Definition
A path-and-terminator leg type used in RNAV procedure coding in which the aircraft flies a specified course (a magnetic track) until reaching a defined slant-range distance from a referenced DME station. The leg ends when that DME distance is reached, at which point the next coded leg begins.
Plain English
Fly this exact direction until you are a certain distance from a specific ground station, then the next part of the procedure starts.
Context Anchor
Seen in instrument procedure coding for approaches, departures, and arrivals, especially when describing path and terminator leg types.
Derivation
In ARINC 424 leg-type coding, each leg is given a two-letter label. The first letter describes the path being flown (C = course); the second letter describes what ends the leg (D = DME distance). So 'CD' literally reads as 'course terminating at a DME distance.'
Why Pilots Care
It tells the navigation system exactly when the current segment ends so the aircraft can begin the next turn or leg at the correct point.
Grounding Statement
The aircraft stays on the specified course until the DME readout reaches the published distance.
Intuition Check
Do not read “course” as the airplane’s heading; here it means the intended path over the ground. Do not read “DME distance” as miles flown along the leg; it is the distance shown from the selected DME station.
Example Sentence 1
After takeoff, the SID coded a CD leg requiring the crew to fly the 090 course until 8 DME from the field, where the next leg began.
Example Sentence 2
After completing the CD leg the aircraft turned inbound to join the final approach course.