Definition
In Traffic Flow Management (TFM), the freeze horizon is the point in time or distance from a constrained airport at which an aircraft's arrival sequence and assigned arrival time become fixed and are no longer adjusted by the automation. Once an aircraft crosses the freeze horizon, its place in the arrival stream is locked.
Plain English
It is the point at which the system stops shuffling aircraft around and locks in each arriving aircraft's slot in line. After that point, your sequence and arrival time will not be changed by the automation.
Context Anchor
Seen in air traffic flow and arrival spacing discussions, especially where controllers are managing a stream of aircraft going to a busy airport.
Derivation
The word 'horizon' is borrowed here in its planning sense — a boundary in time or distance beyond which things become fixed. 'Freeze' means the schedule is locked. Together: the line at which the arrival plan stops changing.
Why Pilots Care
Crossing this altitude in visible moisture can cause rapid ice accumulation on wings and airframe, reducing lift and increasing stall speed.
Analogy
Think of a line of cars approaching a toll booth. Far away, cars may still change lanes and reorder themselves. Close to the booth, the line is effectively set so traffic can keep moving smoothly.
Grounding Statement
Imagine climbing on a warm day; once you pass this height the outside air is suddenly at or below freezing even if rain or clouds are present.
Intuition Check
Freeze Horizon does not refer to ice, weather, or the visual horizon outside the windshield. Here, it means the planning boundary where an arrival sequence becomes fixed.
Example Sentence 1
Once the flight crossed the freeze horizon, its arrival time into the metered airport was fixed and no further sequence changes were applied.
Example Sentence 2
After climbing through the freeze horizon the crew activated the wing boots as light rime began to form on the leading edges.