Definition
An aerodynamic condition in slow flight on the back side of the power curve, where a reduction in airspeed produces an increase in induced drag that exceeds available thrust, causing the airplane to continue decelerating unless pitch is lowered or the airplane is reconfigured. Without corrective action, airspeed diverges further from the trimmed value rather than returning to it.
Plain English
When you're flying very slowly, slowing down a little can actually make the airplane keep slowing down on its own, because drag rises faster than the engine can keep up with. The airplane won't naturally settle back to its old speed -- it will keep falling away from it.
Context Anchor
Encountered in slow flight training and in discussions of how an airplane behaves near the lower end of its safe speed range.
Derivation
Diverge comes from the Latin divergere, meaning 'to bend away' or 'move apart.' Here, airspeed bends away from where you wanted it instead of returning, which is the opposite of how a normally trimmed airplane behaves at cruise.
Why Pilots Care
Unrecognized speed divergence in slow flight can lead to an unintended stall or loss of altitude control.
Grounding Statement
In slow flight, if the airplane begins to slow and nothing is changed, it may keep slowing rather than fix itself.
Intuition Check
Speed divergence does not mean two aircraft have different speeds. It means one airplane’s airspeed is moving away from the speed the pilot is trying to maintain.
Example Sentence 1
During slow flight practice, the instructor demonstrated speed divergence by reducing power slightly and showing how the airspeed kept decaying until the nose was lowered.
Example Sentence 2
Recognizing the first signs of speed divergence helps the pilot add power early and return to the front side of the power curve.