Definition
On a stabilized visual approach, the narrow band of flight conditions — airspeed, descent rate, alignment, and glidepath — that must be met as the airplane crosses the runway threshold. If the airplane arrives at the threshold within these tolerances, a normal landing follows; if it arrives outside them, a go-around is the correct action.
Plain English
A small set of 'must-meet' conditions for the moment you cross the start of the runway. Cross the threshold on speed, on glidepath, lined up, and descending normally — land. Cross it fast, high, low, or off-centerline — go around.
Context Anchor
Used when judging whether an approach is still stabilized as the airplane nears the runway threshold.
Derivation
Threshold' is the beginning of the landing portion of the runway. 'Window' is used here in the sense of a narrow opening in time and space — the same idea as a 'launch window.' Together: the small opening of acceptable conditions you must fit through as you reach the runway.
Why Pilots Care
Remaining inside the threshold window confirms the approach is stabilized, reducing the chance of a runway excursion, hard landing, or unnecessary go-around.
Analogy
It is like aiming to pass through a doorway: being close is not enough if you are too high, too low, or moving at the wrong speed to enter cleanly.
Grounding Statement
Picture the airplane crossing the start of the runway at the planned height, lined up with the center, and moving at a safe landing speed.
Intuition Check
Do not read “window” as a physical window in the airplane. Here it means an acceptable range of position and condition over the runway threshold.
Example Sentence 1
We were slightly fast and high crossing the numbers, outside the threshold window, so the instructor called for a go-around.
Example Sentence 2
Staying inside the threshold window on short final allowed a smooth touchdown in the first third of the runway.