Definition
The condition reached during the developed phase of a spin in which the aerodynamic and inertial forces acting on the airplane have become balanced, so that the spin's rotation rate, vertical descent rate, and pitch attitude remain essentially constant from one turn to the next.
Plain English
The airplane has settled into a steady spin. It keeps rotating and descending at roughly the same rate each turn instead of getting faster, slower, steeper, or flatter.
Context Anchor
Seen in spin training during the developed phase, after the spin has become established and before recovery is started.
Derivation
Equilibrium comes from the Latin aequus (equal) and libra (balance), literally 'equal balance.' In a spin, it means the forces trying to speed up the rotation and the forces trying to slow it down have evened out.
Why Pilots Care
A pilot must recognize when equilibrium has been reached because recovery technique and timing change once the spin is fully developed.
Grounding Statement
In spin equilibrium, the airplane is stuck in a steady rotating fall rather than speeding up, slowing down, or changing shape dramatically.
Intuition Check
Equilibrium does not mean the airplane is level, safe, or recovered. Here it means the spin has settled into a steady balance that will usually continue until the pilot applies recovery controls.
Example Sentence 1
Once the spin reached equilibrium, each turn looked the same as the last, with a steady rotation rate and descent.
Example Sentence 2
The instructor waited for spin equilibrium before demonstrating the recovery inputs.