Definition
The tendency of an airplane to return to its trimmed airspeed after a small disturbance, without pilot input. On the front side of the power curve (normal cruise speeds), an airplane has positive speed stability: if it slows down, drag decreases and it tends to accelerate back; if it speeds up, drag increases and it tends to slow back. On the back side of the power curve (very slow flight, behind the drag curve), the airplane has negative speed stability: a small loss of airspeed increases drag, which slows it further, requiring active pilot intervention to prevent continued deceleration.
Plain English
How well the airplane holds its airspeed on its own. At normal speeds it naturally settles back to the speed you trimmed for. At very slow speeds it does the opposite — once it starts slowing, it keeps slowing unless you do something about it.
Context Anchor
Encountered in slow flight, approach-to-stall practice, and discussions of how the airplane behaves near the low-speed end of its range.
Derivation
Speed comes from an Old English word meaning success or quickness. Stability comes from Latin stabilis, meaning firm or steady. Together, the phrase points to how firmly or steadily an airplane holds its speed after being disturbed.
Why Pilots Care
Poor speed stability increases the risk of unintended airspeed deviations that can lead to stalls or loss of control during approach and landing.
Grounding Statement
If the airplane is trimmed for a slow speed and a gust or pitch change slows it further, speed stability describes whether it tends to recover that speed or keep drifting away from it.
Intuition Check
Speed stability does not mean the airspeed is simply constant right now. It means what the airplane tends to do after the speed is disturbed: return toward the original speed, or move farther away from it.
Example Sentence 1
Because the airplane was operating on the back side of the power curve, it had negative speed stability, so the pilot added power immediately when the airspeed began to decay.
Example Sentence 2
At low speeds near the stall, reduced speed stability required the pilot to pay closer attention to airspeed trends.