Definition
Aeronautical charts published for navigation under Instrument Flight Rules, depicting the airway system, navigation aids, intersections, minimum altitudes, controlled airspace boundaries, communication frequencies, and other information required to fly point-to-point on instruments. Two series exist: Low Altitude charts (covering routes below 18,000 feet MSL) and High Altitude charts (covering routes from 18,000 feet MSL up to FL450).
Plain English
These are the maps pilots use when flying on instruments. They show the highways in the sky -- the airways, the navigation stations that define them, the minimum safe altitudes, and who to talk to along the way.
Context Anchor
You see IFR En Route Charts during instrument flight planning and in the cockpit while navigating the middle portion of an IFR flight.
Derivation
IFR stands for Instrument Flight Rules. 'En route' comes from French, meaning 'on the way' or 'during the journey' -- so these are the charts you use while on the way between airports, as opposed to charts used near the airport for departure, arrival, or approach.
Why Pilots Care
They supply the navigation data needed to fly approved routes safely and maintain required separation from terrain and traffic under instrument conditions.
Analogy
It is like a road map for the highway portion of a trip. It does not show every driveway or parking spot; it shows the main routes and the information needed to travel safely between areas.
Intuition Check
Do not read “en route” as the entire flight from engine start to shutdown. In this term, it means the route portion between the departure phase and the arrival or approach phase.
Example Sentence 1
Before departure, she pulled up the Low Altitude IFR En Route Chart to confirm the minimum en route altitude for the airway segment east of the VOR.
Example Sentence 2
Before departure the instructor reviewed the latest IFR En Route Charts for any temporary flight restrictions along the planned route.