14 C.F.R. § 23.2145 – Stability.

(a) Airplanes not certified for aerobatics must—
1. (1) Have static longitudinal, lateral, and directional stability in normal operations;
2. (2) Have dynamic short period and Dutch roll stability in normal operations; and
3. (3) Provide stable control force feedback throughout the operating envelope.
(b) No airplane may exhibit any divergent longitudinal stability characteristic so unstable as to increase the pilot's workload or otherwise endanger the airplane and its occupants.