Plain English Meaning

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.

Full 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 .