Definition
The portion of an IFR flight that begins after the departure phase ends and continues until the arrival phase begins. It is the cruise segment flown along published airways or random routes between the departure airport area and the destination airport area, conducted under air traffic control on an IFR flight plan.
Plain English
The middle part of the flight — after you've climbed away from the departure airport and before you start setting up to arrive at the destination. It's the cruising segment between the two ends of the trip.
Context Anchor
Seen in instrument procedure discussions that divide a flight into departure, en route, arrival, and approach portions.
Derivation
From the French 'en route,' meaning 'on the way' or 'on the road.' In aviation, it carries the same sense — the part of the flight where you are simply on your way between the start and the end.
Why Pilots Care
Different rules, charts, and procedures apply during each phase of flight. En route charts, en route altitudes, and en route ATC facilities (centers) are distinct from the terminal-area charts and approach controllers used near airports. Knowing which phase you're in tells you which references and procedures apply.
Intuition Check
Do not read “en route” as meaning the whole trip from start to finish. In this context, it means the middle traveling portion of the flight, between the departure and arrival or approach portions.
Example Sentence 1
During the en route phase, the pilot navigated along the airway using VOR stations and remained in contact with the air route traffic control center.
Example Sentence 2
During the en route phase the pilot monitored the airway and reported position to ATC at each fix.