraft becomes harder to control.

A forward CG position makes the aircraft highly stable but less responsive to pilot inputs. Because of this, a forward CG limit is established to ensure the pilot always has enough control authority to manoeuvre the aircraft. If the CG moves too far past this forward limit, the nose becomes incredibly heavy. The pilot will have to constantly pull back on the controls just to fly straight and level, and they might run out of elevator authority completely, for instance, when trying to pull the nose up during the landing flare.

An aft CG position has the opposite effect - it increases controllability but destroys stability. An aft CG limit is set to ensure the aircraft remains naturally stable enough to fly safely. If the CG moves beyond this aft limit, the aircraft becomes highly unstable and unpredictable. The nose will want to pitch up aggressively on its own, and the pilot may not have enough elevator authority to force the nose back down to recover from a dangerous nose up condition or a stall. With an aft CG, the aircraft becomes incredibly sensitive to pitch inputs on the controls. A tiny movement of the yoke results in a massive, violent pitch response.