Fig. 10. The four general types of solution to the longitudinal equations of motion. (A) Monotonic subsidence (stable), corresponding to a negative real root. (B) Monotonic divergence (unstable), corresponding to a positive real root. (C) Damped oscillation (stable), corresponding to a complex conjugate pair of roots with negative real parts. (D) Divergent oscillation (unstable), corresponding to a complex conjugate pair of roots with positive real parts.