Definition
Approaches in which the airplane is not on a stabilized path to the runway by a defined point — meaning airspeed, descent rate, configuration, power, or alignment are not within target ranges — and/or the airspeed is allowed to decay below the target approach speed, increasing the risk of high sink rate, loss of control, or an inability to arrest the descent before touchdown.
Plain English
An approach where the airplane isn't where it should be, doing what it should be doing, at the speed it should be doing it. Either things aren't settled and steady (unstable), or the airplane is flying too slowly (slow), or both — and that combination makes a safe landing much harder.
Context Anchor
Encountered during approach and landing training, especially when discussing energy management on final approach.
Derivation
"Stable" comes from Latin stabilis, meaning steady or firmly fixed. An unstable approach is one that is not steady — parameters are still changing or out of tolerance when they should be settled. "Slow" here is used in the specific sense of below the target approach speed, not just "unhurried."
Why Pilots Care
These approaches are a leading factor in runway excursions, hard landings, and loss-of-control events, often requiring an immediate go-around.
Grounding Statement
Close to the ground, a slow airplane has little height and little extra speed available, so a small mistake can become serious quickly.
Intuition Check
Do not read unstable as only meaning bumpy or uncomfortable. Here it means the approach is not properly controlled and settled. Do not read slow as simply calm or careful. Here it means the airplane may be below the speed needed for a safe landing approach.
Example Sentence 1
The pilot recognized an unstable approach when airspeed dropped ten knots below target on short final and elected to go around.
Example Sentence 2
Slow approaches below target speed can quickly turn into an irreversible sink rate that is difficult to arrest.