Definition
The phase of IFR flight in which an aircraft transitions between assigned altitudes along its route, conducted in compliance with ATC clearances, published altitude restrictions, and standard climb/descent procedures. Unless otherwise instructed, pilots are expected to climb or descend promptly upon receipt of an ATC clearance, at an optimum rate consistent with the operating characteristics of the aircraft, until reaching within 1,000 feet of the assigned altitude, then at a rate between 500 and 1,500 feet per minute to the assigned altitude.
Plain English
Changing altitude while flying along your route under instrument flight rules. When ATC clears you to a new altitude, you climb or descend at a normal rate, slowing the rate as you get within the last 1,000 feet so you arrive smoothly at the new altitude.
Context Anchor
Seen in instrument procedure planning and in flight when a pilot requests, receives, or must follow an altitude change while traveling between departure and arrival areas.
Derivation
Climbing comes from an older English word meaning to go upward. Descending comes from Latin words meaning to climb down. En route comes from French and means “on the way.” Together, the phrase points to altitude changes made while the aircraft is already on its planned way from one place to another.
Why Pilots Care
Proper handling prevents terrain conflicts and loss of separation while allowing efficient altitude changes for performance or weather.
Intuition Check
Do not read this as simply “going up or down anywhere in flight.” In this FAA context, it means altitude changes made during the route portion of an instrument flight, under clearances and published altitude requirements.
Example Sentence 1
After being cleared to FL230, the crew began climbing and descending en route per standard procedure, reducing their rate to 1,000 feet per minute as they passed FL220.
Example Sentence 2
During the descent en route the crew verified the MEA remained below their assigned altitude for obstacle clearance.