Definition
On an instrument approach chart plan view, a circle (typically 10 nautical miles in radius, centered on the airport or another fix) that defines an area within which terrain, obstacles, and navigation features are depicted to scale. Features inside the circle are drawn proportionally; features outside it may be shown out of scale to fit the chart.
Plain English
A circle drawn on an approach chart that shows you which part of the chart is drawn to true scale. Inside the circle, distances and positions are accurate. Outside it, things may be squeezed or stretched to make them fit on the page.
Context Anchor
Seen in instrument-display explanations where the airplane is shown from above, such as a plan-view navigation display.
Derivation
Reference comes from a word meaning to carry or relate back to something. In this term, the circle is the thing you refer back to when judging direction on the display.
Why Pilots Care
Gives instant situational awareness of distance to the airport without needing to measure on the chart during a high-workload phase of flight.
Intuition Check
Do not read reference circle as just any circle drawn on the instrument. Here it means the circle used as the directional guide for interpreting the plan view.
Example Sentence 1
Briefing the approach, the pilot noted the tall tower shown just inside the reference circle and confirmed its position relative to the final approach course.
Example Sentence 2
Once inside the reference circle, the pilot began configuring the aircraft for landing.