Definition
A condition in which an airplane, after being disturbed in roll, tends to continue rolling and enter a progressively tightening descending turn rather than returning to wings-level flight. It results from strong directional (yaw) stability combined with weak lateral (roll) stability: the airplane yaws into the bank, the outer wing travels faster and produces more lift, the bank steepens, the nose drops, and airspeed and rate of descent increase if the pilot does not intervene.
Plain English
If a wing drops and the pilot does nothing, the airplane will not pick the wing back up on its own. Instead, the bank slowly gets steeper, the nose drops, and the airplane winds itself into a tightening descending turn that keeps getting faster.
Context Anchor
Encountered in discussions of airplane stability, turns, and instrument flying, especially when describing what can happen if a pilot lets a bank go unnoticed.
Derivation
‘Spiral’ comes from the Latin spira, meaning a coil or winding shape — describing the tightening, descending corkscrew path the airplane follows. ‘Instability’ means the airplane does not return to its original condition on its own. Together: a tendency to wind into a descending coil rather than recover.
Why Pilots Care
Left uncorrected, spiral instability can lead to an unintentional steep spiral dive, increasing airspeed and load factor beyond safe limits.
Grounding Statement
Picture an airplane left slightly banked: instead of gently leveling, it keeps turning more tightly and descending until the pilot corrects it.
Intuition Check
Spiral instability is not the same as a spin. In spiral instability, the main problem is a bank and descending turn that keep increasing if the pilot does not stop them.
Example Sentence 1
Because of spiral instability, the pilot has to make small aileron corrections every few seconds to keep the wings level when flying without an autopilot.
Example Sentence 2
Because of spiral instability, the student kept a light hand on the controls during the long straight-and-level segment.