Definition
An invisible boundary surrounding an aircraft, used by air traffic control automation to detect potential conflicts with other aircraft, airspace, or terrain. When another aircraft's projected path is calculated to penetrate this boundary, the system alerts the controller to a possible problem requiring resolution.
Plain English
A computer-drawn safety bubble around each aircraft. If another aircraft is predicted to enter that bubble, the controller's screen flags it as a possible conflict so the controller can act before it becomes a real problem.
Context Anchor
Seen in FAA glossary and air traffic control automation discussions. It is not a cockpit instrument, pilot clearance, or route boundary the pilot navigates by.
Derivation
"Boundary" comes from Old French bodne meaning a limit or marker. Here, the boundary isn't a physical line — it's a calculated zone the automation watches around each aircraft.
Why Pilots Care
Automation systems use these boundaries to alert controllers to developing issues before they affect your flight.
Grounding Statement
Picture a computer-drawn warning zone around controlled airspace where the system starts checking flights for possible future traffic conflicts.
Intuition Check
Do not think of this boundary as a visible line, a charted route limit, or a clearance limit for the pilot. In this context, the boundary belongs to air traffic control software and tells the computer where to start looking for possible problems.
Example Sentence 1
The controller issued an early turn after the automation showed a projected APB intrusion between the two arrivals.
Example Sentence 2
APB settings are adjusted during high traffic periods to give earlier detection of potential problems.