Definition
A flight condition on the back side of the power curve where, at airspeeds below the speed for minimum drag, any decrease in airspeed causes total drag to increase, requiring more power just to hold altitude. Without prompt corrective action, the airplane tends to keep slowing and sinking rather than returning to its trimmed speed.
Plain English
When the airplane is flying very slowly, it does not naturally settle back to a steady speed. If it slows down a little, it wants to slow down more, and the pilot has to add power or lower the nose to stop the trend.
Context Anchor
Encountered during slow flight practice, especially when the airplane is flying near stall speed while maintaining altitude.
Derivation
Instability comes from the idea of something not being stable. In aviation, a stable condition tends to return toward where it was after a small change; an unstable condition tends to move farther away unless the pilot corrects it.
Why Pilots Care
Understanding this prevents unintentional stalls and helps maintain control during low-speed maneuvers.
Analogy
It is like riding a bicycle very slowly. At normal speed, small wobbles are easy to correct, but at very low speed, a small change can grow quickly unless you actively control it.
Grounding Statement
In slow flight, the airplane may need more power, not less, when it gets slower and starts to settle.
Intuition Check
Speed instability does not mean the airspeed indicator is broken or that the airplane is suddenly uncontrollable. It means the airplane’s speed can keep moving away from the desired speed unless the pilot corrects it.
Example Sentence 1
During slow flight practice, the instructor demonstrated speed instability by reducing power slightly and showing how the airplane continued to decelerate without prompt pitch and power correction.
Example Sentence 2
Recognizing speed instability early allows the pilot to add power and lower the nose before a stall develops.