Definition
Aeronautical charts used for IFR navigation below 18,000 feet MSL, depicting the airway system, navigation aids (VORs, NDBs), intersections, minimum altitudes, airway distances, controlled airspace boundaries, airports with instrument approaches, and communication frequencies needed for en route flight under instrument flight rules.
Plain English
The IFR road maps pilots use when flying below 18,000 feet. They show the published airways, the radio navigation stations that define those airways, the lowest altitudes you're allowed to fly on each segment, and the frequencies you'll need to talk to controllers.
Context Anchor
Seen during instrument flight planning and while flying the cruise portion of an IFR flight below 18,000 feet.
Derivation
"En route" comes from French, meaning "on the way" or "along the route." "Low-altitude" distinguishes these charts from the high-altitude charts used at and above FL180. Together, the name simply tells you what the chart is for: navigating along your route at lower altitudes.
Why Pilots Care
They supply the route, altitude, and communication details required for safe navigation when flying solely by instruments.
Intuition Check
Low-altitude does not mean any chart used while flying close to the ground. Here it means the FAA chart designed for IFR route navigation in the lower altitude system, generally below 18,000 feet above mean sea level.
Example Sentence 1
Before departing on the IFR flight to Asheville, she pulled up the en route low-altitude chart on her tablet to review the airways and minimum altitudes along the route.
Example Sentence 2
Before takeoff she reviewed the en route low-altitude charts to confirm the navigation fixes along her planned path.