Definition
A proposed change to an aircraft's flight trajectory entered into an air traffic control automation system so the controller can evaluate it against surrounding traffic and conditions before deciding whether to issue it as a clearance. The trial plan is processed by the system to detect potential conflicts but is not transmitted to the aircraft and does not alter the active flight plan.
Plain English
A 'what if' check the controller runs in the computer to see whether a possible new route, altitude, or speed for an aircraft would cause a problem. Nothing happens to the flight until the controller actually issues the instruction.
Context Anchor
Seen in air traffic control automation and traffic management discussions, especially when a controller is checking a possible route or altitude change.
Derivation
From 'trial' (a test or try) and 'plan' (a proposed course of action). The name reflects exactly what it is: a tested-but-not-yet-committed plan.
Why Pilots Care
It lets pilots and controllers discover routing or altitude problems early, before an official plan is submitted.
Intuition Check
A Trial Plan is not a training plan or a rough guess. In ATC use, it is a proposed flight-plan change tested by the computer before the controller uses it.
Example Sentence 1
The controller ran a trial plan to see if descending the arrival ten miles earlier would conflict with departing traffic.
Example Sentence 2
Before filing his IFR clearance, the pilot ran a trial plan in the flight-planning software to confirm the routing was acceptable.