The short-period mode is an oscillation with a period of only a few seconds that is usually heavily damped by the existence of lifting surfaces far from the aircraft’s center of gravity, such as a horizontal tail or canard.
Ability to quickly self damp when the stick is briefly displaced is one of the many criteria for general aircraft certification.
If the Dutch roll is very lightly damped or unstable, the yaw damper becomes a safety requirement, rather than a pilot and passenger convenience.
Most aircraft trimmed for straight-and-level flight, if flown stick-fixed, will eventually develop a tightening spiral-dive.
A spiral dive is not a spin; it starts, not with a stall or from torque, but with a random perturbation, increasing roll and airspeed.
Without prompt intervention by the pilot this can lead to structural failure of the airframe, either as a result of excess aerodynamic loading or flight into terrain.
The pilot's "down" sensation continues to be with respect to the bottom of the airplane, although the aircraft actually has increasingly rolled off the true vertical.
Under VFR conditions, the pilot corrects for small deviations from level by automatically using the true horizon, but in IMC or dark conditions the deviations can go unnoticed: the roll will increase and the lift, no longer vertical, is insufficient to support the airplane.
Now examine the resulting forces one at a time, calling any rightward influence yaw-in, leftward yaw-out, or roll-in or -out, whichever applies.
The sequence is: Oscillations can be caused longitudinally or laterally by fuel slosh, a phenomenon known to have affected aircraft including the Douglas A4D, Lockheed P-80, Boeing KC-135, Cessna T-37 and the North American YF-100.