Definition
The time, calculated by an air traffic facility, at which an aircraft is expected to cross from one ATC sector or facility's airspace into the next, based on the proposed (filed) flight plan rather than an active flight already underway.
Plain English
An estimated time, worked out from your filed flight plan, of when your aircraft will pass from one controller's airspace into the next controller's airspace.
Context Anchor
Seen in air traffic control planning, flight progress, and handoff coordination between control facilities or sectors.
Derivation
"Proposed" because the time is based on the proposed flight plan, not on an aircraft already airborne and being tracked. "Boundary" refers to the edge of a controller's assigned airspace. Together: the predicted moment the aircraft will cross that edge.
Why Pilots Care
It lets controllers plan handoffs, spacing, and workload before you actually reach the boundary.
Intuition Check
Do not read “proposed” as a pilot request or a clearance. Here it means a predicted planning time used by ATC for coordination.
Example Sentence 1
The center used the proposed boundary crossing time from the flight plan to coordinate the handoff with the adjacent facility before the aircraft departed.
Example Sentence 2
ATC requested an updated Proposed Boundary Crossing Time after the pilot accepted a direct routing that would change the arrival time at the sector edge.